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Article

Winter Bird Diversity and Community Structure in Relation to Shrub Cover and Invasive Exotic Natal Grass in Two Livestock Ranches in the Chihuahuan Desert, Mexico

by
Mieke Titulaer
1,*,
Cielo Marisol Aragón Gurrola
1,
Alicia Melgoza Castillo
1,
Angela A. Camargo-Sanabria
2 and
Nathalie S. Hernández-Quiroz
1
1
Facultad de Zootecnia y Ecología, Universidad Autónoma de Chihuahua, Chihuahua 31453, Mexico
2
Consejo Nacional de Humanidades, Ciencias y Tecnologías (CONAHCYT), Universidad Autónoma de Chihuahua, Chihuahua 31453, Mexico
*
Author to whom correspondence should be addressed.
Birds 2024, 5(3), 404-416; https://doi.org/10.3390/birds5030027
Submission received: 26 June 2024 / Revised: 27 July 2024 / Accepted: 29 July 2024 / Published: 31 July 2024
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Abstract

:

Simple Summary

Grasslands are one of the most threatened terrestrial biomes leading to a loss of grassland biodiversity, including birds. Two important factors contributing to the loss of grasslands and their birds are the expansion of woody vegetation and invasive exotic grasses into native grassland habitats. To be able to protect grassland bird communities, it is essential to know how they are affected by these two threats. In this study, we determined the wintering grassland bird diversity and community structure in two private livestock ranches in the Chihuahuan Desert, Mexico, and related this to vegetation measurements, such as grass cover and height, shrub cover and height, as well as the cover of the invasive exotic natal grass. We found that the amount of bare ground (no vegetation) negatively affected the grassland bird diversity on a local scale. We also found that shrub cover, grass height and natal grass cover affected the grassland bird community composition at the ranch level, indicating that shrub cover and natal grass appeared to negatively affect the presence of grassland specialist bird species in the bird community. The results, therefore, indicated that it is important to halt the invasion of shrubs and natal grass into grasslands because they alter the grassland bird community.

Abstract

Grasslands are one of the most threatened terrestrial biomes leading to a loss of grassland biodiversity, including birds. Here, we studied the wintering grassland bird diversity and community structure in two private livestock ranches in the Chihuahuan Desert, Mexico. We explored how bird communities are related to vegetation structure, including shrub cover and invasive exotic natal grass (Melinis repens), two drivers of grassland bird habitat degradation. We used Hill’s numbers to estimate taxonomic species richness and diversity and related these metrics to vegetation covariables using linear models. We employed nonmetric dimensional scaling (NMDS) to explore the importance of vegetation covariates in structuring bird communities. We found that bird species diversity was significantly and negatively related to bare ground cover at a plot level. At the ranch level, shrub cover, grass height and natal grass were important in structuring avian communities, negatively affecting the presence and abundance of grassland specialist bird species. Our results indicated that shrub encroachment and invasive exotic natal grass cover may alter grassland bird communities and should, therefore, be considered in grassland bird conservation management.

1. Introduction

Habitat degradation and land conversion because of human activities are a main cause of global biodiversity loss [1,2]. Grasslands are one of the most threatened terrestrial ecosystems in the world [3,4]. Agriculture, livestock production and overgrazing, a change in historical disturbance regimes and invasive species are some of the main reasons for grassland degradation [5,6]. Few grasslands are protected [7], while healthy grasslands are essential for supporting unique biodiversity [8], as well as providing material and non-material human benefits [9].
As a result of habitat loss and degradation, grassland birds are among the most threatened group of birds [10,11]; more than 50% of all North American grassland bird populations have declined since 1970 [12], and European grassland birds experience similar threats [13]. Migratory North American grassland birds spend up to 8 months of their life cycle away from their breeding grounds, necessitating an understanding of factors influencing grassland birds on the nonbreeding grounds [14]. Eighty-five percent of grassland bird species that breed in the Northern Great Plains spend the winter in the Chihuahuan Desert, where population declines are stronger compared to birds wintering elsewhere [15]. The land extension in the Chihuahuan Desert is much smaller than the breeding grounds. Moreover, the available land is patchy and subject to a high level of perturbation and land change [16,17]. This means that wintering grassland birds congregate into small patches where competition for resources such as food and cover is high. This effect is furthermore aggravated by the effects of climate change [18,19].
Grassland bird communities that have evolved over hundreds of years are under pressure from recent changes occurring over a relatively short time span [11]. Following population trends is essential for tracking species numbers and monitoring species population status and declines. However, ecological communities have evolved in an interplay with abiotic and biotic factors over thousands of years. Recent perturbations, such as climate change [18,20], agricultural expansion [21,22], management practices including fire and grazing regimes [23,24] and shrub encroachment [25], affect the fragile balance of the grassland ecological community [26]. For example, shrub invasion may benefit some shrub-dependent species while eliminating grassland-obligate bird species [27]. Thus, some changes might even result in an overall increase in taxonomic species richness, while community composition changes drastically [28]. The study of bird communities in terms of species richness as well as community composition, considering the relative abundances of bird species, is, therefore, essential.
Bird communities are affected by changes such as shrub invasion, which may lead in a shift toward more shrub-dependent bird species [29]. Other shifts are less clear. For example, invasive exotic grasses can replace native grass species, lead to structural changes in the grassland vegetation and reduce plant species richness and diversity [30]. At least in breeding habitats, territory selection is influenced by the presence of exotic plant species, especially for bird species with a smaller habitat breadth [31]. Also, for wintering birds with more narrow habitat requirements, the presence of non-native vegetation might negatively influence their abundance [32]. Shrub encroachment and invasive grass species may also interact in propagating structural changes to the vegetation [33]. The grassland bird diversity and community structure are dictated by the underlying vegetational heterogeneity of the landscape [34,35].
Natal grass (Melinis repens), also known as Red natal grass, Rose natal grass or Natal redtop, is one of the most rapidly expanding exotic grasses in the Chihuahuan Desert of northern Mexico [36,37]. One potential mechanism through which natal grass could affect grassland birds at the individual level, particularly granivorous grassland birds, is through food availability [38]. Furthermore, this effect is not necessarily negative for all bird species [39]. For example, grassland bird species in captivity were found to prefer natal grass seeds over other native grass seeds due to its handling efficiency [40], whereas the same species primarily consume native grass seeds while overwintering in the Chihuahuan Desert [41]. However, the effects of invasive grass species on individual birds, whether positive or negative, may lead to changes in the grassland bird community. Furthermore, information on the effects of natal grass on grassland bird communities in the field is lacking.
In this study, we look at the grassland bird community structure and estimate wintering grassland bird species richness and diversity in two private livestock ranches in the Chihuahuan Desert in northern Mexico. Both ranches have a light to moderate invasion of natal grass and one experiences heavy shrub encroachment, while the other is located in a natural woodland area. In Chihuahua, Mexico, more than 90% of grasslands are privately owned, and 51% of their territory is used for livestock production [42]. Our study sites, therefore, represent the landscapes available to wintering grassland birds in the Chihuahuan Desert. Our objectives were to estimate bird species richness and diversity in both properties and evaluate bird–habitat relationships at a local (1 km2) and landscape (ranch) scale (15–20 km2) to determine if and how shrub cover and natal grass cover are related to avian diversity and community structure. To our knowledge, this is the first study that specifically looks at natal grass cover in relation to wintering grassland birds in the field.
We hypothesized that shrub cover and natal grass would affect grassland bird communities. Specifically, we predicted that shrub cover would be negatively related to grassland-obligate bird species. We also predicted that the bird community structure would shift towards more generalist and shrub-dependent species with an increase in shrub cover. We predicted that natal grass would not affect bird species at lower densities, but would negatively affect bird species diversity and impact community structure at higher cover because of changes in structural vegetation characteristics.

2. Materials and Methods

2.1. Study Sites

The study took place at two private livestock ranches (Laja and Tinaja) in the State of Chihuahua, Mexico (Figure 1). Both ranches were sampled in the winter of 2021–2022 (Laja and Tinaja 1) and Tinaja was additionally sampled in the winter of 2022–2023 (Tinaja 2). Laja is located in the municipality of Satevó (27°37′53.27″ N, −106°17′17.49″ E) and comprises 4535.87 ha, of which approximately 2000 ha was considered suitable for our study. The other part of the ranch consists of a dense forest and difficult terrain with high slopes. The closest town is Satevó (6 km), with a population of 3141 inhabitants. Tinaja is located in the municipality of Santa Isabel (28°21′29.44″ N, −106°17′17.28″ E) and comprises 1425.96 ha. Santa Isabel has a population of 3791 inhabitants. The most common grasses in both ranches are Bouteloua spp. and Aristida spp. The most common shrubs are Mimosa aculeaticarpa and Prosopis spp., and at Laja, trees from Quercus spp. are abundant. Both are grazed by cattle year-round. The mean annual temperature is 17.3 °C with a minimum temperature of −8 °C, and maximum of 46 °C. During the study, minimum temperatures reached −10 °C and −8 °C for the first and second winter seasons, respectively. Annual precipitation is 260 mm with the majority of rainfall in the months of August–October.

2.2. Study Design

Bird and vegetation surveys followed the protocol designed by the Bird Conservancy of the Rockies [43] using a generalized random tessellation stratification (GRTS) design [44]. Following this procedure, a grid of 1 km2 cells was placed over the study sites using ArcMap 10.8. We randomly numbered grid cells and then selected the first 10 for sampling. If we were unable to sample more than half of a selected grid cell, we replaced it with the one next in line. Reasons for not being able to sample a grid cell included unsafe terrain or impenetrable vegetation. In each grid cell, we placed three evenly spaced 800 m line transects from East to West. Due to some grid cells needing to be partially discarded due to unfavorable conditions, the total number of completed line transects in Tinaja was 21 and in Laja 22. Sampling in Tinaja 2 (2022–2023) was conducted in the same grid cells as in Tinaja 1 (2021–2022).
During the first winter season of the study, Tinaja 1 was sampled in December 2021 and Laja in January 2022. During the second winter, Tinaja 2 was sampled in January 2023. All 10 grid cells per site were sampled on consecutive days unless weather conditions were unfavorable. Therefore, sampling was completed within 15 days at each study site. By performing the bird surveys within such a short time frame, we believe it was safe to assume a closed bird community at each site. Bird surveys were conducted along the 800 m line transects (3 per grid cell). Surveys started at sunrise (approximately 7:30 a.m.) and did not continue later than 11.00 a.m. Only one grid cell was surveyed per day and bird surveys were completed within this time frame for the particular grid cell. At the start and end of each transect, we noted the hour, temperature, sky condition and wind. Surveys did not take place under adverse weather conditions. A trained observer (CMAG) walked along the 800 m line transect and noted down all bird species observed, as well as the group size, perpendicular distance to the line transect within a 100 m range for the transect line and how the bird was detected (visual, call, song, wings, other). Overflying individuals were not taken into account because they did not use the habitat and were not considered part of the sampled bird community.
Vegetation surveys were completed on the same day for each grid as the bird surveys. Line transects (800 m) were divided in 200 m segments. Upon finishing the three line transects in a grid cell, the observer walked back the same way and conducted a vegetation survey at 5 m and 50 m radius circles at the center of each 200 m segment (i.e., 4 vegetation plots per transect, 12 per grid cell). At the 5 m radius plot, we estimated the grass cover and height, forb cover and height, shrub and tree cover and height (plants > 30 m), woody vegetation cover and height (plants < 30 m), bare ground cover and other cover (organic matter, rocks, feces). We also determined the three dominant grass and shrub species, and we estimated the percentage of grass cover that was composed of natal grass. At the 50 m radius plot, we estimated the shrub and tree (>30 m) cover and height. The vegetation cover was estimated using the reference manual from the Bird Conservancy of the Rockies [43] and the height was measured using a ruler.

2.3. Statistical Analysis

All statistical analyses were performed in R 4.2.0 [45]. First, we calculated the bird diversity for each site using Hill’s numbers or true diversity (qD) with the following equation [46]:
D q = ( ∑ i = 1 R p i q ) 1 / ( 1 − q )
where q is the order of the Hill numbers, R denotes the total number of species and p is the relative abundance of each species. The parameter q determines the relative weight given to common and rare species; q = 0 represents species richness, the total number of species without weighing their relative abundances; q = 1 is the same as the exponential of the Shannon–Wiener diversity index, where every species is weighed according to its relative abundance; q = 2 is the inverse of the Simpson diversity index and places emphasis on dominant species. For each value of q, we plotted the species accumulation curve and extrapolated until reaching a horizontal asymptote to estimate the total diversity for each site. We also estimated the 95% confidence intervals around the curve. qD was calculated using the iNEXT function from the package iNEXT [47,48], and plots were created using the function ggiNEXT, which is an extension from the ggplot2 package [49] for iNEXTobject.
To explore the relationship between the vegetation structure and bird diversity, we first estimated qD for each level of q at the grid level and then modelled the three measures of D as a function of the vegetation covariates (estimated at the 5 m vegetation plot and averaged over all vegetation plots in each grid cell: grass cover, grass height, natal grass cover, shrub cover, shrub height, woody plant cover, forb cover, forb height, bare ground; at 50 m vegetation plot: shrub cover, shrub height) using a linear model (one model for each level of q). We excluded other covers from the analysis due to being strongly correlated to grass cover (r = 0.68). The site (Laja, Tinaja 1, Tinaja 2) was added as a factor to account for differences between sites and years. We used the lm function of the stats package. We considered a vegetation covariate important at p < 0.05. Model assumptions were evaluated graphically and with a Shapiro–Wilk normality test for the residuals. With this approach, we tested whether the bird diversity at the grid level was dependent on the grid level vegetation structure. Thus, with this approach, we explored whether vegetation cover and, specifically, natal grass were somehow related to a higher or lower bird diversity at a local scale.
To explore the relationship between the vegetation structure and bird community structure at a landscape scale (ranch level), we used nonmetric dimensional scaling (NMDS), separating communities by site (Laja, Tinaja 1, Tinaja 2). We used the function vegdist from the package Vegan [50] to calculate the dissimilarity matrix using the Bray–Curtis dissimilarity index. We performed the NMDS analysis with the function metaMDS. We then fitted the vegetation covariates on the 2-dimensional space (NMDS 1 and 2) with the function envfit to explore whether bird communities were associated with certain vegetation variables, i.e., if vegetation variables were associated strongly with one direction of the NMDS dimensions. Statistical significance (p < 0.05) of the vegetation covariates in structuring the bird communities was tested with a permutation test within the envfit function. We also used the function envfit to see how grassland birds were associated with the ordination. We used the classification from Bernath-Plaisted et al. [11] to identify grassland birds and classify them as obligate and facultative. The results were plotted using the ggplot function from the ggplot2 package [49].

3. Results

We detected 2430 individuals from 63 species, of which 42 species were in Laja 2021–2022, 37 were in Tinaja 2021–2022 and 39 were in Tinaja 2022–2023 (Table S1). In total, 15 of the 63 species (24%) that were detected could be classified as grassland birds. Of these, seven birds were grassland-obligate species: Grasshopper Sparrow (Ammodramus savannarum), Meadowlark (Sturnella sp.), Cassin’s Sparrow (Peucaea cassinii), Chestnut-collared Longspur (Calcarius ornatus), Savannah Sparrow (Passerculus sandwichensis), Lark Bunting (Calamosiza melanocorys) and Northern Harrier (Circus hudsonius). Meadowlarks could not always be reliably identified to the species level and were, therefore, grouped together. The other eight facultative grassland bird species were as follows: Vesper Sparrow (Pooecetes grammineus), Clay-colored Sparrow (Spizella pallida), Rufous-crowned Sparrow (Aimiphila ruficeps), Lark Sparrow (Chondestes grammacus), Say’s Phoebe (Sayornis saya), Loggerhead Shrike (Lanius ludovicianus), American Kestrel (Falco sparverius) and Turkey Vulture (Cathartes aura).
All common and dominant (most abundant) bird species were detected at both ranches as indicated by the horizontal asymptote of the curves at q = 1 and q = 2. The bird diversities 1D and 2D were significantly higher at Laja than Tinaja, and Tinaja was more diverse in year two compared to year one of the study (i.e., 95% confidence intervals did not overlap; Figure 2). The species richness (0D) was not significantly different between sites, but did appear higher at Laja (Figure 1). However, not all rare species at Laja were detected (Figure 2), as indicated by the failure of the curve (0D) to reach the asymptote. This meant that there were more uncommon species at Laja that could have been detected with a higher sampling effort.
Table 1 shows the vegetation data for each site. The mean grass cover over all sites was 23.44 ± 8.13%. Of all grass covers, the mean natal grass cover was only 8.29 ± 7.42%. The natal grass cover was, on average, a little higher at Laja than at Tinaja (Table 1). Other notable differences between the sites were a higher total grass cover at Tinaja 2 compared to Laja and Tinaja 1, taller forbs at Laja, more shrub cover at Tinaja 2, more bare ground cover at Laja and fewer other covers (organic matter, rocks, feces, etc.) at Tinaja 2.
The natal grass cover was not significantly related to bird diversity at the grid level at any of the diversity orders (p > 0.05 for all levels of q as the dependent variable). None of the other vegetation covariates were significantly related to the species richness (q = 0; all p > 0.05). For the species diversity (1D and 2D), there were significant effects of bare ground (linear model: B = −0.31, p = 0.04 and B = −0.29, p = 0.02, respectively), indicating a negative relationship between bare ground cover and species diversity at the grid level.
The NMDS ordination converged on a two-dimensional solution with a stress of 0.19. The ordination showed that communities were separated by the study site (Figure 3). The shrub cover (p = 0.03) and grass height (p = 0.02) were statistically significant in structuring bird communities, and natal grass showed a tendency towards significance (p = 0.07; Figure 3).
Figure 4 shows how the grassland bird species were located in the two-dimensional ordination plot. Interestingly, grassland-obligate and -facultative species were separated. Grassland-obligate birds appeared strongly separated by the direction of grass height, with the Meadowlark (Sturnella sp.) and Grasshopper Sparrow (Ammodramus savannarum) towards the top left, indicating an association with tall grass, and Cassin’s Sparrow (Peucaea cassinii), Lark Bunting (Calamospiza melanocorys), Savannah Sparrow (Passerculus sandwichensis) and Chestnut-collared longspur (Calcarius ornatus) towards the bottom-right, indicating an association with short grass.

4. Discussion

Habitat loss and degradation through invasive plant species, such as shrub encroachment and exotic grasses, are a major threat to grassland bird biodiversity and grassland specialist bird communities. Our results showed that at the local (grid) level, shrub and natal grass cover did not affect the bird species richness or diversity. Here, only bare ground cover was negatively related to bird diversity (1D and 2D). However, when looking at the bird community from a landscape (ranch) perspective, we found shrub cover, grass height and a tendency for natal grass to be important in structuring bird communities. Obligate and facultative grassland birds were separated along the ordination, indicating different bird–habitat associations for each group.
The results confirmed our hypothesis that shrub cover is important in structuring bird communities. A negative effect of shrub cover on grassland bird communities was previously documented [51,52,53]. Studies have shown a displacement of open grassland specialist bird species toward shrub-tolerant and -dependent bird species [27], and a lower winter survival of grassland birds [54]. Although some shrub cover could be important for thermoregulation due to less extreme temperatures under shrub cover compared to grass cover [55], many grassland specialist bird species have certain shrub cover thresholds above which they do not use grasslands [56,57]. A possible reason for the negative effect of shrub cover on grassland birds is increased predation [54,58].
The results showed some support in favor of our hypothesis that natal grass is important in structuring wintering grassland bird communities. However, the bird species richness (0D) and diversity (1D, 2D) at the grid level were unrelated to natal grass cover. In agreement with this, some grassland birds even prefer natal grass seeds in captivity [40]. Other research works also showed that invasive grasses do not necessarily have a negative effect on bird communities when mixed with native grass species [39,59]. Nevertheless, effects can be species-dependent [60], and multiple studies have shown that non-native grasses are harmful when becoming dominant [61,62,63]. Natal grass cover in both ranches was relatively low. Furthermore, both ranches presented a heterogeneous vegetation cover of multiple grass species. Therefore, more research is needed in locations where natal grass cover has become dominant and where it is displacing other native seed resources.
The ordination showed a separation of obligate and facultative grassland birds. Whereas obligate grassland birds depend exclusively on grassland for at least part of their annual cycle, facultative grassland birds can also use other types of habitats [11]. Grass height appeared to be an important variable in separating grassland-obligate birds. The association of the Grasshopper Sparrow with tall grass is in agreement with the literature [64,65,66], as is the association of the Chestnut-collared Longspur with short grass [67,68]. However, surprisingly, the Chestnut-collared Longspur usually avoids shrub cover [64,68,69], but was located together with shrub-associated grassland birds in the direction of more shrub cover (Lark Bunting and Cassin’s Sparrow). A possible explanation could be that, on our scale of measurement, short grass and shrub cover were partly associated, but that birds used slightly different patches within the 1 km2 grid. In addition, it could simply be an effect of the study site. Chestnut-collared longspurs were only detected at Tinaja, which is located to the right of the ordination. In this sense, most grassland-obligate and -facultative species were found at Tinaja. Historically, Tinaja was a grassland that is now heavily perturbed by shrub encroachment, while Laja contains a small stretch of grassland surrounded by woodlands. This indicates the importance of preserving the grassland habitat at Tinaja. In addition, this showed that generalizations from our two study sites to other grassland areas should be considered with caution, as these results may be site-specific.
Shrub cover and natal grass were important in structuring bird communities when considering the community from a landscape (ranch) perspective, but were not related to taxonomic richness or diversity at a local (grid) scale. The spatial scale is an important variable in habitat selection for migratory birds [70,71] that affects the community structure and diversity [23]. The habitat is selected on multiple scales where different variables could play a role [72]. For example, at a landscape scale, general habitat features such as overall shrub cover could play a more important role [56,73]. On a finer scale, the habitat heterogeneity regarding grass cover vs. bare ground, grass height or specific plant species is more important [64,74]. In line with this, our results showed that different variables were important based on the spatial scale examined. It should be noted that other variables that were out of the scope of this research, such as climate or land use history, could be affecting bird communities at different spatial scales, and should be considered for inclusion in future studies on grassland bird communities.
Bare ground negatively influenced the bird diversity at a local (grid) scale. This result was in agreement with other studies on grassland bird–habitat relationships [75]. Furthermore, bare ground represents the absence of vegetation, and multiple studies have shown that an increased structural heterogeneity or complexity in vegetation is associated with a higher bird species richness and diversity [35,76,77]. More heterogeneity in vegetation can attract a higher bird diversity [78]. Some grassland bird species are associated with moderate degrees of bare ground cover [11]. However, a more complex vegetation structure with multiple layers offers a more niche diversity for different species to occupy and increase in resource availability [79]. Grazing can be an important management tool for creating a heterogeneous habitat structure for communities of grassland bird species with slightly different needs [80,81,82]. Thus, the fact that our study sites were both used for livestock grazing is not necessarily an obstacle for grassland bird conservation, and could even be an important tool in combating invasive shrub and grass species [83,84].
The synergic effect of habitat features and spatial scale on grassland birds has implications for the study of grassland bird community ecology. Importantly, diversity and richness should not be the only metrics used to evaluate community status or management effects, because they do not reflect a shift in community structure. For example, specialist species that are lost and replaced by generalists go unnoted [85]. Also, an overall shift in abundance of common species could actually increase diversity metrics and not reflect the loss of individuals [28]. We, therefore, recommend the use of multiple metrics in the study of biodiversity and its relationship to environmental features. For example, a functional approach focused on a group of bird response traits can be used to examine the effects of invasive plant species on grassland bird communities across different spatial scales.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/birds5030027/s1, Table S1: Bird species and their abundances detected during the winter season at the study sites in Chihuahua, Mexico.

Author Contributions

Conceptualization, M.T.; methodology, M.T.; validation, M.T., A.M.C., A.A.C.-S. and N.S.H.-Q.; formal analysis, M.T., C.M.A.G., A.A.C.-S. and N.S.H.-Q.; investigation, C.M.A.G. and M.T.; resources, M.T. and A.M.C.; data curation, C.M.A.G.; writing—original draft preparation, M.T.; writing—review and editing, M.T., C.M.A.G., A.A.C.-S., A.M.C. and N.S.H.-Q.; visualization, M.T., C.M.A.G. and A.A.C.-S.; supervision, M.T. and A.M.C.; project administration, M.T.; funding acquisition, M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Ethical review and approval were waived for this study because the study did not involve the capturing or manipulation of birds, only their observation.

Data Availability Statement

Data are available upon request from the corresponding author.

Acknowledgments

The authors thank the owners of the ranches where the study took place for allowing the study to take place on their land and logistical support for data collection. The authors thank Kimberly Alcala and Sara Saenz for helping with field work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the study sites in Chihuahua, Mexico.
Figure 1. Location of the study sites in Chihuahua, Mexico.
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Figure 2. The Hill numbers indicating species richness (q = 0) and diversity (q = 1: considering relative abundances of species; and q = 2: emphasizing dominant species) at Laja (2021–2022), Tinaja 1 (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico. The continuous curve represents the actual sample size, while the dashed lines represent extrapolations. Shaded areas represent 95% confidence intervals.
Figure 2. The Hill numbers indicating species richness (q = 0) and diversity (q = 1: considering relative abundances of species; and q = 2: emphasizing dominant species) at Laja (2021–2022), Tinaja 1 (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico. The continuous curve represents the actual sample size, while the dashed lines represent extrapolations. Shaded areas represent 95% confidence intervals.
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Figure 3. Nonmetric multi-dimensional scaling (NMDS) of the wintering bird community at Laja (2021–2022) and Tinaja (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico, and the most important vegetation covariates (p < 0.1) in structuring the communities along the two dimensions. The direction of the arrows represents the association of the vegetation covariates with the two NMDS dimensions and the length of the arrows represents the strength of the relationship.
Figure 3. Nonmetric multi-dimensional scaling (NMDS) of the wintering bird community at Laja (2021–2022) and Tinaja (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico, and the most important vegetation covariates (p < 0.1) in structuring the communities along the two dimensions. The direction of the arrows represents the association of the vegetation covariates with the two NMDS dimensions and the length of the arrows represents the strength of the relationship.
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Figure 4. Nonmetric multi-dimensional scaling (NMDS) of the wintering bird community at Laja (2021–2022) and Tinaja (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico, and the location of all detected obligate (bold) and facultative grassland birds within the two-dimensional ordination. The direction of the arrows represents the association of the bird species with the two NMDS dimensions and the length of the arrows represents the strength of the relationship. Bird codes: STU_SP—Sturnella sp.; AMMSAV—Ammodramus savannarum; AIMRUF—Aimiphila ruficeps; LANLUD—Lanius ludovicianus; POOGRA—Pooecetes gramineus; SPIPAL—Spizella pallida; CHOGRA—Chondestes grammacus; CIRHUD—Circus hudsonius; SAYSAY—Sayornis saya; CALMEL—Calamospiza melanocorys; PASSAN—Passerculus sandwichensis; PEUCAS—Peucaea cassinii; FALSPA—Falco sparverius; CAEAUR—Cathartes aura; CALORN—Calcarius ornatus.
Figure 4. Nonmetric multi-dimensional scaling (NMDS) of the wintering bird community at Laja (2021–2022) and Tinaja (2021–2022) and Tinaja 2 (2022–2023) in the Chihuahuan Desert of northern Mexico, and the location of all detected obligate (bold) and facultative grassland birds within the two-dimensional ordination. The direction of the arrows represents the association of the bird species with the two NMDS dimensions and the length of the arrows represents the strength of the relationship. Bird codes: STU_SP—Sturnella sp.; AMMSAV—Ammodramus savannarum; AIMRUF—Aimiphila ruficeps; LANLUD—Lanius ludovicianus; POOGRA—Pooecetes gramineus; SPIPAL—Spizella pallida; CHOGRA—Chondestes grammacus; CIRHUD—Circus hudsonius; SAYSAY—Sayornis saya; CALMEL—Calamospiza melanocorys; PASSAN—Passerculus sandwichensis; PEUCAS—Peucaea cassinii; FALSPA—Falco sparverius; CAEAUR—Cathartes aura; CALORN—Calcarius ornatus.
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Table 1. Mean (±SD) vegetation covariates at the three study sites measured at 5 m radius circle vegetation plots and shrub and tree cover at 50 m radius circle plot. Letters indicate significant differences (p < 0.05) between sites for each vegetation covariate based on a one-way ANOVA and a Tukey test for pairwise mean comparisons.
Table 1. Mean (±SD) vegetation covariates at the three study sites measured at 5 m radius circle vegetation plots and shrub and tree cover at 50 m radius circle plot. Letters indicate significant differences (p < 0.05) between sites for each vegetation covariate based on a one-way ANOVA and a Tukey test for pairwise mean comparisons.
LajaTinaja 1Tinaja 2
Natal grass cover (%)10.85 (13.52) a7.54 (5.78) a6.47 (3.00) a
Grass cover (%)20.89 (4.31) a22.78 (2.86) a29.16 (8.02) a
Grass height (cm)54.44 (7.05) a47.46 (3.19) a53.18 (6.26) a
Forb cover (%)15.68 (3.17) a15.16 (2.14) a14.54 (2.54) a
Forb height (cm)42.38 (8.28) a39.93 (4.94) a27.08 (5.09) a
Shrub and tree (>30 m) cover (%)15.85 (2.41)a17.00 (5.12) a22.75 (5.01) b
Shrub and tree (>30 m) height (m)1.71 (0.32) a1.42 (0.26) a3.85 (12.00) b
Woody plant (<30 m) cover (%)0.42 (1.45) a3.16 (2.24) ab1.49 (2.55) b
Bare ground (%)28.18 (6.23) a22.77 (6.01) a21.37 (3.30) a
Other cover (%)19.17 (5.98) a19.08 (3.59) a11.54 (2.39) a
Shrub and tree cover (50 m plot; %)26.95 (3.38) a26.18 (3.22) a27.94 (5.00) a
Shrub and tree height (50 m plot; m)2.52 (1.43) a2.02 (7.23) a2.41 (6.70) a
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Titulaer, M.; Aragón Gurrola, C.M.; Melgoza Castillo, A.; Camargo-Sanabria, A.A.; Hernández-Quiroz, N.S. Winter Bird Diversity and Community Structure in Relation to Shrub Cover and Invasive Exotic Natal Grass in Two Livestock Ranches in the Chihuahuan Desert, Mexico. Birds 2024, 5, 404-416. https://doi.org/10.3390/birds5030027

AMA Style

Titulaer M, Aragón Gurrola CM, Melgoza Castillo A, Camargo-Sanabria AA, Hernández-Quiroz NS. Winter Bird Diversity and Community Structure in Relation to Shrub Cover and Invasive Exotic Natal Grass in Two Livestock Ranches in the Chihuahuan Desert, Mexico. Birds. 2024; 5(3):404-416. https://doi.org/10.3390/birds5030027

Chicago/Turabian Style

Titulaer, Mieke, Cielo Marisol Aragón Gurrola, Alicia Melgoza Castillo, Angela A. Camargo-Sanabria, and Nathalie S. Hernández-Quiroz. 2024. "Winter Bird Diversity and Community Structure in Relation to Shrub Cover and Invasive Exotic Natal Grass in Two Livestock Ranches in the Chihuahuan Desert, Mexico" Birds 5, no. 3: 404-416. https://doi.org/10.3390/birds5030027

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