Prof Andrew Gosler FLS, FRSB is Associate Professor in Applied Ethnobiology and Conservation at Oxford University; a joint position between the Edward Grey Institute of Field Ornithology and the Institute of Human Sciences; and is Fellow in Human Sciences at Mansfield College. With over 30 years' experience of research and teaching in ornithology, ecology, evolution and conservation he refocused his research in Ethno-ornithology after publication of the book "Ethno-Ornithology: Birds, Indigenous Peoples, Culture and Society" which he co-edited with Sonia Tidemann in 2010. He is a strong advocate for community empowerment in conservation, and is Research Director of EWA - the Ethno-ornithology World Atlas (formerly Archive); see: https://EWAtlas.net Phone: +44 7778302488 Address: Institute of Human Sciences, The Pauling Centre, 58a Banbury Rd., Oxford OX2 6QS
Many small passerine birds worldwide lay white eggs speckled with red, brown and black proto-porp... more Many small passerine birds worldwide lay white eggs speckled with red, brown and black proto-porphyrin pigment spots (maculation). Unlike some patterns of avian eggshell pigmentation which clearly serve a crypsis or signalling function, the ubiquity of maculation among passerines suggests that its origins lie in another function, not speciWc to any particular ecological or behavioural group. Elsewhere, we have presented evidence that protoporphyrin pigments serve a structural function related to eggshell thickness and calcium availability: eggshell maculation in the great tit Parus major increases with decreasing soil calcium levels, pigments demarcate thinner areas of shell, and both the pigment intensity and distribution are related to shell thickness. Here we show that macula-tion also aVects the rate of water loss from the egg during incubation (t Mass Loss per Day or MLD, which is critical to egg viability), but not that of unincubated eggs. We also demonstrate, both by observation and experiment, that the eVect of female incubation behaviour on MLD compensates in some way for variation in egg characteristics, and that diVerences between females in the degree of such compensation are related to diVerences in clutch maculation. Our results suggest that, while a principal function of maculation in this species may be to strengthen the eggshell, it may also reduce eggshell permeability when large amounts of pigment are used, and that this necessitates a behavio-ural adjustment from the female during incubation. We discuss these Wndings and make further testable predictions from our model.
Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines... more Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines, which lay white eggs speckled with reddish spots of protoporphyrin) the pigmentsÕ function is unknown. We studied a bird population at a geologically variable site, and considered a hitherto untested hypothesis: that protoporphyrin pigments might compensate for reduced eggshell-thickness (caused partly by calcium deficiency), which is known to reduce eggshell-strength and increase eggshell-permeability. We found that pigment spots specifically demarcated thinner areas of shell, with darker spots marking yet thinner shell than paler spots. Variation in pigmentation was thus associated with variation in shell thickness both within and between clutches, so accounting for the eggshell's characteristic spot patterns. Geological variability at this site has resulted in a great range of calcium availability and, as predicted by the hypothesis, variation in calcium availability was found to affect between-clutch variation in both eggshell-mass (+) and pigmentation characteristics ()). We suggest a physiological mechanism and some important implications of these findings.
Perktaş U., Gosler A. G. 2010. Measurement error revisited: its importance for the analysis of si... more Perktaş U., Gosler A. G. 2010. Measurement error revisited: its importance for the analysis of size and shape of birds. Abstract. Measurement error in morphological characters is an important issue for many ornithological studies (e.g. ecomorphology, quantitative studies of heritability, studies of systematic and geographic variation). The variation in external morphological characters, such as wing and tarsus length, is usually evaluated using multivariate statistical methods such as principal component analysis (PCA). These are often considered better than univariate statistical methods for explaining size and shape variation in bird populations because they reduce the 'dimensionality' of the data — the size of individual measures (wing etc.) are assumed to contain a component reflecting a general character 'size'. However, the effect of measurement error on principal components has not been formally assessed with respect to such data. Here we report three examples in order to assess the importance of measurement error for analyses within and between bird populations. The effect of measurement error on PCA is also discussed in relation to the importance of levels of error in shape components. Our results indicate that, in relation to size (PC1), principal component scores are affected less by measurement error if the covariance matrix is used rather than the correlation matrix. However, the effects of relative measurement error were substantially greater in subsequent axes, which represent shape variation rather than size, than they were in the size axis (PC1). Measurement error may, therefore, be a more important issue for shape axes than for the size axis and this problem may be exacerbated further if very few characters are used in the PCA. Our results also indicate that PCA is especially sensitive to issues relating to sample size. We recommend that if reducing the measurement error in size and shape measures is not possible, and the sample size is small (≤ 30), principal component scores should be derived using the covariance matrix, as these are more likely to give more robust results.
In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of ed... more In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of edge environments is thus essential for the conservation of forest communities. The reproductive output of forest passerines is often reduced close to forest edges. Possible explanations include overcrowding by conspecifics, elevated rates of predation, and the occurrence of lower-quality habitat and/or individuals at forest edges. We attempted to separate these processes by examining edge effects in the absence of nest predation and by effectively controlling for differences in breeding density and the quality of habitats and individuals. We used an edge distance index (EDI), which accounts for the number and distribution of edges in close proximity to a breeding location, to help explain variation in breeding density, nesting success, and reproductive traits of 8308 pairs of Great Tits (Parus major) breeding between 1965 and 2005, in Wytham, near Oxford, United Kingdom. Results from linear mixed modeling confirmed higher breeding density and a higher proportion of immigrant individuals at forest edges. Nevertheless, independently of these effects, we also found that birds laying later, with smaller clutches but larger eggs, were typical of edge environments. The number of offspring recruited to the breeding offspring per breeding attempt was also reduced at edges, both directly and mediated through changes in clutch size and laying date. Edge effects on life histories were detectable within individual females and up to 500 m from the woodland edge. Woodland edges are increasingly common in contemporary fragmented landscapes. Therefore these results, which suggest a pervasive effect of edges on reproduction, are of considerable importance to the management and conservation of forest communities. Resumen: En los paisajes fragmentados contemporáneos, los bordes son frecuentes, y por lo tanto el en-tendimiento de los efectos de borde es esencial para la conservación de las comunidades forestales. A menudo, la productividad de paserinos se reduce cerca de los bordes de los bosques. Las explicaciones posibles incluyen hacinamiento, tasas de depredación elevadas y la ocurrencia de hábitat y/o individuos de menor calidad en los bordes del bosque. Intentamos separar estos procesos mediante el examen de los efectos de borde en ausen-cia de depredación de nidos y el control efectivo de las diferencias en la densidad reproductiva y de la calidad de los hábitats y de los individuos. Utilizamos uníndice de distancia al borde (IDB), que incluye el número y distribución de bordes cercanos a una localidad de reproducción, para tratar de explicar la variación en la densidad reproductiva, eí exito de anidación y las características reproductivas de 8308 parejas de Parus major entre 1965 y 2005, en Wytham, cerca de Oxford, Reino Unido. Los resultados de la modelación lineal mixta confirmaron una mayor densidad reproductiva y una mayor proporción de individuos inmigrantes en los bordes de bosques. Sin embargo, independientemente de estos efectos, también encontramos que aves con nidadas menores, pero huevos más grandes, eran típicas de los ambientes de borde. El número de crías reclutadas por intento reproductivo también fue más reducido en los bordes, tanto directamente como por medio de cambios en el tamaño de nidada y de fecha de puesta. Los efectos de borde sobre las historias de ‡email teddy.wilkin@zoo.ox.ac.uk
Calcium is an essential micronutrient for birds during egg formation and for skeletal development... more Calcium is an essential micronutrient for birds during egg formation and for skeletal development in nes-tlings. Habitat level studies suggest that birds breeding in low-calcium areas may be limited in the size or number of eggs they lay and in the quality of their nestlings. However, as birds forage non-randomly and may travel considerable distances to acquire calcium, describing different breeding environments in terms of their calcium availability is problematic. Here we explore the spatial relationships between 300-fold variation in soil calcium and the life-history traits of ca. 6,000 pairs of great tits breeding in a single continuous woodland over 41 years. Controlling for other habitat differences , we found strong positive associations between soil calcium, clutch size and recruitment at spatial scales of over 300 m from each nestbox, suggesting that females may have been travelling inter-territorially to acquire calcium during egg formation. Soil calcium near each nestbox (mean distance = 58 m) was a strong positive predictor of mean fledgling mass, suggesting that local calcium was more important during nestling stages. We found no effect of soil calcium on lay-date or egg mass. This study is the first to provide evidence that small woodland passerines are limited by calcium availability at several different spatial scales. However, experimental work is necessary to test the cau-sality of these spatial patterns.
1. Population density often has strong effects on the population dynamics and reproductive proces... more 1. Population density often has strong effects on the population dynamics and reproductive processes of territorial animals. However, most estimates of density-dependent effects use the number of breeding pairs per unit area in a given season and look for correlations across seasons, a technique that assigns the same density score to each breeding pair, irrespective of local spatial variation. 2. In this study, we employed GIS techniques to estimate individual breeding densities for great tits breeding in Wytham Woods UK, between 1965 and 1996. We then used linear mixed modelling to analyse the effect of density on reproductive processes. 3. The areas of Thiessen polygons formed around occupied nestboxes were used to approximate territory size (necessarily inverse of breeding density). There were significant , independent and positive relationships between clutch size, fledging mass and the number of offspring recruited to the population, and territory size (all P < 0·001), but no effect of territory size on lay-date or egg mass. 4. Thiessen polygons are contiguous and cover all of the available area. Therefore, at low nest densities territory polygons were excessively oversized. Using a novel procedure to address this limitation, territory sizes were systematically capped through a range of maxima, with the greatest effect in the models when territories were capped at 0·9–2·3 ha. This figure approximates to the maximum effective territory size in our population and is in close agreement with several field-based studies. This capping refinement also revealed a significant negative relationship between lay-date and territory size capped at 0·9 ha (P < 0·001). 5. These density-dependent effects were also detected when analyses were restricted to changes within individual females, suggesting that density effects do not merely result from either increased proportions of low-quality individuals, or increased occupation of poor sites, when population density is high. 6. Overall, these results suggest that, in the current population, great tits with territories smaller than c. 2 ha independently lay smaller and later clutches, have lighter fledglings, and recruit fewer offspring to the breeding population. These analyses thus suggest a pervasive and causal role of local population density in explaining individual reproductive processes.
1. Individual animals optimize their body mass to enhance fitness and mass is expected to be opti... more 1. Individual animals optimize their body mass to enhance fitness and mass is expected to be optimized over both short and long timescales. On the short timescale small birds increase mass as they build up energy reserves over each day to avoid starving at night. Theory predicts that starvation risk should be minimized by gaining mass at the start of the day thus insuring against unpredictable food supply later. However, if predation risk is mass-dependent due to reduced escape flight performance, birds should delay mass gain until later in the day to minimize predation risk. 2. Regulation of mass change over longer timescales has been well documented and a number of studies have been able to show that over weeks, months and years mass is regulated consistent with the starvation–predation risk trade-off being mass-dependent. However, it is crucial to mass-dependent predation theory that it functions over shorter timescales as well and that birds are able to regulate their diurnal mass gain strategies in response to predation risk. This has not yet been investigated and recent studies of flight performance have been unable to show that small-scale mass change (< 10%) over short timescales (e.g. diurnal mass change) affects escape flight performance as predicted. 3. We used a unique design of automated identification and weighing system that for the first time allowed the diurnal mass gain of individual great tits Parus major to be monitored remotely in the wild. While automatically tracking diurnal body mass changes of individuals we manipulated perceived predation risk by using model sparrowhawks. We were thus able to investigate experimentally the effect of increased perceived predation risk on diurnal mass gain patterns. 4. We show, for the first time, that birds are capable of manipulating their diurnal body mass gain strategy in response to heightened predation risk and that they do so by delaying mass gain until later in the day, as predicted by mass-dependent predation risk theory. Furthermore, the results are consistent with changing flight performance rather than changing exposure time to predators being the driving force for mass-dependent predation risk.
In animals such as birds that are capable of adjusting energy reserves behaviourally in response ... more In animals such as birds that are capable of adjusting energy reserves behaviourally in response to starvation risk and predation risk, mass is a measure of an individual's perception of the risks in its environment. Although it is often necessary to capture and handle animals to allow the scientific study of animal behaviour , little is known about how this affects the individual's perception of the risks in its environment or how this changes subsequent behaviour. In this study we remotely monitored the mass of individual wild great tits, Parus major, for 10 days after capture, handling and release and then used starvation–pre-dation risk trade-off theory to investigate how individuals perceived the risks in their environment. Mass increased significantly after capture and remained higher for 7 days before returning to precapture levels. We conclude that the capture and handling event was perceived as an interruption to foraging induced by an encounter with a predator which heightened starvation risk. Although this forced interruption to foraging was a relatively small proportion of available foraging time (<6%), the subsequent response produced a large (66% of normal diurnal mass gain) and sustained change in mass. The results highlight the key role that foraging interruptions play in an individual's assessment of risks when trading off starvation against predation. They may also help to explain why some investigations of the effect of perceived predation risk on body mass in captive birds have produced inconsistent results.
Protoporphyrin pigmentation is a common feature of passerine eggshells worldwide, typically givin... more Protoporphyrin pigmentation is a common feature of passerine eggshells worldwide, typically giving red to brown coloration. Although these may form a background colour to the egg, protoporphyrin more commonly forms speckles, blotches or patches of colour (maculation) against a white (unpigmented) or blueygreen (biliverdin-pigmented) background. Conventional explanations for the presence of such pigmentation have focused on the eggs' appearance, and include crypsis and strategies to deter brood parasites, such as cuckoos (Cuculidae). More recently, evidence has emerged that there may also be a sexually-selected visual function. However, the pattern of occurrence across taxa and upon and within the eggshell, suggest that non-visual functions may be at least as important, or even the principal function of some pigmentation. In particular, correlations between protoporphyrin maculation, eggshell thickness, rates of water-loss and local calcium availability in the Great Tit (Parus major), suggest that the pigment could serve a structural function. Despite strong circumstantial evidence, however, a direct relationship between eggshell strength and protoporphyrin pigmentation has not previously been available. Here, we present preliminary findings from a comparison of fracture toughness (resistance to fracture) and brittleness of pigmented and unpigmented shell from the same Great Tit eggshells. These suggest that while pigmented shell is as brittle as unpigmented shell, the fracture toughness is significantly greater and more than compensates for the reduced shell thickness observed at a pigment spot. The data also suggest that although the first two principal components of pigmentation influence fracture toughness, neither influences how the material responded to the propagation of a crack through its surface. We suggest that pigmentation provides structural overcompensation for shell thinning caused, for example, by a deficit of dietary calcium, and the reasons for this are discussed.
Research in urban ecology is growing rapidly in response to the exponential growth of the urban e... more Research in urban ecology is growing rapidly in response to the exponential growth of the urban environment. However, few studies have focused on tropical megacities, and on the interplay between predators' habitat selection and human socioeconomic aspects, which may mediate their resilience and coexistence with humans. We examined mechanisms of breeding habitat selection by a synanthropic raptor, the Black Kite Milvus migrans, in Delhi (India) where kites mainly subsist on: (1) human refuse and its associated prey-fauna, and (2) ritualised feeding of kites, particularly practised by Muslims. We used mixed effects models to test the effect of urban habitat configuration and human practices on habitat selection, site occupancy and breeding success. Kite habitat decisions, territory occupancy and breeding success were tightly enmeshed with human activities: kites preferred areas with high human density, poor waste management and a road configuration that facilitated better access to resources provided by humans, in particular to Muslim colonies that provided ritual subsidies. Furthermore, kites bred at 'clean' sites with less human refuse only when close to Muslim colonies, suggesting that the proximity to ritual-feeding sites modulated the suitability of other habitats. Rather than a nuisance to avoid, as previously portrayed, humans were a keenly-targeted foraging resource, which tied a predator's distribution to human activities, politics, history, socio-economics and urban planning at multiple spatio-temporal scales. Many synurbic species may exploit humans in more subtle and direct ways than was previously assumed, but uncovering them will require greater integration of human socio-cultural estimates in urban ecological research.
2006. Eggshell-pigmentation, soil calcium and the local abundance, distribution and diversity of ... more 2006. Eggshell-pigmentation, soil calcium and the local abundance, distribution and diversity of woodland snails (Mollusca) Recent studies of Great Tit Parus major eggs in Wytham Woods, UK, indicate that the eggshell's pigmentation is related to the eggshell's thickness and so also to calcium availability (indicated by soil analysis) near to the nest. Previous work elsewhere has shown that snails are the principal source of calcium for eggshell formation in the Great Tit. Although these observations suggest that snail abundance and/or size (mass) is related to local soil calcium availability, this has never been demonstrated at the fine spatial scale required to support the observations on eggshell characteristics. We therefore examined the effect of soil calcium concentration on snail distribution and size across a series of sites of similar habitat, along a soil calcium gradient in Wytham Woods, UK. Soil and litter samples were collected from 30 sites and any snails present extracted by intensive sorting. The abundance, biometrics and dry mass of shells were recorded, and modelled against log 10 soil calcium using General Linear Modelling. Our analyses confirmed the existence of strong correlations between snail abundance and size, and soil calcium at this fine spatial scale, presumably due to the snails' requirement for calcium for shell formation and growth, general health and successful reproduction. Snail diversity was also correlated significantly with soil calcium, implying that calcium scarcity limits the abundance of particular snail species. We believe that these observations provide an important brick in the wall of our understanding of calcium limitation in breeding birds. They are also important to understanding the ecology of snails.
—Many birds lay eggs speckled with black or reddish-brown spots of protoporphyrin pigment, but th... more —Many birds lay eggs speckled with black or reddish-brown spots of protoporphyrin pigment, but the function of these spots is debated. Two recent hypotheses have received considerable attention. Under the " signaling-function hypothesis, " speckling reflects female quality and influences allocation of male parental care; under the " structural-function hypothesis, " the speckles strengthen the eggshell when calcium is scarce. The evidence for each is taxonomically uneven. For instance, evidence for the signaling-function hypothesis comes from cavity-nesting and uniparentally incubating species in which the viewing conditions and possibilities for males to assess the speckling are limited, and male response to speckling is indirect (i.e., female-feeding) or postponed (i.e., chick-feeding). We tested the predictions of both hypotheses in a biparentally incubating ground-nesting shorebird, the Northern Lapwing (Vanellus vanellus), in which the potential for a direct male response to eggshell speckling exists and visual inspection of the eggs is not hindered by light availability in a nest cavity. The speckling parameters (spot intensity, distribution, and cover) were repeatable within Northern Lapwing clutches, a requirement for a sexually selected signal, but we found no relationship between male incubation and speckling. However, the spots were associated with thinner areas of eggshell, which strongly supports the suggestion of a structural (strengthening) function. Our results do not support the signaling-function hypothesis of eggshell speckling, but extend the structural-function hypothesis and prior findings (that speckling occurs primarily at thinner areas of the shell) to other avian taxa. Furthermore, if the generally accepted view that Northern Lapwing eggs are cryptically marked is correct, our findings suggest that more than one significant function can drive the evolution of avian eggshell pigmentation. Las Manchas en las Cáscaras no Predicen la Incubación por los Machos pero Demarcan Áreas más Delgadas en los Huevos de un Ave Playera Resumen.—Muchas aves ponen huevos manchados con puntos negros, rojizos o cafés del pigmento protoporfirina, pero la función de tales manchas es debatida. Dos hipótesis recientes han recibido atención considerable. De acuerdo a la " hipótesis de función de señalización " , los puntos reflejan la calidad de la hembra e influencian la asignación del cuidado parental del macho. De acuerdo a la " hipótesis de función estructural " , las manchas fortalecen la cáscara del huevo donde el calcio es escaso. La evidencia que apoya cada una de las hipótesis es taxonómicamente desigual. Por ejemplo, la evidencia a favor de la hipótesis de función de señalización proviene de anidantes en cavidades y de especies en las que sólo uno de los padres incuba los huevos, situaciones en las que las condiciones de visión y las posibilidades de los machos de evaluar el manchado son limitadas y la respuesta de los machos al manchado es indirecta (i.e., alimentación de la hembra) o pospuesta (i.e., alimentación de los polluelos). Probamos las predicciones de ambas hipótesis en Vanellus vanellus, un ave playera que anida en el suelo y presenta incubación biparental. En esta especie existe el potencial para una respuesta directa del macho hacia el machado de la cáscara y la inspección visual de los huevos no se ve impedida por la disponibilidad de luz en el lugar donde se ubica el nido. Los parámetros del manchado (intensidad de los puntos, distribución y cobertura) fueron repetibles dentro de las nidadas de V. vanellus, un requisito de una señal sexualmente seleccionada, pero no encontramos ninguna relación entre la incubación del macho y el manchado. Sin embargo, los puntos se asociaron con áreas más delgadas en la cáscara, lo que sustenta fuertemente la sugerencia de una función estructural (fortalecimiento). Nuestros resultados no sustentan la hipótesis de función de señalización del machado de las cáscaras, pero
Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin whi... more Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin which is produced during biosynthesis of blood haem. Its functional signifi cance has only received empirical attention in recent years. is interest has generated a number of hypotheses of which some remain untested partly because the quantifi cation of protoporphyrin is analytically challenging and can be prohibitively expensive. Many studies have therefore used the extent of eggshell spotting as a proxy for total eggshell protoporphyrin concentration, although this has not been formally tested. Pigment scoring involves recording visible eggshell pigment attributes, such as spot intensity, distribution and size. Since even immaculate eggs can contain some protoporphyrin, there remains doubt over the degree to which visible pigment correlates with total pigment content of the shell. In this study, we test whether visible pigment scoring can be used as a proxy for protoporphyrin concentration of an eggshell. We use pigmented eggshells of two common British passerine species to compare eggshell spot intensity, distribution and spot size (as used by the visual pigment scoring method) with direct measures of eggshell protoporphyrin concentration. In addition, we compared an alternative method of pigment scoring, the pixel pigment scoring method, using a computer programme to quantify the number of pixels exceeding a specifi ed colour threshold. We demonstrate that although results from both scoring methods were positively correlated with eggshell protoporphyrin concentrations, the correlations were not suffi ciently strong to be used as surrogates in studies where actual pigment concentrations are required.
Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce... more Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce wing-loading during nestling feeding) or result from physiological stress. To test which might be more important in determining mass loss in breeding Savi's Warblers (Locu-stella luscinioides), we used a new approach in which the variation in four indices of body condition was described: weight, fat score, muscle score and lean weight (i.e. excluding fat and muscle). We expected weight variations to be adaptive if they involved changes in fat and lean weight, whereas physiological stress should influence the muscle score to a greater extent. As in other species, females showed a greater variation in weight, and carried more fat, than males during the breeding cycle. During incubation, females had greater weight and fat score than males. The weight remained constant and lean weight declined in both sexes, whereas females increased in muscle , which probably reflects the regression of the reproductive organs. During the nestling stage, both sexes declined significantly in all four indices of condition, showing evidence of physiological stress. However, the greater decline in weight in females than in males is consistent with the flight-adaptation hypothesis, as are the cyclic changes in lean weight associated with the various nesting attempts. The fact that both sexes declined significantly in weight, muscle and lean weight with an increasing number of nesting attempts, but not in fat, which was recovered after each nestling period, also indicates that both reproductive stress and adaptive changes occur during breeding. When the whole breeding season was considered, females showed a greater decline in muscle than males, which we interpret to be evidence for a greater reproductive stress in females. We suggest that the small breast muscle size and depleted protein reserves at the end of the breeding period might influence future survival through impaired flight ability and a compromised post-breeding moult.
2006. Using stable isotope analysis to determine the winter moult extent in migratory birds: the ... more 2006. Using stable isotope analysis to determine the winter moult extent in migratory birds: the complex moult of Savi's warblers Locustella luscinioides. Á/ J. Avian Biol. 37: 117 Á/124. Patterns of feather wear in birds captured in spring have traditionally been analysed to describe the extent of winter moult in long-distance migrants. However, the interpretation of feather wear may be rendered extremely difficult due to long moult periods, by the progress of the season, and by the existence of complex moult patterns. Here, stable isotope analysis is used to determine the origin of the wing feather generations present in Savi's warblers Locustella luscinioides captured in Portugal. Carbon, nitrogen and hydrogen isotope ratios of feathers of known European origin differed significantly from those known to have grown in Africa. A discriminant analysis, in which 91.1% of the cross validated samples were correctly classified, was used to determine the origin of tail and wing feathers collected from birds caught when they returned to the breeding quarters. The interpretation of feather-wear generally agreed with the stable isotope analysis, but some inconsistencies were identified. The extent of winter moult in Savi's warblers is described and its moult strategy discussed.
Many small passerine birds worldwide lay white eggs speckled with red, brown and black proto-porp... more Many small passerine birds worldwide lay white eggs speckled with red, brown and black proto-porphyrin pigment spots (maculation). Unlike some patterns of avian eggshell pigmentation which clearly serve a crypsis or signalling function, the ubiquity of maculation among passerines suggests that its origins lie in another function, not speciWc to any particular ecological or behavioural group. Elsewhere, we have presented evidence that protoporphyrin pigments serve a structural function related to eggshell thickness and calcium availability: eggshell maculation in the great tit Parus major increases with decreasing soil calcium levels, pigments demarcate thinner areas of shell, and both the pigment intensity and distribution are related to shell thickness. Here we show that macula-tion also aVects the rate of water loss from the egg during incubation (t Mass Loss per Day or MLD, which is critical to egg viability), but not that of unincubated eggs. We also demonstrate, both by observation and experiment, that the eVect of female incubation behaviour on MLD compensates in some way for variation in egg characteristics, and that diVerences between females in the degree of such compensation are related to diVerences in clutch maculation. Our results suggest that, while a principal function of maculation in this species may be to strengthen the eggshell, it may also reduce eggshell permeability when large amounts of pigment are used, and that this necessitates a behavio-ural adjustment from the female during incubation. We discuss these Wndings and make further testable predictions from our model.
Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines... more Birds are unique in laying eggs with pigmented shells, but for most species (e.g. most passerines, which lay white eggs speckled with reddish spots of protoporphyrin) the pigmentsÕ function is unknown. We studied a bird population at a geologically variable site, and considered a hitherto untested hypothesis: that protoporphyrin pigments might compensate for reduced eggshell-thickness (caused partly by calcium deficiency), which is known to reduce eggshell-strength and increase eggshell-permeability. We found that pigment spots specifically demarcated thinner areas of shell, with darker spots marking yet thinner shell than paler spots. Variation in pigmentation was thus associated with variation in shell thickness both within and between clutches, so accounting for the eggshell's characteristic spot patterns. Geological variability at this site has resulted in a great range of calcium availability and, as predicted by the hypothesis, variation in calcium availability was found to affect between-clutch variation in both eggshell-mass (+) and pigmentation characteristics ()). We suggest a physiological mechanism and some important implications of these findings.
Perktaş U., Gosler A. G. 2010. Measurement error revisited: its importance for the analysis of si... more Perktaş U., Gosler A. G. 2010. Measurement error revisited: its importance for the analysis of size and shape of birds. Abstract. Measurement error in morphological characters is an important issue for many ornithological studies (e.g. ecomorphology, quantitative studies of heritability, studies of systematic and geographic variation). The variation in external morphological characters, such as wing and tarsus length, is usually evaluated using multivariate statistical methods such as principal component analysis (PCA). These are often considered better than univariate statistical methods for explaining size and shape variation in bird populations because they reduce the 'dimensionality' of the data — the size of individual measures (wing etc.) are assumed to contain a component reflecting a general character 'size'. However, the effect of measurement error on principal components has not been formally assessed with respect to such data. Here we report three examples in order to assess the importance of measurement error for analyses within and between bird populations. The effect of measurement error on PCA is also discussed in relation to the importance of levels of error in shape components. Our results indicate that, in relation to size (PC1), principal component scores are affected less by measurement error if the covariance matrix is used rather than the correlation matrix. However, the effects of relative measurement error were substantially greater in subsequent axes, which represent shape variation rather than size, than they were in the size axis (PC1). Measurement error may, therefore, be a more important issue for shape axes than for the size axis and this problem may be exacerbated further if very few characters are used in the PCA. Our results also indicate that PCA is especially sensitive to issues relating to sample size. We recommend that if reducing the measurement error in size and shape measures is not possible, and the sample size is small (≤ 30), principal component scores should be derived using the covariance matrix, as these are more likely to give more robust results.
In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of ed... more In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of edge environments is thus essential for the conservation of forest communities. The reproductive output of forest passerines is often reduced close to forest edges. Possible explanations include overcrowding by conspecifics, elevated rates of predation, and the occurrence of lower-quality habitat and/or individuals at forest edges. We attempted to separate these processes by examining edge effects in the absence of nest predation and by effectively controlling for differences in breeding density and the quality of habitats and individuals. We used an edge distance index (EDI), which accounts for the number and distribution of edges in close proximity to a breeding location, to help explain variation in breeding density, nesting success, and reproductive traits of 8308 pairs of Great Tits (Parus major) breeding between 1965 and 2005, in Wytham, near Oxford, United Kingdom. Results from linear mixed modeling confirmed higher breeding density and a higher proportion of immigrant individuals at forest edges. Nevertheless, independently of these effects, we also found that birds laying later, with smaller clutches but larger eggs, were typical of edge environments. The number of offspring recruited to the breeding offspring per breeding attempt was also reduced at edges, both directly and mediated through changes in clutch size and laying date. Edge effects on life histories were detectable within individual females and up to 500 m from the woodland edge. Woodland edges are increasingly common in contemporary fragmented landscapes. Therefore these results, which suggest a pervasive effect of edges on reproduction, are of considerable importance to the management and conservation of forest communities. Resumen: En los paisajes fragmentados contemporáneos, los bordes son frecuentes, y por lo tanto el en-tendimiento de los efectos de borde es esencial para la conservación de las comunidades forestales. A menudo, la productividad de paserinos se reduce cerca de los bordes de los bosques. Las explicaciones posibles incluyen hacinamiento, tasas de depredación elevadas y la ocurrencia de hábitat y/o individuos de menor calidad en los bordes del bosque. Intentamos separar estos procesos mediante el examen de los efectos de borde en ausen-cia de depredación de nidos y el control efectivo de las diferencias en la densidad reproductiva y de la calidad de los hábitats y de los individuos. Utilizamos uníndice de distancia al borde (IDB), que incluye el número y distribución de bordes cercanos a una localidad de reproducción, para tratar de explicar la variación en la densidad reproductiva, eí exito de anidación y las características reproductivas de 8308 parejas de Parus major entre 1965 y 2005, en Wytham, cerca de Oxford, Reino Unido. Los resultados de la modelación lineal mixta confirmaron una mayor densidad reproductiva y una mayor proporción de individuos inmigrantes en los bordes de bosques. Sin embargo, independientemente de estos efectos, también encontramos que aves con nidadas menores, pero huevos más grandes, eran típicas de los ambientes de borde. El número de crías reclutadas por intento reproductivo también fue más reducido en los bordes, tanto directamente como por medio de cambios en el tamaño de nidada y de fecha de puesta. Los efectos de borde sobre las historias de ‡email teddy.wilkin@zoo.ox.ac.uk
Calcium is an essential micronutrient for birds during egg formation and for skeletal development... more Calcium is an essential micronutrient for birds during egg formation and for skeletal development in nes-tlings. Habitat level studies suggest that birds breeding in low-calcium areas may be limited in the size or number of eggs they lay and in the quality of their nestlings. However, as birds forage non-randomly and may travel considerable distances to acquire calcium, describing different breeding environments in terms of their calcium availability is problematic. Here we explore the spatial relationships between 300-fold variation in soil calcium and the life-history traits of ca. 6,000 pairs of great tits breeding in a single continuous woodland over 41 years. Controlling for other habitat differences , we found strong positive associations between soil calcium, clutch size and recruitment at spatial scales of over 300 m from each nestbox, suggesting that females may have been travelling inter-territorially to acquire calcium during egg formation. Soil calcium near each nestbox (mean distance = 58 m) was a strong positive predictor of mean fledgling mass, suggesting that local calcium was more important during nestling stages. We found no effect of soil calcium on lay-date or egg mass. This study is the first to provide evidence that small woodland passerines are limited by calcium availability at several different spatial scales. However, experimental work is necessary to test the cau-sality of these spatial patterns.
1. Population density often has strong effects on the population dynamics and reproductive proces... more 1. Population density often has strong effects on the population dynamics and reproductive processes of territorial animals. However, most estimates of density-dependent effects use the number of breeding pairs per unit area in a given season and look for correlations across seasons, a technique that assigns the same density score to each breeding pair, irrespective of local spatial variation. 2. In this study, we employed GIS techniques to estimate individual breeding densities for great tits breeding in Wytham Woods UK, between 1965 and 1996. We then used linear mixed modelling to analyse the effect of density on reproductive processes. 3. The areas of Thiessen polygons formed around occupied nestboxes were used to approximate territory size (necessarily inverse of breeding density). There were significant , independent and positive relationships between clutch size, fledging mass and the number of offspring recruited to the population, and territory size (all P < 0·001), but no effect of territory size on lay-date or egg mass. 4. Thiessen polygons are contiguous and cover all of the available area. Therefore, at low nest densities territory polygons were excessively oversized. Using a novel procedure to address this limitation, territory sizes were systematically capped through a range of maxima, with the greatest effect in the models when territories were capped at 0·9–2·3 ha. This figure approximates to the maximum effective territory size in our population and is in close agreement with several field-based studies. This capping refinement also revealed a significant negative relationship between lay-date and territory size capped at 0·9 ha (P < 0·001). 5. These density-dependent effects were also detected when analyses were restricted to changes within individual females, suggesting that density effects do not merely result from either increased proportions of low-quality individuals, or increased occupation of poor sites, when population density is high. 6. Overall, these results suggest that, in the current population, great tits with territories smaller than c. 2 ha independently lay smaller and later clutches, have lighter fledglings, and recruit fewer offspring to the breeding population. These analyses thus suggest a pervasive and causal role of local population density in explaining individual reproductive processes.
1. Individual animals optimize their body mass to enhance fitness and mass is expected to be opti... more 1. Individual animals optimize their body mass to enhance fitness and mass is expected to be optimized over both short and long timescales. On the short timescale small birds increase mass as they build up energy reserves over each day to avoid starving at night. Theory predicts that starvation risk should be minimized by gaining mass at the start of the day thus insuring against unpredictable food supply later. However, if predation risk is mass-dependent due to reduced escape flight performance, birds should delay mass gain until later in the day to minimize predation risk. 2. Regulation of mass change over longer timescales has been well documented and a number of studies have been able to show that over weeks, months and years mass is regulated consistent with the starvation–predation risk trade-off being mass-dependent. However, it is crucial to mass-dependent predation theory that it functions over shorter timescales as well and that birds are able to regulate their diurnal mass gain strategies in response to predation risk. This has not yet been investigated and recent studies of flight performance have been unable to show that small-scale mass change (< 10%) over short timescales (e.g. diurnal mass change) affects escape flight performance as predicted. 3. We used a unique design of automated identification and weighing system that for the first time allowed the diurnal mass gain of individual great tits Parus major to be monitored remotely in the wild. While automatically tracking diurnal body mass changes of individuals we manipulated perceived predation risk by using model sparrowhawks. We were thus able to investigate experimentally the effect of increased perceived predation risk on diurnal mass gain patterns. 4. We show, for the first time, that birds are capable of manipulating their diurnal body mass gain strategy in response to heightened predation risk and that they do so by delaying mass gain until later in the day, as predicted by mass-dependent predation risk theory. Furthermore, the results are consistent with changing flight performance rather than changing exposure time to predators being the driving force for mass-dependent predation risk.
In animals such as birds that are capable of adjusting energy reserves behaviourally in response ... more In animals such as birds that are capable of adjusting energy reserves behaviourally in response to starvation risk and predation risk, mass is a measure of an individual's perception of the risks in its environment. Although it is often necessary to capture and handle animals to allow the scientific study of animal behaviour , little is known about how this affects the individual's perception of the risks in its environment or how this changes subsequent behaviour. In this study we remotely monitored the mass of individual wild great tits, Parus major, for 10 days after capture, handling and release and then used starvation–pre-dation risk trade-off theory to investigate how individuals perceived the risks in their environment. Mass increased significantly after capture and remained higher for 7 days before returning to precapture levels. We conclude that the capture and handling event was perceived as an interruption to foraging induced by an encounter with a predator which heightened starvation risk. Although this forced interruption to foraging was a relatively small proportion of available foraging time (<6%), the subsequent response produced a large (66% of normal diurnal mass gain) and sustained change in mass. The results highlight the key role that foraging interruptions play in an individual's assessment of risks when trading off starvation against predation. They may also help to explain why some investigations of the effect of perceived predation risk on body mass in captive birds have produced inconsistent results.
Protoporphyrin pigmentation is a common feature of passerine eggshells worldwide, typically givin... more Protoporphyrin pigmentation is a common feature of passerine eggshells worldwide, typically giving red to brown coloration. Although these may form a background colour to the egg, protoporphyrin more commonly forms speckles, blotches or patches of colour (maculation) against a white (unpigmented) or blueygreen (biliverdin-pigmented) background. Conventional explanations for the presence of such pigmentation have focused on the eggs' appearance, and include crypsis and strategies to deter brood parasites, such as cuckoos (Cuculidae). More recently, evidence has emerged that there may also be a sexually-selected visual function. However, the pattern of occurrence across taxa and upon and within the eggshell, suggest that non-visual functions may be at least as important, or even the principal function of some pigmentation. In particular, correlations between protoporphyrin maculation, eggshell thickness, rates of water-loss and local calcium availability in the Great Tit (Parus major), suggest that the pigment could serve a structural function. Despite strong circumstantial evidence, however, a direct relationship between eggshell strength and protoporphyrin pigmentation has not previously been available. Here, we present preliminary findings from a comparison of fracture toughness (resistance to fracture) and brittleness of pigmented and unpigmented shell from the same Great Tit eggshells. These suggest that while pigmented shell is as brittle as unpigmented shell, the fracture toughness is significantly greater and more than compensates for the reduced shell thickness observed at a pigment spot. The data also suggest that although the first two principal components of pigmentation influence fracture toughness, neither influences how the material responded to the propagation of a crack through its surface. We suggest that pigmentation provides structural overcompensation for shell thinning caused, for example, by a deficit of dietary calcium, and the reasons for this are discussed.
Research in urban ecology is growing rapidly in response to the exponential growth of the urban e... more Research in urban ecology is growing rapidly in response to the exponential growth of the urban environment. However, few studies have focused on tropical megacities, and on the interplay between predators' habitat selection and human socioeconomic aspects, which may mediate their resilience and coexistence with humans. We examined mechanisms of breeding habitat selection by a synanthropic raptor, the Black Kite Milvus migrans, in Delhi (India) where kites mainly subsist on: (1) human refuse and its associated prey-fauna, and (2) ritualised feeding of kites, particularly practised by Muslims. We used mixed effects models to test the effect of urban habitat configuration and human practices on habitat selection, site occupancy and breeding success. Kite habitat decisions, territory occupancy and breeding success were tightly enmeshed with human activities: kites preferred areas with high human density, poor waste management and a road configuration that facilitated better access to resources provided by humans, in particular to Muslim colonies that provided ritual subsidies. Furthermore, kites bred at 'clean' sites with less human refuse only when close to Muslim colonies, suggesting that the proximity to ritual-feeding sites modulated the suitability of other habitats. Rather than a nuisance to avoid, as previously portrayed, humans were a keenly-targeted foraging resource, which tied a predator's distribution to human activities, politics, history, socio-economics and urban planning at multiple spatio-temporal scales. Many synurbic species may exploit humans in more subtle and direct ways than was previously assumed, but uncovering them will require greater integration of human socio-cultural estimates in urban ecological research.
2006. Eggshell-pigmentation, soil calcium and the local abundance, distribution and diversity of ... more 2006. Eggshell-pigmentation, soil calcium and the local abundance, distribution and diversity of woodland snails (Mollusca) Recent studies of Great Tit Parus major eggs in Wytham Woods, UK, indicate that the eggshell's pigmentation is related to the eggshell's thickness and so also to calcium availability (indicated by soil analysis) near to the nest. Previous work elsewhere has shown that snails are the principal source of calcium for eggshell formation in the Great Tit. Although these observations suggest that snail abundance and/or size (mass) is related to local soil calcium availability, this has never been demonstrated at the fine spatial scale required to support the observations on eggshell characteristics. We therefore examined the effect of soil calcium concentration on snail distribution and size across a series of sites of similar habitat, along a soil calcium gradient in Wytham Woods, UK. Soil and litter samples were collected from 30 sites and any snails present extracted by intensive sorting. The abundance, biometrics and dry mass of shells were recorded, and modelled against log 10 soil calcium using General Linear Modelling. Our analyses confirmed the existence of strong correlations between snail abundance and size, and soil calcium at this fine spatial scale, presumably due to the snails' requirement for calcium for shell formation and growth, general health and successful reproduction. Snail diversity was also correlated significantly with soil calcium, implying that calcium scarcity limits the abundance of particular snail species. We believe that these observations provide an important brick in the wall of our understanding of calcium limitation in breeding birds. They are also important to understanding the ecology of snails.
—Many birds lay eggs speckled with black or reddish-brown spots of protoporphyrin pigment, but th... more —Many birds lay eggs speckled with black or reddish-brown spots of protoporphyrin pigment, but the function of these spots is debated. Two recent hypotheses have received considerable attention. Under the " signaling-function hypothesis, " speckling reflects female quality and influences allocation of male parental care; under the " structural-function hypothesis, " the speckles strengthen the eggshell when calcium is scarce. The evidence for each is taxonomically uneven. For instance, evidence for the signaling-function hypothesis comes from cavity-nesting and uniparentally incubating species in which the viewing conditions and possibilities for males to assess the speckling are limited, and male response to speckling is indirect (i.e., female-feeding) or postponed (i.e., chick-feeding). We tested the predictions of both hypotheses in a biparentally incubating ground-nesting shorebird, the Northern Lapwing (Vanellus vanellus), in which the potential for a direct male response to eggshell speckling exists and visual inspection of the eggs is not hindered by light availability in a nest cavity. The speckling parameters (spot intensity, distribution, and cover) were repeatable within Northern Lapwing clutches, a requirement for a sexually selected signal, but we found no relationship between male incubation and speckling. However, the spots were associated with thinner areas of eggshell, which strongly supports the suggestion of a structural (strengthening) function. Our results do not support the signaling-function hypothesis of eggshell speckling, but extend the structural-function hypothesis and prior findings (that speckling occurs primarily at thinner areas of the shell) to other avian taxa. Furthermore, if the generally accepted view that Northern Lapwing eggs are cryptically marked is correct, our findings suggest that more than one significant function can drive the evolution of avian eggshell pigmentation. Las Manchas en las Cáscaras no Predicen la Incubación por los Machos pero Demarcan Áreas más Delgadas en los Huevos de un Ave Playera Resumen.—Muchas aves ponen huevos manchados con puntos negros, rojizos o cafés del pigmento protoporfirina, pero la función de tales manchas es debatida. Dos hipótesis recientes han recibido atención considerable. De acuerdo a la " hipótesis de función de señalización " , los puntos reflejan la calidad de la hembra e influencian la asignación del cuidado parental del macho. De acuerdo a la " hipótesis de función estructural " , las manchas fortalecen la cáscara del huevo donde el calcio es escaso. La evidencia que apoya cada una de las hipótesis es taxonómicamente desigual. Por ejemplo, la evidencia a favor de la hipótesis de función de señalización proviene de anidantes en cavidades y de especies en las que sólo uno de los padres incuba los huevos, situaciones en las que las condiciones de visión y las posibilidades de los machos de evaluar el manchado son limitadas y la respuesta de los machos al manchado es indirecta (i.e., alimentación de la hembra) o pospuesta (i.e., alimentación de los polluelos). Probamos las predicciones de ambas hipótesis en Vanellus vanellus, un ave playera que anida en el suelo y presenta incubación biparental. En esta especie existe el potencial para una respuesta directa del macho hacia el machado de la cáscara y la inspección visual de los huevos no se ve impedida por la disponibilidad de luz en el lugar donde se ubica el nido. Los parámetros del manchado (intensidad de los puntos, distribución y cobertura) fueron repetibles dentro de las nidadas de V. vanellus, un requisito de una señal sexualmente seleccionada, pero no encontramos ninguna relación entre la incubación del macho y el manchado. Sin embargo, los puntos se asociaron con áreas más delgadas en la cáscara, lo que sustenta fuertemente la sugerencia de una función estructural (fortalecimiento). Nuestros resultados no sustentan la hipótesis de función de señalización del machado de las cáscaras, pero
Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin whi... more Eggshell maculation of most passerines is due to the deposition of the pigment protoporphyrin which is produced during biosynthesis of blood haem. Its functional signifi cance has only received empirical attention in recent years. is interest has generated a number of hypotheses of which some remain untested partly because the quantifi cation of protoporphyrin is analytically challenging and can be prohibitively expensive. Many studies have therefore used the extent of eggshell spotting as a proxy for total eggshell protoporphyrin concentration, although this has not been formally tested. Pigment scoring involves recording visible eggshell pigment attributes, such as spot intensity, distribution and size. Since even immaculate eggs can contain some protoporphyrin, there remains doubt over the degree to which visible pigment correlates with total pigment content of the shell. In this study, we test whether visible pigment scoring can be used as a proxy for protoporphyrin concentration of an eggshell. We use pigmented eggshells of two common British passerine species to compare eggshell spot intensity, distribution and spot size (as used by the visual pigment scoring method) with direct measures of eggshell protoporphyrin concentration. In addition, we compared an alternative method of pigment scoring, the pixel pigment scoring method, using a computer programme to quantify the number of pixels exceeding a specifi ed colour threshold. We demonstrate that although results from both scoring methods were positively correlated with eggshell protoporphyrin concentrations, the correlations were not suffi ciently strong to be used as surrogates in studies where actual pigment concentrations are required.
Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce... more Current theory suggests that mass change in adult birds while breeding may be adaptive (to reduce wing-loading during nestling feeding) or result from physiological stress. To test which might be more important in determining mass loss in breeding Savi's Warblers (Locu-stella luscinioides), we used a new approach in which the variation in four indices of body condition was described: weight, fat score, muscle score and lean weight (i.e. excluding fat and muscle). We expected weight variations to be adaptive if they involved changes in fat and lean weight, whereas physiological stress should influence the muscle score to a greater extent. As in other species, females showed a greater variation in weight, and carried more fat, than males during the breeding cycle. During incubation, females had greater weight and fat score than males. The weight remained constant and lean weight declined in both sexes, whereas females increased in muscle , which probably reflects the regression of the reproductive organs. During the nestling stage, both sexes declined significantly in all four indices of condition, showing evidence of physiological stress. However, the greater decline in weight in females than in males is consistent with the flight-adaptation hypothesis, as are the cyclic changes in lean weight associated with the various nesting attempts. The fact that both sexes declined significantly in weight, muscle and lean weight with an increasing number of nesting attempts, but not in fat, which was recovered after each nestling period, also indicates that both reproductive stress and adaptive changes occur during breeding. When the whole breeding season was considered, females showed a greater decline in muscle than males, which we interpret to be evidence for a greater reproductive stress in females. We suggest that the small breast muscle size and depleted protein reserves at the end of the breeding period might influence future survival through impaired flight ability and a compromised post-breeding moult.
2006. Using stable isotope analysis to determine the winter moult extent in migratory birds: the ... more 2006. Using stable isotope analysis to determine the winter moult extent in migratory birds: the complex moult of Savi's warblers Locustella luscinioides. Á/ J. Avian Biol. 37: 117 Á/124. Patterns of feather wear in birds captured in spring have traditionally been analysed to describe the extent of winter moult in long-distance migrants. However, the interpretation of feather wear may be rendered extremely difficult due to long moult periods, by the progress of the season, and by the existence of complex moult patterns. Here, stable isotope analysis is used to determine the origin of the wing feather generations present in Savi's warblers Locustella luscinioides captured in Portugal. Carbon, nitrogen and hydrogen isotope ratios of feathers of known European origin differed significantly from those known to have grown in Africa. A discriminant analysis, in which 91.1% of the cross validated samples were correctly classified, was used to determine the origin of tail and wing feathers collected from birds caught when they returned to the breeding quarters. The interpretation of feather-wear generally agreed with the stable isotope analysis, but some inconsistencies were identified. The extent of winter moult in Savi's warblers is described and its moult strategy discussed.
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