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J Coast Conserv
DOI 10.1007/s11852-015-0394-8
Use of artificial intertidal sites by birds in a Mediterranean lagoon
and their importance for wintering and migrating waders
Francesco Scarton 1 & Marco Montanari 2
Received: 2 November 2014 / Revised: 16 May 2015 / Accepted: 19 May 2015
# Springer Science+Business Media Dordrecht 2015
Abstract Man-made habitats provide suitable nesting, resting and feeding habitats for many birds at coastal sites. Despite intensive study outside Europe, very few data are available to date on the bird communities that exploit artificial
intertidal sites along the European coasts. Between July 2009
and December 2010, 32 ornithological surveys were performed at six artificial intertidal sites in the lagoon of Venice
(Italy). Overall, 101 species (with 23,399 birds) were observed, about a third of those occurring in the lagoon of Venice; 58 of these species were of conservation concern. Ten
species comprised about 80 % of the total; bird abundance
was the highest during the post-breeding migration and wintering period. Multivariate analysis identified similarity
among sites, seasons and observed behaviours. Dredge islands
were mostly used by waders as foraging sites, when surrounding tidal flats were still flooded. In winter the six dredged
islands supported about 2 % of the dunlins Calidris alpina
occurring in the whole lagoon of Venice and about 10 % of
the grey plovers Pluvialis squatarola. Since the study sites
comprised just the 6 % of the total dredge island area available
in 2009–2010, the results indicate as large fractions of these
shorebird populations might exploit the dredge island habitats
for foraging. Among the newly created habitats, intertidal
ponds and inner tidal flats were the most heavily used by
birds, followed by dykes and mounds with ruderal vegetation.
Maintenance of the habitat mosaic is a need for a
conservation- based management of the dredge islands.
* Francesco Scarton
scarton@selc.it
1
SELC Soc. Coop., Via dell’Elettricità 3/d, Venezia, Italy
2
SISSAD, Via C.Colombo 5, 34144 Trieste, Italy
Keywords Dredge islands . Habitat selection . Man-made
habitats . Waders . Venice lagoon . Waterbirds
Introduction
Artificial islands created by the controlled disposal of sediments dredged from sea inlets, channels and lagoons have
often been termed Bdredge islands^, and many studies have
been published concerning their use by birds. Almost all studies in the scientific literature deal with sites in the USA (Soots
and Parnell 1975; Soots and Landin 1978; Melvin and Webb
1998; Delaney et al. 2000; Shafer and Streever 2000; Zedler
2000; Erwin et al. 2001; Perry et al. 2001; Neckles et al. 2002;
Erwin et al. 2003; Darnell and Smith 2004; Yozzo et al. 2004;
Guilfoyle et al. 2007; Spear et al. 2007; Golder et al. 2008;
Emslie et al. 2009). However, very few studies exist for European countries, where these particular types of man-made
sites are far less common and of very small size (ABP
Southampton 1998; Atkinson et al. 2001; de Jonge and
de Jong 2002; Atkinson 2003; Bakker and Piersma
2006; Gallego Fernández and García 2007). Data reported in the previously quoted literature indicate that birds
rapidly colonise dredge islands, and that nesting populations at these sites might reach levels of regional or
national importance (e.g. supporting >1 % of the local
population or national population), at least for some
species.
It is already well known that man-made wetlands such as
salt ponds, water-treatment plants, agricultural ponds, fishfarms and gravel pits might become suitable and alternative
sites to natural ones, especially as feeding sites or roosts, enhancing the survival of overwintering birds (Davidson and
Evans 1986; Warnock et al. 2002; Santoul et al. 2004;
Sebastián-González et al. 2010; Mander et al. 2007). In the
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F. Scarton, M. Montanari
Lagoon of Venice (NE Italy), the total area of salt marshes has
fallen from approximately 12,000 ha to less than 4000 ha
between 1900 and the present day, due to reclamation, erosion, natural and human-induced subsidence (Cecconi 2005;
Day et al. 2011). A large dredging program undertaken for the
Italian Ministry of Public Works has been underway since
1984 to maintain channel depths for the purposes of navigation and to increase tidal flushing in the inner lagoon (Scarton
2005). The resulting dredged material has been used to build
artificial salt marshes (hereafter called dredge islands); in
2013, about 120 dredge islands were present, over a total area
of about 1200 ha and using a total volume of sediment in
excess of 13,000,000 cubic meters.
From July 2009 until December 2010, regular bird surveys
were carried out at six dredge islands in the Venice Lagoon.
As far as we know, this is the first study in Europe that explores bird occurrence and habitat use on intertidal artificial
islands. The aims of this paper were:
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to analyse bird occurrence throughout the yearly cycle;
to analyse the use of the different habitats available at
dredge islands;
to suggest criteria for the optimum management of
existing sites and the planning of new ones.
Study area
The Venice Lagoon is a large (550 km2) shallow coastal lagoon, located along the Adriatic Sea, 45°26′N, 12°19′E. Two
barrier islands, each about 10 km long, separate the lagoon
from the sea. Most of the lagoon consists of an open water
body (about 400 km2), with peripheral fish farms for about an
additional 10,000 ha along the borders. The mean depth of the
lagoon is 1.1 m (all the elevations refer to the Venice local
datum) and the tidal range during spring tides is about 1 m,
with a mean tidal range of 0.6 m, one of the highest observed
in the whole Mediterranean (Ferrarin et al. 2013). The highest
tides are observed in November, and the lowest tides in February. Salt marshes cover an area of about 3800 ha and are
regularly flooded during high tides; tidal flats encompass an
area of additional 5000 ha. The climate is temperate, with a
mean annual temperature of 14.5 °C; the lowest mean values
occur in January (4.1 °C) and the maximum in July (25.2 °C;
Ferrarin et al. 2013). Rainfall is around 800 mm per year, with
the peak in March and the minimum in May.
In the period 2006–2010 the lagoon of Venice supported
about 350,000 waterbirds in mid-January; because of this it is
the most important wintering site in Italy and likely the most
important one around the Mediterranean (Bon and Scarton
2012). As a breeding site, the lagoon hosts significant fractions (i.e., >1 % of the Italian populations) of several
waterbirds, such as Little Tern Sternula albifrons, Sandwich
Tern Sterna sandvicensis, Redshank Tringa totanus, Oystercatcher Haematopus ostralegus and Kentish Plover
Charadrius alexandrinus (Hale et al. 2005; Scarton et al.
2013a). Due to its importance for birds, the whole Venice
Lagoon has been recognized since 2007 as a Special Protection Area, according to the European Union Birds Directive
2009/147/EC.
Dredge islands built in the lagoon of Venice so far consist
of a containment cell formed using wooden piles along the
exterior. These areas of shallow water are then filled with
sediments originating from the regular dredging of lagoon
channels or inlets. Sediments discharged into dredge islands
are thus confined by a row of posts, with a sheet of geotextile
along the inner side, or by two or three rows of gabions, filled
with stones. At some dredge islands, posts or gabions were
removed along selected sectors, to promote tidal creek formation. After sediment compaction, these islands reach a mean
elevation above sea level of between 0.5 and 1.2 m; for this
reason, the area flooded by mean high tides is variable, ranging from about 50 % to almost 100 % (Scarton 2005). Despite
their very low elevation, the topography of dredge islands is
not completely flat; small mounds and depressions coexist,
producing an array of microhabitats. Soil composition and
elevation above sea level of dredge islands drive the processes
of vegetation colonization and succession. Throughout the
years, remarkable modifications occur to the vegetation cover
and structure of dredge islands. From almost a bare surface,
with the occurrence of just a few annual species mostly of the
genus Salicornia, several phases lead to an almost continuous
coverage of halophytes, mostly Sarcocornia
(=Arthrocnemum) fruticosa, Aster tripolium, Limonium
narbonense, Puccinellia palustris or, less commonly, ruderal
and halo-nitrophilous species (genus Suaeda, Elymus,
Oenothera, Atriplex). Bushes or small trees (Tamarix gallica
and Populus alba) are extremely rare, occurring only at a few
sites that have higher elevations. Shallow tidal ponds and tidal
creeks, both naturally formed and man-made, have a different
extent and complexity at each site.
Materials and methods
Surveys were carried out at six dredge islands (Fig. 1). They
ranged in size from 6.86 to 14.59 ha, had a mean area of
10.82 ha (1 SD = ±3.06 ha), a total combined area of
64.9 ha and an age of 8–16 years (Table 1). The distance of
the dredge islands from the mainland ranged between 250 m
and 3.3 km. The six dredge islands were visited between July 2009 and December 2010, twice-monthly between August
to April and monthly between May and July. Each island was
thus visited 32 times during the survey; all the islands were
visited on the same day. Visits took place between 7:00 and
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Use of artificial intertidal sites by birds in a Mediterranean lagoon and their importance for wintering...
Fig. 1 Study sites (in black) with the surrounding salt marshes (dark grey) and islands (light grey); channels are dotted
14:00, avoiding days with strong wind, heavy fog or rain.
Tidal levels ranged between −0.20 and +1.00 m, as recorded
by tidal gauges located near the sites; data were downloaded
from the Venice Municipality web site (http://www.comune.
venezia.it). With tides exceeding +1.00 m, all the sites were
completely flooded, so they appeared unsuitable for birds.
Habitat maps at the scale 1:2500 were made in 2009 from
field observations made by plant ecologists and with the use of
recent aerial pictures. Dominant habitat features were
categorised as follows (Table 1):
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Bdykes and mounds, with ruderal or halo-nitrophilous
vegetation^ (code: RV). These were areas slightly more
elevated than the surroundings, with vegetation cover being from scarce to high. These habitats occurred along the
edge of some sites or, with small extensions, within them.
Bushes were very scarce, whereas trees were not present at
any of the six sites.
Table 1
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Bareas with typical salt marsh vegetation^. This was the
dominant habitat feature at all sites and it was divided into
three groups: 1) dominated by BSalicornia sp.^ (SA), 2)
dominated by BLimonium/Puccinellia/Aster/Juncus/
Spartina^ (LP) and 3) dominated by BSarcocornia/
Halimione^ (SR).
Btidal pools, tidal creeks and interior tidal flats^ (PC), the
last term indicating some depressions, always along the
edge, that originated through surficial erosion. There was
some aquatic vegetation, including both algae (Ulva sp.,
Enteromorpha sp.) and seagrasses (Nanozostera noltii and
Ruppia sp.).
Bbare areas^ (BA), surfaces with very scarce vegetation
cover, almost completely flat and with higher elevation
and coarser sediments than PC.
Characterisation of the six dredge island in terms of extensions of their habitats was analysed by applying principal
Morphological characteristics for the six dredge islands considered in the study
Site
Age
(years)
Size (ha)
RV (Dykes
and mounds, ha)
SA (Salicorniadominated, ha)
LP (Limoniumdominated, ha)
SR (Sarco corniadominated, ha)
PC Ponds
and creeks, ha)
BA (Bare
areas, ha)
Campalto
Canale Tessera
Detregani
Fusina 2
Ravaggio 3
Tezze Fonde
Total
16
14
14
8
11
16
6.86
9.49
8.31
13.41
14.59
12.31
64.97
0.38
1.05
0.07
2.95
1.55
0.05
6.05
2.35
1.06
2.47
0.95
0.79
2.92
10.54
1.36
4.2
0.11
1.64
1.2
2.55
11.06
0.56
2.06
1.16
3.2
2.96
2.89
12.83
2.2
0.82
4.35
1.68
0.52
3.54
13.11
0.01
0.30
0.15
2.99
7.57
0.36
11.38
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F. Scarton, M. Montanari
components analysis (PCA) to the matrix of the surface values
(ha) of the six habitats * six dredge islands. The PCA obtained
identifies three groups of dredge islands in the ordination
graph: the first one includes Tezze Fonde, Detregani and
Campalto, dominated by SA and PC; the second group
(Ravaggio 3 and Fusina 2), opposed to the first, shows
spatial dimensions associate to BA, SR and RV; the
third group is made only by Canale Tessera and it is
heavily characterized by LP.
Some birds were not observed in any of the abovedescribed habitats, but were observed resting, perched on the
containment poles; these birds (about 6 % of the total) were
excluded from the habitat–species relationships analysis reported below.
Initially, sites were surveyed from the boat at a distance of 100–150 m and birds were spotted with binoculars and counted whenever possible. This allowed us to
count species such as the Curlew Numenius arquata and
a few other larger waterbirds, which often flew away as
soon as the boat reached the sites. Once landed on a
dredge island, the observer walked slowly throughout
the whole site, recording all the birds observed and their
behaviour (as in Gregory et al. 2004). Visits lasted 40–
45 min at each site, thus allowing subsequent comparisons of the data. Data analysed were raw, meaning that
no attempts were made to take the different detectability
of birds in different habitats into account; we are aware
that some elusive species, such as the Common Snipe
Gallinago gallinago might be under-represented in the
results.
The behaviour of birds was classified as follows: 1)
Bfeeding^, 2) Brepeatedly flying over the site^; 3) “making anxiety calls, distraction displays or seen with
young”: this was considered a clue for birds nesting in
the study sites; 4) Bsinging^; 5) Bresting, preening^; 6)
Bunknown^, i.e. birds flushed by the observers before it
was possible to record their behaviour. We named the
periods of the year as follows, considering the prevailing bird phenology in the lagoon of Venice: 1) wintering period: December–January; 2) pre-breeding migration period: February–April; 3) nesting season: May–July; 4) post-breeding migration August–November. This
classification is purely indicative and periods in reality
overlap, due to the different phenology among the species and even among individuals of the same species.
Species observed during the study were grouped into
the following functional guilds, according to their ecology and systematics: 1) ducks (Anatidae); 2) cormorants
(Phalacrocoracidae); 3) herons (Ardeidae); 4) raptors
(Falconiformes and Strigiformes); 5) waders (all the
Charadriiformes except Laridae and Sternidae); 6) gulls and
terns (Laridae and Sternidae); 7) aeroplankcton feeders (swallows, bee-eaters, swifts); 8) Passerines.
Statistical analysis
The analysis of the relationships between species occurrence,
their behaviour and habitat characteristics was performed
using multivariate techniques, such as cluster analysis, principal component analysis, correspondence analysis and multidimensional scaling (n-MDS). Comparison among waterbird
assemblages in relation to tidal level and seasons were made
with an analysis of similarity (ANOSIM), a non-parametric
multidimensional ordination method for detecting differences
between groups of community samples. ANOSIM generates
the R statistic which ranges from 0 to 1 and gives an absolute
measure of how separated groups are (R > 0.75, well separated; 0.25 < R < 0.75, overlapping to some extent but different;
R < 0.25, barely separable: Liordos 2010). Similarity percentages (SIMPER) analysis, based on the similarity matrix obtained from the Bray-Curtis index, was used to calculate the
contribution of each species to the dissimilarity between sites
or seasons.
Relationships among relative species abundance were studied with k-dominance curves (Lambshead et al. 1983) and
diversity indices (Magurran 2004):
1) Shannon diversity H’ = − Σ fr ln(fr) where fr is the frequency of records of each species (ni/n; where ni is the
number of records of the species i);
2) Pielou index J = H’/H’max, where H’max = ln S and H’ is
the Shannon diversity index;
3) Simpson’s Index of Diversity 1 – D, where D = Σ(n/N)2,
n = the total number of organisms of a particular species
and N = the total number of organisms of all species.
Correspondence analysis has been applied to the abundance
values (no. of birds) of 100 species (i.e., all the species observed apart from one, which was recorded only perched on
the containment poles) * six habitats. Species–habitat relationships were evaluated with the Density Kernel application.
Bird habitat selections explored using the Jacobs preference
index. The index (D) was calculated for/with each habitat type
as follows:
D ¼ ðr−pÞ=ðr þ p−2rpÞ;
where:
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r is the proportion of birds observed in a particular habitat
type;
p is the proportion of the surface of this habitat out of the
total available area.
The index may range between −1 (habitat completely
avoided) and +1 (exclusive habitat). Values equal to 0 indicate
that the habitat is used in proportion to its availability. Chi-
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Use of artificial intertidal sites by birds in a Mediterranean lagoon and their importance for wintering...
square test and ANOVA were used to compare differences in
distribution and among groups. Numerical and statistical
analysis were performed using Microsoft Excel 2007,
PAST version 2.05 (Hammer et al. 2001) and
STAXS2000 version 1.0.
Results
During the study, 101 species were observed, of which 63
were non-Passerines and 38 were Passerines. The number of
individuals counted was 23,399; the most abundant order was
that of Charadriiformes (17,225 birds, 74 % of the total),
followed by that of Passerines (4175 birds, 18 %) and
Anseriformes (923 birds, 4 %). Altogether, these three
orders accounted for the 96 % of the total number of
observed birds.
Composition of the bird community
The number of birds ranged between 230 and 2774 per survey
(Fig. 2); the number of species ranged between 12 and 40. The
periods with the highest number of species (86) were the postbreeding migration, followed by the pre-breeding migration
and wintering season (51 species each); the nesting season had
the lowest number of species (45). Nineteen species were
resident and were observed in all four periods; 35 species were
observed in only one period, 23 of which were observed only
in the post-breeding migration. Multivariate ordination (nMDS) showed that the bird communities differed among sites,
with Fusina, Canale Tessera and Campalto grouped apart from
Detregani, Tezze Fonde and Ravaggio (Fig. 3). The first group
of sites were used by less waders and more Passerines than the
remaining three. The ANOSIM test gave a general R of 0.44
(with P < 0.0001), a value which indicate distinct groups with
Fig. 2 Total number of birds and
species observed per visit
some overlapping. The SIMPER analysis between this two
groups of sites shows an average dissimilarity of 80.4 %, with
Dunlin Calidris alpina, Yellow-legged gull Larus
michahellis, Black-headed gull Chroicocephalus ridibundus
and Curlew representing about 60 % of this dissimilarity.
Considering all the 101 observed species, about 50 % of the
birds counted refer only to two species, i.e., Dunlin (31.7 % of
the total) and Yellow-legged Gull (18.2 %). The first ten species, in decreasing order of abundance, comprised 82.2 % of
the overall number of birds; 16 species were observed with
just one individual. The k-dominance curve for each site is
shown in Fig. 4. We observed marked differences among sites,
with the Dunlin representing only 20 % of the total at Canale
Tessera, but about 70 % of the total at Detregani. Number of
species ranged between 48 (Campalto) and 63 (Ravaggio 3),
and diversity (H’) between 1.24 (Ravaggio 3) and 1.80
(Fusina 2). Two sites (Ravaggio 3 with 6827 birds and
Detregani, with 5692 birds), accounted for 53 % of the individuals observed; the poorest site was Campalto (1689 birds).
Bird numbers were not dependent on the size of the dredge
islands: Campalto, Fusina 2 and Tezze Fonde had fewer birds
than expected, with the remaining three sites having more
birds than expected (Chi-squared test:χ2 = 4.89, P < 0.001,
d.f. = 5). Trend of the three diversity indices is reported in
Fig. 5. Highest H’ were reached between the end of July and
that of September, and the lowest between the beginning of
October and that of February. Diversity values were different
across sites (ANOVA: F5,186; P always <0.001) but the
ANOSIM tests failed to identify well separated groups
of sites. Moreover, comparisons among the four periods
of the year showed overlapping but different bird communities (ANOSIM: global R = 0.32, P < 0.001) with
the autumn migration and nesting season showing the
highest and significant differences with the remaining
periods (Table 2).
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F. Scarton, M. Montanari
Fig. 3 N-MDS ordination plot
showing distribution of study
sites according to bird occurrence
observed over 18 months (Stress
value: 0.05)
Abundance and temporal variation of bird guilds
At our study sites, ducks occurred throughout the year, but
with only two to four species; the maximum was recorded
during the post-breeding migration, as was also the case for
herons (eight species observed). Waders richness peaked (22
species) during the post-breeding migration, with the secondrichest period (15 species) being the pre-breeding migration.
A similar pattern with the highest richness during postbreeding migration was shown by gulls and terns (nine species
overall), and birds of prey (nine species). Among the 43 species of Passerines, only four were observed throughout the
whole year, whereas 36 occurred only during the postbreeding migration.
Fig. 4 K-dominance plot for bird
assemblages on the six dredge
islands
Among ducks, the most abundant species were the Mallard
Anas platyrhynchos (583 birds; 2.5 %) and the Shelduck
Tadorna tadorna (293 birds; 1.3 %), both which were observed
resting, feeding or nesting. Among cormorants (177 birds,
0.75 %), the occurrence of the Great Cormorant Phalacrocorax
carbo was negligible, whereas the Pigmy Cormorant P. pygmeus
was observed several times feeding inside the ponds during high
tides or resting on the containment poles. Herons were far more
abundant (788 birds; 3.4 %); the commonest species was the
Little Egret Egretta garzetta (711 birds, 3.0 %), followed by
the Grey Heron Ardea cinerea (57 birds, 0.2 %), both which
were observed feeding on the ponds and resting. Diurnal and
nocturnal raptors accounted for 0.3 % of the total, with 73 birds;
among these, the majority were Western Marsh Harriers Circus
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Use of artificial intertidal sites by birds in a Mediterranean lagoon and their importance for wintering...
Fig. 5 Diversity indices (mean
values at six sites ± SE) for each
survey. Abbreviations: D
Simpson index, H’ Shannon
index, J Pielou index
aeruginosus (22 birds, 0.1 %), Common Buzzards Buteo buteo
(19 birds, 0.1 %) and Hen Harriers Circus cyaneus (11 birds,
<0.1 %). Only one nocturnal raptor, the Short-eared Owl Asio
flammeus was observed three times, hunting over the islands or
resting throughout the vegetation.
Waders were the most abundant guild, with 11,109 birds
(47.5 %): the commonest species were the Dunlin (7417 birds,
31.7 %), the Curlew (1174 birds, 5 %) and the Grey Plover
Pluvialis squatarola (909 birds, 3.9 %). These three species used
dredge islands mostly for foraging in the interior tidal flats or in
the tidal ponds and secondarily for resting, often atop the containment poles. The abundance of waders increased into the
winter period with peaks observed during the mid-winter period
(Fig. 6). The density (mean ± SE) of waders feeding during the
wintering season (six surveys made in December and January)
for the three most abundant species was 50.1 ± 19.9 birds/ha for
Dunlin, 6.9 ± 5.4 for Grey Plover and 4.8 ± 1.5 for Curlew.
The mean ± SE (N = 192, i.e. 6 sites*32 visits) density of
feeding waders per ha of suitable habitat and according to tidal
levels is shown in Fig. 7; waders were found within a high
range of tide levels, with the lowest values either at levels
below 0 cm, i.e., when the natural tidal flats surrounding the
dredge islands began to become exposed, and at high tide
levels, above +80 cm when dredge islands became mostly or
totally flooded. Differences among densities were highly significant (χ2 = 141.9, d.f. = 5, P < 0.001).
Table 2 Results of ANOSIM
pairwise comparisons among
seasons: R values are reported,
with P values in brackets.
Significant comparisons are in
bold
Among the remaining waders, the Common Snipe (234
birds, 1 %) was observed in most of the surveys, with birds
along the edge of tidal pools or amidst the salt-marsh vegetation, most often when the ground surface was covered with a
few centimeters of water. Among gulls and terns, the Yellowlegged Gull, was the most abundant (4269 birds: 18.2 %) and
was frequently observed perched on the containment poles or
sitting on the ground. Less abundant were the Black-headed
Gull (1568 birds; 6.7 %), the Little Tern and the Black Tern
Chlidonias niger (both 77 birds, 0.3 %). Aeroplankton feeders
were represented by the Barn Swallow Hirundo rustica (243
bird, 1 %), the Bee-eater Merops apiaster (22 birds, 0.1 %)
and the Sand Martin Riparia riparia (44 birds, 0.2 %). Among
Passerines, the most abundant species were the Meadow Pipit
Anthus pratensis (987 birds; 4.2 %), followed by the Common
Reed Bunting Emberiza schoeniclus (686 birds, 2.9 %), the
European Starling Sturnus vulgaris (874 birds, 3.7 %), European Goldfinch Carduelis carduelis (648 birds; 2.8 %) and
Yellow Wagtail Motacilla flava (153 birds, 0.7 %).
Several species observed at our study sits had high
conservation status: 28 of them were included in the Annex 1 of the EU Birds Directive, 14 were listed as SPEC
1, 2 or 3 (Birdlife International 2004) and 20 species are
considered as Bthreatened^ (i.e., classified as Critically Endangered, Endangered or Vulnerable) in the Italian Red
List (Peronace et al. 2012). Among the species of
Spring migration
Spring migration
Nesting season
Autumn migration
Wintering season
0.06 (0.007)
0.47 (0.007)
0.18 (0.10)
Nesting season
Autumn migration
Wintering season
0.06 (0.26)
0.47 (0.006)
0.70 (0.003)
0.18 (0.10)
0.52 (0.005)
0.05 (0.25)
0.70 (0.003)
0.52 (0.005)
0.054 (0.25)
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F. Scarton, M. Montanari
Fig. 6 Number of wader species
and individuals observed at the
six dredge islands
European concern, the regular roccurrence of the Pigmy
Cormorant (SPEC 1) as well as the Lapwing Vanellus
vanellus, Curlew, Redshank and Corn Bunting Emberiza
calandra (all SPEC 2), is notable.
Breeding species
Fifteen species nested at the study sites; among these, several
species were of conservation concern. The most threatened of
these were the Kentish plover (EN) and the Shelduck (VU).
The commonest species that nested at each site were Blackwinged Stilt Himantopus himantopus, Redshank and Yellow
Wagtail Motacilla flava. The number of breeding species at
each dredge island ranged between three (Detregani) and ten
(Canale Tessera and Ravaggio 3). Overall, we estimated the
occurrence of 243 breeding pairs: the most abundant species
Fig. 7 Density (mean ± SE,
birds/1 ha of suitable habitat) of
feeding shorebird and tidal levels,
all 32 surveys considered. Below
the 0 level natural surrounding
tidal flats become exposed,
whereas above 80 cm dredge
islands are totally inundated
(Table 3) were Yellow-legged Gull (119 pairs), Black-winged
Stilt (20 pairs), Redshank (14 pairs).
According to the occurrence of nesting species, the cluster
analysis (not shown) identifies three possible site groups:
Detregani-Campalto, Fusina 2-Canale Tessera, Tezze FondeRavaggio 3. ANOSIM test performed on these groups gave a
global R of 0.47, with a P value (=0.06) only at the threshold
of significance. The SIMPER analysis shows the largest cumulative contribute, about 60 %, to the overall dissimilarity
was due to the Black-winged Stilt, Zitting Cisticola Cisticola
juncidis, Redshank and Mallard.
Species and habitat relationships
The simultaneous ordination of the six habitats, as described
by the 100 species, was obtained using the first two axes
Author's personal copy
Use of artificial intertidal sites by birds in a Mediterranean lagoon and their importance for wintering...
Table 3
Density of breeding species: no. of pairs/10 ha, total surface of dredge islands considered
Tadorna tadorna
Anas platyrhynchos
Haematopus ostralegus
Humantopus himantopus
Recurvirostra avosetta
Charadrius alexandrinus
Tringa totanus
Larus michahellis
Galerida crestata
Alauda arvensis
Motacilla flava
Saxicola torquatus
Cisticola juncidis
Carduelis carduelis
Emberiza calandra
Total
Campalto
Canale Tessera
Detregani
Fusina 2
Ravaggio 3
Tezze Fonde
Mean
± 1 SE
0
2.92
1.46
2.92
1.46
0
2.92
0
0
0
1.46
1.46
0
0
0
14.58
1.05
4.21
1.05
10.54
1.05
0
5.27
0
0
1.05
3.16
0
3.16
0
1.05
31.61
0
0
0
3.61
0
0
6.02
0
0
0
2.41
0
0
0
0
12.03
0
0.75
0
1.49
0
0.75
0.75
0
0.75
1.49
2.24
0
1.49
0
1.49
11.19
0
1.37
0.69
1.37
0
0
0.69
75.39
0
1.37
4.11
0.69
1.37
0.69
0
87.73
0
1.62
0.81
0.81
0
0
2.44
7.31
0
0.81
3.25
0
0
0
0
17.06
0.18
1.81
0.67
3.46
0.42
0.12
3.01
13.78
0.12
0.79
2.77
0.36
1.00
0.11
0.42
29.03
0.18
0.62
0.24
0.27
1.48
0.12
0.91
12.38
0.12
0.27
0.38
0.25
0.52
0.11
0.27
12.13
extracted from the first two eigenvectors of the correspondence analysis (Fig. 8). The first two axes explain the
81.6 % of the total variability (axis 1 = 58.7 %, axis
2 = 22.9 %) The Bdensity^ of species (dots in Fig. 8) shows
the preference of the observed species for the PC habitat (first
group, bottom left), opposed to another group of several
habitats (RV, BA and SR) with a high density of species. Fewer species were found in the SA and LP
habitats.
The Jacobs’ index was used to evaluate a possible preference of the observed species for the habitats occurring at
dredge islands, considering the abundance of birds that used
the different habitats. The highest preference was shown for
the habitat PC (D = 0.71) and RV (D = 0.34). Habitats that
were less used, comparing their availability, were LP
(D = −0.19), BA (D = −0.48), SR (D = −0.74) and SA
(D = −0.94). In other words, the habitat BTidal pools, tidal
creeks and interior tidal flats^, covered about 20 % of the total
extension and was used by 60 % of the individuals observed.
The Jacobs’ index was also calculated for each of the 100
species considered; 86 of these showed a strong preference
for a single habitat, with a Jacobs’ index >0.8. Using a reduced
list of 37 species (those with at least 32 individuals, i.e., a
mean of 1 bird/survey), we obtained the dendrogram of the
hierarchical classification of the species, according to their
similarity (Fig. 9). Species or groups of species clearly preferred one particular habitat: we observed six groups of species, each of which associated with a single habitat. Seventeen
species, mostly waders, preferred the PC habitat, whereas ten
species, mostly Passerines, preferred the RV habitat.
Smaller groups of species were associated with the remaining habitats.
Behaviour of birds
Fig. 8 Simultaneous ordination of the 100 species and the six habitats,
using the kernel density estimator with the first two axis extracted from
the first two eigenvalues of CA. Symbols and abbreviations: ● = species;
grey areas, maximum density of species; BA bare areas; LP Limonium/
Puccinellia/Aster/Juncus/Spartina dominated; PC tidal pools, tidal creeks
and interior tidal flats; RV dykes and mounds, with ruderal or halonitrophilous vegetation; SA Salicornia sp. dominated; SR Sarcocornia/
Halimione-dominated
About 67 % of the birds observed were feeding, 13 % were
resting and 7 % breeding; for the remaining 13 %, behaviour
could either not be ascertained or it was classified as Bflying
over the site^. Differences of different behaviours among the
guilds were statistically significant (χ2 = 12,537, P < 0.001,
d.f. = 21). Feeding behaviour was particularly common (more
than 80 % of birds involved) among waders, herons and
Author's personal copy
F. Scarton, M. Montanari
Fig. 9 Dendrogram of the
hierarchical classification of the
species obtained applying the
mean linkage to the matrix of the
correlation coefficients calculated
from the abundance values of the
matrix 37 species * 6 habitats.
The most preferred habitat for
each species is reported in the
third column, with the observed
Jacobs’ index values (D) in the
fourth column. Abbreviations: BA
bare areas; LP Limonium/
Puccinellia/Aster/Juncus/
Spartina dominated; PC tidal
pools, tidal creeks and interior
tidal flats; RV dykes and mounds,
with ruderal or halo-nitrophilous
vegetation; SA Salicornia sp.
dominated; SR Sarcocornia/
Halimione
Passerines. Resting was the commonest behaviour among
gulls and terns, which rarely fed on the dredge islands, apart
from the Little Tern. The cluster analysis identified three
groups: ducks-herons, raptors-aeroplancton feeders-cormorants and gulls + terns-Passerines-waders. These three groups
were well separated (ANOSIM test: global R = 0.89,
P < 0.05).
The percentage of birds observed while feeding, out of the
total observed, was high in all surveys and reached the highest
values in late autumn/winter, and the minimum in April–June.
Considering only feeding birds, the habitat PC had a greater
use than expected from its relative availability (D = 0.87),
whereas RV was used in proportion to its availability
(D = 0.04). In contrast, SA and BA were almost completely
avoided (D = −0.96, for both). Both PC and BA habitats
offered a clear visibility of the birds occurring there to the
surveyors, whereas the other habitats did not; it is thus likely
that this somewhat reduced the number of birds actually seen
foraging in the latter habitats.
Discussions
We observed 101 species of birds, a result that compares well
with other similar studies performed at dredge islands: 48
species in 1 year by Melvin and Webb (1998), 91 species over
3 years (Gallego Fernández and García 2007), or 121 species
(Erwin and Beck 2007). Much higher values, such as 280
species (Erwin and Beck 2007) or 289 species (Guilfoyle
et al. 2007) reflect the very long period of those surveys performed at dredge islands in subtropical coastal wetlands,
which are richer in species than temperate wetlands. Other
types of man-made wetlands are known to support a rich bird
community: 103 species were observed over 3 years in a
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Use of artificial intertidal sites by birds in a Mediterranean lagoon and their importance for wintering...
restored fresh-water site in Belgium (Beauchard et al. 2013),
48 species in agricultural ponds in Spain (Sebastián-González
et al. 2010), 69 species in a saltpans-lagoon complex in NE
Italy (Boldreghini and Dall’Alpi 2008) and 67 species in salt
evaporation ponds and other artificial wetlands in Sri Lanka
(Bellio et al. 2009). Considering that the lagoon of Venice is
used throughout the year by about 300 species, about 30 % of
these were observed at least once at the six dredge islands. The
high number of species observed at our study sites, despite
their relative small size, was certainly due to the occurrence of
different suitable habitats and to the relatively low degree of
anthropogenic disturbance, which we deem to be of particular
relevance in a wetland that is as intensively used by man as the
lagoon of Venice. The importance of disturbance, both
human- and naturally induced, in shaping bird communities
at wetland sites has been often studied elsewhere (see for
Italian examples Malavasi et al. 2009) but detailed analysis
are lacking for our study area.
The occurrence, and sometimes even the reproduction, of
several species of conservation concern among the 101 species we observed emphasizes the importance of our study sites
for bird conservation and confirms the findings of other studies, i.e., that dredge islands may offer suitable habitats for
species that are uncommon elsewhere and that usually exploit
only particular wetland habitats (Erwin and Beck 2007;
Guilfoyle et al. 2007). Over the last decade it has been observed (Scarton 2008, 2010) an increase in the number of
breeding pairs at dredge islands, and a concomitant decrease
of their number at natural saltmarshes, which were the traditional nesting sites; it is likely this is an adaptive response to
the raise in mean sea level and high tides frequency which
have been documented very recently in the lagoon of Venice
(about 10 cm between the years 2000 and 2013: Comune di
Venezia 2014). Due to the expected sea level rise in the next
decades (Ferrarin et al. 2013), the importance of artificial intertidal sites as an alternative nesting sites for waterbirds may
become more and more evident.
Overall, the abundance and richness values observed at the
study sites are typical of temperate wetland sites, with a few
dominant species and many other species with few or very few
individuals (see Redolfi De Zan et al. 2011 and Malavasi et al.
2009 for Italian wetlands; Tourenq et al. 2001 for the Camargue; Ysebaert et al. 2000 for the Schelde estuary). Diversity reached a peak during the post-breeding migration, when
the whole lagoon of Venice is used as a stopover site by a very
large number of birds migrating south; the availability of additional feeding and resting habitats, such as those occurring
in the dredge islands, may thus play a significant role in enhancing survival of the individuals during migrations.
Among the most abundant wintering species the Dunlin,
Curlew and Grey Plover regularly used dredge island habitats,
in particular, the inner tidal flats and the tidal ponds for foraging. Due to the their mean elevation above sea level, these
feeding habitats are still available to waders even when the
surrounding tidal flats are completely submerged by the tide.
According to our results, the feeding areas located within
dredge islands might play an important role for waders at a
lagoon scale, especially in winter. This confirms the importance of man-made coastal wetlands for the foraging of
waders, as already observed at salt ponds (Múrias et al.
2002; Dias et al. 2014), abandoned basins for underground
salt deposits (Davidson and Evans 1986), rice fields (Fasola
and Ruiz 1996) and newly created intertidal habitats (Evans
et al. 1998; Mander et al. 2007).
The densities of the three most common waders observed
feeding at the dredge islands are in the range of those reported
for the tidal flats of the whole lagoon of Venice (4.5 birds/ha:
Scarton and Bon 2009) or of those observed elsewhere in
Europe (0.4–98.0 birds/ha: van de Kam et al. 2004). Comparisons must be made cautiously, due to large differences in the
size of the tidal flats considered by the different authors, but
they indicate that dredge islands have at least some functional
attributes similar to those of tidal flats. The mean daily occurrence (totaled for all six dredge islands) of dunlins, grey plovers and curlews in the winter months was 607, 97 and 53
birds, respectively. Comparing these data with the totals
censused in the whole lagoon in the same winters (Bon and
Scarton 2012), fractions between 2.3 % (Dunlin) and 9.6 %
(Grey Plover) of the whole wintering populations of these
waders were observed during a single survey. Since our study
sites comprised about 6 % of the total dredge island area
available in 2009–2010 in the lagoon, it is likely that large
fractions of these shorebird populations exploit the dredge
island habitats for foraging during a single winter. These results agrees with findings of Evans et al. (1998) and Dias et al.
(2014) on the use of aquatic man made habitats as alternative
feeding sites.
Nevertheless, our field observations showed that when tidal flats became exposed during the receding tide, waders began to leave the dredge islands, indicating that the surrounding
tidal flats must be more profitable as foraging locations. A
very likely causal factor is that tidal flats are richer in preys
than the sediments contained inside dredge islands. This is
probably due to different grain size composition, which in turn
affects the distribution of many benthic wader preys (Spruzen
et al. 2008; Jing et al. 2007). Evans et al. (1998) observed that
after 4 years, sediments of man-made intertidal habitats were
still poorer in prey biomass than tidal flats. Our study sites had
a mean age of 13 years, which indicate that reaching comparable biomass densities may take even more years; comparative data between tidal flats and dredge islands prey biomass at
our study area are lacking at the moment to support this
hypothesis.
Considering only Passerines, the ten most abundant species
included typical wetland species (such as A. pratensis,
E. schoeniclus, M. flava, C. juncidis) as well as non-wetland
Author's personal copy
F. Scarton, M. Montanari
species, in particular, several that are well-adapted to human
presence, such as the European Starling, the Hooded Crow
C. cornix and the Magpie P. pica. This was likely due to the
generally low distance of the dredged islands from the densely
human-populated mainland and to the occurrence at dredge
islands of patches of ruderal and halo-nitrophilous vegetation.
Among non-wetland species, we did not observe an intensive
use of the salt marsh habitats by the House Sparrow Passer
europaeus, in contrast to the findings of Malavasi et al. (2009)
for a Mediterranean costal wetland. Moreover, our study sites
also lacked most of the typical reedbed dwellers since the
occurrence of Phragmites australis was negligible and was
limited to only two dredge islands.
The nesting community was low in species (15), while it
comprised several species of conservation concern. Between
2005 and 2007, Scarton et al. (2013a, b) found that populations of national relevance were nesting in the 88 dredge
islands existing at that time, and this still holds true; nevertheless, the large occurrence in these artificial sites of Yellowlegged Gull, a species which may negatively affects survival,
fecundity, foraging ecology and nesting habitat availability for
many species (see for a detailed review of this topic Oro and
Martınez-Abrain 2007), is a cause of concern and should be
limited.
Birds were not randomly distributed, either across sites or
habitats. Jacobs’ index showed that birds disproportionally
used tidal ponds, creeks, inner tidal flats and, far less, dykes
and mounds with ruderal vegetation, whereas Salicornia- or
Sarcocornia-dominated areas were almost avoided. This was
even more evident considering only the observations of feeding birds, with tidal ponds showing almost the maximum positive selection. This was clearly due the large percentage of
waders among the birds recorded, whereas Passerines preferably used areas with non-halophytic vegetation. It is certainly
possible that the lower detectability of birds among some habitats might have slightly biased these results, but walking
throughout the site allowed us to flush out and count all the
birds occurring there, even those that were more reluctant to
leave (such as the Common Snipe). A high availability of
feeding resources and clear visibility of possible predators
are the probable reasons for the prevalent use of ponds, creeks
and especially inner tidal flats, as observed elsewhere (Jing
et al. 2007; Van Dusen et al. 2012).
but always in the center of the containment site, which usually
produced lower elevation areas around the edge), or due to
surface erosion of the softer sediment areas. Given the high
values of inner tidal flats for foraging waders, their occurrence
should be planned at each site; moreover, reshaping of the soil
at already existing dredge islands, which do not show this
habitat feature, would greatly increase their value for birdlife.
Ruderal vegetation and bushes, which are confined to areas
slightly elevated and thus less prone to flooding, were used by
many Passerines as foraging or resting sites, mostly during
pre- and post-breeding migration, or in winter. The planning
or construction in new dredge islands of a few mounds with a
higher elevation than the surrounding area may thus increases
the diversity and value of these artificial habitats islands for
this group of birds. On the other side, one should note that
bushes are also used by predators such as Corvidae or falcons,
which need high perch to search for prey; the occurrence of
these species, in particular falcons, may depress wader abundance at salt marshes otherwise suitable as feeding site (see
Jing et al. 2007).
Finally, sedimentation due to tidal submersion and especially salt marsh vegetation encroachment, natural processes
that occur during the ageing of dredge islands, reduce the
extension of the open habitats; this contributes to making
dredge islands more aesthetically, and maybe functionally,
similar to the surrounding salt marshes. However, it greatly
reduces the number of individuals and species that might find
suitable sites for feeding and resting. For these reasons an
active management, such as vegetation cutting, soil profiling
and the disposal of new freshly dredge sediments should be
performed to maintain a mosaic of suitable habitats and
the resulting high value of dredge islands for a large
number of birds.
Acknowledgments Data were gathered within the framework of a
study carried out for the Provveditorato Interregionale per le Opere
Pubbliche per il Triveneto - Ministero dei Lavori Pubblici (Italian Ministry of Public Works) through its concessionary Consorzio Venezia Nuova.
The assistance of C. Cerasuolo, F. Turco and P. Nascimbeni is warmly
acknowledged. M. Baldin, P. Bertoldo and E. Checchin were of great help
during field visits. An anonymous referee greatly improved the text.
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