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Analyses the distribution of Calidris alpina from variations in the numbers of birds and their age ratios as determined in 6 localities. Migration routes are also analysed. Results are discussed in relation to the theory of habitat distribution of Fretwell & Lucas (1970).-from Authors
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... The fact that proportional abundance was greatest for the same subsite at both tidal stages for the wader species (Redshank and Grey Plover) suggests that some of the birds may have been foraging preferentially close to their roost sites. This has been shown in previous studies of Oystercatcher (Swennen 1984) and Dunlin Calidris alpina (Have et al. 1984, Dias et al. 2006a where higher status birds tended to roost closer to the best feeding areas. This strategy minimises the distance between roosting and feeding areas and reduces the energy costs of flying between sites (Rogers 2003). ...
... data). Similar observations were reported by Van der Have et al. (1984) for Dunlin in the Wadden Sea of the Netherlands, in autumn, where the juveniles staged in smaller groups close to the mainland shores. Age-related mortalities in Dunlin in a similar context have been reported by Kus et al. (1984). ...
... For example, van den Hout et al. (2014) showed that interference competition from adults forced juvenile red knots (Calidris canutus) to poorer and more dangerous feeding areas. We are as yet unable to assess whether this applies to Pacific dunlins, though van der Have et al. (1984) showed that among dunlins in the Dutch Wadden Sea, juveniles were over-represented in areas with low dunlin density, suggesting that they were somehow excluded from better feeding areas. An alternative hypothesis is that some other site attribute is negatively correlated with foraging rate, so that sites better in foraging terms are poorer in some other respect. ...
Many studies have investigated how foraging behavior such as prey choice varies with factors such as prey size or density.
Models of such relationships can be applied “in reverse” to translate easily observed foraging behaviors into assays of habitat
attributes that cannot (easily) be measured directly. One such model analyzes the speed of a forager flying between patches,
where it captures prey. Faster flight shortens the travel time and hence elevates the intake rate, but is increasingly expensive.
The model shows that the net intake rate is maximized at the point at which the energetic cost of flight is equivalent to
the net rate of intake. Easy-to-measure flight speeds can thus be translated into hard-to-measure foraging intake rates using
established flight power relationships. We studied nonbreeding Pacific dunlins (Calidris alpina pacifica) at 4 intertidal sites on the Fraser River estuary, British Columbia, Canada. These sites differed sufficiently that we expected
food availability and hence the attainable foraging rate to differ. We measured interpatch flight speeds of dunlins foraging
along the tideline within each site. The measured ground speed, calculated airspeed, and the statistically derived zero-wind
effect airspeed all differed significantly between sites, matching in rank order our expectation of habitat quality based
on their physical differences. Intake rate estimates ranged from 4.10W (best mudflat) to 3.48W (poorest). We think it unlikely
that we would have been able to find such small differences using direct measures of foraging intake.
... De valken mijden ook hier de open wadvlaktes om zoveel mogelijk hun prooien vanuit een hinderlaag te kunnen overrompelen. Juveniele steltlopers worden vaak aangetroffen op plaatsen waar de meerderheid van hun soortgenoten niet komt (van der Have 1984, Swennen 1984, vermoedelijk omdat het voor de meeste individuen vanuit een afweging van voedsel tegen veiligheid niet loont om daar te foerageren. Ook hier lijken inferieure foerageervaardigheden en verschillen in competitieve kracht een rol te spelen (Swennen 1984, Cresswell 1994, Cresswell & Whitfield 2008. ...
The recovery of Peregrine populations in the last few decades triggers the question what impact these raptors have on shorebirds using the Wadden Sea as a staging or wintering site. To answer this question, we took advantage of results of a four-winter study on raptor prédation on shorebirds on the Banc d'Arguin, a key wintering site for shorebirds on the East-Atlantic Flyway. We suggest that in the Wadden Sea, as on the Banc d'Arguin, populations of wintering shorebirds are not regulated through direct consumption by predators. Instead, raptors may have a profound intimidating impact on (groups of) shorebirds, which induces the latter to engage in a wide spectrum of anti-predation behaviours and body composition changes. This means that shorebirds pay predation costs by means of foregone opportunities affecting their long-term survival and reproduction, rather than by direct mortality. In a predator-prey game of anti-predation measures against stealth, predators will try to exploit vulnerabilities of their prey. Because of this, especially inexperienced birds or migrants which are unfamiliar with the habitat are prone to being depredated. However, also fattening (spring) migrants may suffer considerable predation costs, and these may increase in the near future, as growing numbers of Peregrines breed in the Wadden Sea region.
... One states that yearlings are forced to use suboptimal habitats due to competition with adults. This is the commonest explanation for age-specific habitat partitioning in birds, although evidence is based on observations rather than experiments (Gauthreaux 1982, van der Have et al. 1984. A second hypothesis suggests that yearlings and adults are specialized in their use of habitat types. ...
... However, to obtain a representative estimate, there are issues to address, such as: do the adults and young birds spend the non-breeding season together and are there any biases associated with the catching method? There are examples of age segregation across sites within a wintering area (Summers et al. 2005, van der Have et al. 1984). Therefore, one needs to take care when combining data from different sites, and weight the data accordingly. ...
... In fact, an extra energy expenditure on such flights may represent a heavy burden in the tight energy budget of shorebirds during the winter (Rehfisch et al., 1996) and may even affect their survival (Rehfisch et al., 1996;Drake et al., 2001;Durell et al., 2005), particularly in the case of small species, such as the dunlin (Piersma et al., 1993). Previous works with dunlins (Have et al., 1984) and oystercatchers (Haematopus ostralegus) (Swennen, 1984) in the Wadden Sea showed that higher status birds tend to roost closer to the best feeding areas than lower status birds, which also suggests a preference for minimizing the distance between roosting and feeding areas. ...
Shorebirds are declining all around the world, mostly due to deterioration of the estuarine habitats used in winter and migration. Estuaries cover small areas, so it is essential to guarantee that shorebirds can access all the tidal flats where they usually feed at low-tide.Studying use of space by dunlins (Calidris alpina) in the Tagus estuary (Portugal), we noted that lack of suitably located high-tide roosts can limit the access of shorebirds to feeding habitats. Density of dunlins on foraging areas declined significantly with distance to the nearest roost, and fewer than 20% individuals foraged more than 5 km from two roosts where they were dye-marked.So to permit full access to feeding areas it is important to maintain a network of suitably located high-tide roosts. We developed a GIS modelling methodology to evaluate the adequacy of existing roost networks, and to estimate the consequences of losing or creating new roosts. The methodology requires maps with the location of roosts and foraging habitats, and knowledge of the distances that birds are willing to fly to reach foraging areas. It quantifies the proportion of foraging areas close to the existing roosts and the average distance that birds have to fly to reach potential feeding sites.Applying this methodology to the Tagus estuary we concluded that lack of roosts probably explains why the intertidal flats in the north-west of the estuary are underused by shorebirds. A modelling exercise suggested that this gap could be eliminated by creating a roost in an old drained wetland area. We also modelled the impact of the loss of two roosts that are currently threatened. Without them almost half of the available feeding areas will be too far from roosts to be efficiently used by dunlins, and possibly by other shorebirds.
... Preliminary indices on the numbers of Oystercatcher, Grey Plover and Knot, based on data from Texel, Terschelling, Ameland, Balgzand, and the Friesland and Groningen mainland coast (data IWRB Wader Research Group, largely supplied by counts organized by P. Zegers) juveniles often use different areas. This phenomenon has also been demonstrated in the Wadden Sea (van der Have et al., 1984; Swennen, 1984). The annual chick production can be highly variable between years (Boere, 1974; Furness & Bailhe, 1981; Summers & Underhill, 1987), which may be important information when counts reveal a dramatic drop in numbers of certain species (cf. the example for Knot on p. 374). ...
Over the past 20 years, a tradition has developed in western Europe of carrying out shorebird surveys in estuaries. These
counts are mostly conducted by amateur ornithologists, usually well acquainted with the area in which they are active. The
results of these surveys are well reproducible for at least the abundant species, and information is available on the size
of the errors which occur. In several parts of Europe, shorebird surveys have been carried out long enough to allow trend
analysis of population size. Four examples from Great Britain demonstrate that monitoring schemes may also be used for management
purposes, especially if combined with specific research projects. Long-term shorebird surveys can be used not only for simple
analysis of trends in bird numbers, but also to provide a deeper understanding of the functioning of the ecosystem. They may
also act as a tool to document the effects of management activities in the area concerned. Shorebird surveys in the Wadden
Sea have not only revealed the extremely large importance of the area, especially for waders, but also show that different
areas are exploited in different ways. They have also provided data on changes in bird numbers throughout the year. Apart
from the counts carried out in January, the frequency of counts in most areas in the Wadden Sea has been too low to allow
the results to be used for monitoring purposes. In an area as large as the Wadden Sea, some sites, often supporting large
numbers of birds, may not always be covered during simultaneous surveys. This may seriously affect the completeness of the
count. The best way to overcome this problem is to calculate index values, using only those sites which have been surveyed
in successive years. Two monitoring schemes are proposed which may be effective in providing relevant information for management
From 1989 to 1999, I examined intraseasonal and annual chan-es in the abundance and local distribution of two groups of wintering Dunlin (Calidris alpina) occupying different positions along hydrographic gradient: in the Tomales Bay estuary, California. Reciprocal translocation of color-banded Dunlin indicated a discrete separation of wintering Dunlin between the north and south of Tomales Bay. Local abundance in both groups was significantly associated with cumulative seasonal rainfall within and among years. Increased variation in annual and intra-seasonal abundance was related to reduced tidal circulation, greater freshwater stream flow, and increased salinity variation. On average, winter Dunlin use peaked later but declined earlier in the southern part of the bay (near the head of the bay) than in the northern part of the bay (near the mouth of the bay). Shifts in distribution associated with changes in hydrographic conditions and weather revealed consistent intra-seasonal changes in habitat preference on temporal scales other than tidal cycles. In the northern part of the bay, changes in habitat preference by Dunlin corresponded to thresholds of 20-25 cm cumulative rainfall and 0.5-1.0 m(3) sec(-1) stream flow, whereas in the south bay changes were relatively continuous. Rainfall, wind velocity and direction, and temperature also influenced habitat preferences. Flocking behavior dominated over habitat choice at low levels of Dunlin abundance, resulting in contrasting patterns of habitat use relative to overall trends. These patterns suggested the loss of habitat quality as the criterion for patch use. In general, this study indicates that winter shorebird use of temperate estuaries can be predicted by differences in weather and hydrographic regimes, including rates and variances of freshwater inflow, estuarine circulation, and water residence times unique to each system.
Characteristics of Sex and Age Composition of Calidris Alpine (Aves, Charadriiformes) Migrating Across Sivash
The paper shows features of distribution based on regular captures and measurements since 1986 on two sites of Sivash (Central and Eastern), which are different in their hydrological regime and composition of forage macrozoobenthos. Shorter-billed males reliably prefer Central Sivash for storage of fat reserves at the expense of feeding on larvae of chironomids and possibly the brine shrimp. For longer-billed females it is more profitable to concentrate on Eastern Sivash, where they prefer to feed on polychaete worms. On basis of size characteristics of the captured birds and their time dynamics, the paper considers suggested terms of passage of different subspecies and populations of the Dunlin across the Azov-Black Sea coast of Ukraine. Also differences in age composition of migratory waders on different sites of Sivash are shown compared to other water bodies of the near Black Sea area.
Durell, S.E.A. le V. dit. The implications for conservation of age-and sex-related feeding specialisations in shorebirds. Wader Study Group Bull. 100: 35-39. Age-and sex-related feeding specialisations have been found in several shorebird species. The phenotypic con-straints to age-and sex-related feeding specialisations mean that birds may not be able to change diets or feed-ing methods when faced with any deterioration in their feeding conditions. Furthermore, should any increase in mortality affect a particular age group, or one sex, more than another, the resulting reduction in population size may be greater than if all birds were affected to the same extent. For this reason, it is important that con-servation managers are aware of any age-or sex-related feeding specialisations in their study species.
Evidence is presented for the age-related distribution of Bar-tailed Godwits in Moreton Bay, Queensland, during the northward migration in 1989. The age-related distribution was deduced from plumage characteristics, sex ratio patterns and the general distribution of Bar-tailed Godwits in the Bay. A 'natural experiment' at a sewage outfall is described to help explain the age-related distribution. Despite the high densities of birds on the intertidal area adjacent to the outfall, the area appeared to be used by a relatively small proportion of adults. It is suggested that the nature of the intertidal habitat near the sewage outfall is less suited to godwits than island locations in the Bay. Qualitative differences in habitat are suggested to play an important role in determining the overall distribution of Bar-tailed Godwits in Moreton Bay.
Western Sandpipers Calidris mauri are the most numerous shorebird species in the San Francisco Bay estuary during winter. A sample of 106 Western Sandpipers was captured in mist nets and radio-marked with 1-g transmitters to examine their wintering site fidelity and movements. Differences in distances moved, home range extent and core area size were examined by age, sex, season, site, time of day and tide. All birds remained in the south San Francisco Bay region during winter and exhibited strong site fidelity, with a mean home range of 22.0 km2 or only 8% of the study area. First-year birds had larger home ranges (26.6 ± 3.6 km2) than adults (17.2 ± 2.5 km2) in winter, but home range sizes of males and females were not significantly different in any period. Home range sizes were similar between seasons, but core areas were smaller in spring (6.3 ± 1.2 km2) than in early (9.6 ± 4.0 km2) or late (11.6 ± 1.6 km2) winter. Movements and home range size were similar for radio-marked birds located during day and night. The high degree of regional and local site fidelity demonstrated that the mixture of natural mud fiats and artificial salt ponds in southern San Francisco Bay provided sufficient resources for large wintering populations of Western Sandpipers.
The existence of possible differences in homing performances between adult and juvenile Dunlins was investigated at the beginning of two wintering seasons, when 49 adults (1992) and 51 first-year birds (1993) were caught in the Lagoon of Venice and displaced to Cervia Salines. Both sites (133 km apart) are important winter resorts for the species at the Italian and Mediterranean level. In a previous season, a control series of 47 birds belonging to both age classes were caught and released at Cervia Salines. The patterns of return at the capture site by the displaced birds did not statistically differ between the two age groups and the estimated percentage of homed birds after 50 days in the Lagoon of Venice was similar to the resighting rate of control birds in Cervia. Juvenile Dunlins seem to have developed a wintering site attachment by early December, i.e. 1-2 months after their arrival on the wintering grounds.
Annual breeding productivity of Knots Calidris canutus was estimated by the proportion of first-year birds in winter ringing samples. Significant associations were found between the productivity of Knots, other species that are known to breed on the Taimyr Peninsula, and lemming abundance in that region. It is inferred from this that Knots wintering in southern Africa are of the Russian subspecies canutus unlike British wintering birds which do not show these correlations and are subspecies islandica.
Birds with uniparental incubation may face a time allocation problem between incubation and feeding. Eggs need regular warming to hatch successfully, but the parent must leave the nest to feed and safeguard its own survival. Time allocation during incubation is likely to depend on factors influencing egg cooling rates, parental energy requirements and feeding intake rate. How this allocation problem is resolved was subject of this study on arctic-breeding shorebirds. We compared incubation rhythms between four uniparental shorebird species differing in size and expected to find both species differences and weather effects on the organisation of incubation.
Predators may influence many aspects of the daily life and seasonal movements of their prey. Here we quantify direct, and evaluate indirect effects of predation by three falcon species (Lanner Falcon Falco biarmicus, Barbary Falcon Falco pelegrinoides and Peregrine Falcon Falco peregrinus) on coastal shorebirds wintering on the Banc d’Arguin, Mauritania, an area hosting approximately 30% of the East Atlantic Flyway population of shorebirds. On the basis of 754 h of observation over five winters, 97 witnessed attacks and 585 collected prey remains, we show that shorebirds were safer in larger flocks, which tended to be attacked less often. Furthermore, species that forage relatively close to shore and in small flocks were depredated more often than expected from their relative abundance. In three species, Red Knot Calidris canutus canutus, Bar-tailed Godwit Limosa lapponica taymyrensis and Dunlin Calidris alpina, the juveniles were more vulnerable than adults. We estimated that on average 1% of the juvenile and 0.1% of the adult Red Knots present were killed by large falcons each winter. For Red Knots we simultaneously quantified annual survival on the basis of an individual colour-marking programme: mortality due to predation by falcons accounted for an estimated 6.2% (juveniles) and 0.8% (adults) of annual mortality. We suggest that juvenile Red Knots are 10 times as likely to be killed by falcons because they use riskier habitats, i.e. early and late tide foraging areas closer to shores where surprise attacks are both more common and more successful. These results indicate that the strength of indirect effects of predation operating in a shorebird population largely outweigh the effects of mortality per se.
Due to the ‘double-clutch’ mating system found in the arctic-breeding Little Stint Calidris minuta, each parent cares for a clutch and brood alone. The resulting constraint on feeding time, combined with the cold climate and a small body size, may cause energetic bottlenecks. Based on the notion that mass stores in birds serve as an ‘insurance’ for transient periods of negative energy balance, but entail certain costs as well, body mass may vary in relation to climatic conditions and stage of the breeding cycle. We studied body mass in Little Stints in relation to breeding stage and geographical location, during 17 expeditions to 12 sites in the Eurasian Arctic, ranging from north Norway to north-east Taimyr. Body mass was higher during incubation than during chick-rearing. Structural size, as estimated by wing length, increased with latitude. This was probably caused by relatively more females (the larger sex) incubating further north, possibly after leaving a first clutch to be incubated by a male further south. Before and after correction for structural size, body mass was strongly related to latitude during both incubation and chick-rearing. In analogy to a similar geographical pattern in overwintering shorebirds, we interpret the large energy stores of breeding Little Stints as an insurance against periods of cold weather which are a regular feature of arctic summers. Climate data showed that the risk of encountering cold spells lasting several days increases with latitude over the species’ breeding range, and is larger in June than in July. Maintaining these stores is therefore less necessary at southern sites and during the chick-rearing period than in the incubation period. When guarding chicks, feeding time is less constrained than during incubation, temperatures tend to be higher than in the incubation period, reducing energy expenditure, and the availability of insect prey reaches a seasonal maximum. However, the alternative interpretation that the chick-tending period is more energetically stressful than the incubation period, resulting in a negative energy balance for the parent, could not be rejected on the present evidence. Also published In 2007 as chapter 5 in 'The Arctic Pulse', the PhD thesis of Ingrid Tulp (University of Groningen), page 98-117.
The Schelde estuary is an important wintering area and stop-over place for waders and waterfowl using the East Atlantic fly-way. The port of Antwerp is situated in the Lower Zeeschelde, the transition area between the brackish- and freshwater tidal part. Three intertidal areas in this zone (Groot Buitenschoor, Galgenschoor and Schor Ouden Doel) are protected under several international and national legislations. In this study long-term datasets (1982–1998) on water birds in these intertidal areas were analysed and attempts were made to assess the impact of two container terminals, constructed during the covered period. Overall abundance of water birds in the study area did not show any significant trends. Looking at individual areas, maxima and winter means on the Galgenschoor and Groot Buitenschoor were very variable but the peak seasons levelled out with time. On the Schor Ouden Doel, where hunting was banned, maximal numbers increased by a factor of four between 1985 and 1990, mainly due to an increase in Greylag Goose numbers. The trophic composition of the bird populations showed major shifts. Initially, species compositions in winter differed considerably between the three areas, but they became more similar during the study period. The results suggest that the area became more important as wintering and resting place for herbivores such as Greylag Goose and Wigeon while its function as feeding ground and stop-over site for migrating benthivorous birds became relatively less important, especially on the right bank. The water bird populations in the study area varied greatly with the geographic and regional trends for the different species and were influenced by winter severity but no direct proof of impacts from the container terminals could be established. Nearly every season the international 1 level was exceeded by one or more species, but the species of international importance changed with time. National and international protection measures are valuable but insufficient tools for the conservation of these estuarine habitats. A more comprehensive conservation strategy is proposed.
Western sandpiper (Calidris mauri) predation was examined by concurrent experiments and direct observations of foraging behaviour on high intertidal mudflats
of the Fraser River estuary, British Columbia. Western sandpipers foraged by either “pecking” on the surface (64% of observational
time) or probing into sediment (29%). The first experiment (probe-mark method) consisted of collecting small-volume cores
(21.2 cm3) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min feeding period. The second
experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collection
of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal
and meiofaunal size fractions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control
sediments revealed significant differences, attributed to shorebird predation, for both experiments. The probe-mark experiment
detected the removal of large infaunal polychaetes (∼ 20 mm), while the exclosure experiment showed depletion of epifaunal
harpacticoid copepods (0.063–0.5 mm). Predation on macrofaunal cumaceans was detected in both experiments. Invertebrates selected
by western sandpipers neither fell within traditional infaunal size classifications (macro- vs. meiofauna; 500 μm delineation)
nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western
sandpipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal
size ranges and (2) selective probing for larger infauna, such as polychaetes. These findings were facilitated by the combination
of methodologies employed.
The buffer effect predicts that where the reproductive success and survivorship of a species vary between potential habitats, sites will be sequentially filled according to a preference hierarchy. Once favoured sites reach saturation, numbers on the less-suitable/poorer quality sites will show a greater rate of increase compared with those on favoured sites. Supporting evidence for a buffer effect is readily available in the literature for many species, although this is generally restricted to small-scale analyses. In this paper we test for a buffer effect on a national scale for 19 species of waterbirds regularly wintering in the UK for which populations have increased nationally. The results provide little support for the effect, with only four species showing significant negative correlations. Nonetheless, a number of factors are likely to confound the occurrence/identification of a buffer effect for these species, including site area and data limitations. By contrast, for the majority of these 19 species, those sites where initial population totals are largest are also those with the fastest rates of population increase. Encouragingly, these sites are, therefore, more likely to be classified and managed as Special Protection Areas (SPAs) or Ramsar sites using the current numerical criteria for identifying such conservation areas.
Het is alweer enkele jaren geleden dat Alterra Texel onderzoek deed naar lopende monitorprogramma's in zoute wateren in Nederland. Dit onderzoek is in 2007 en 2008 besproken met vertegenwoordigers van zowel LNV als Waterstaat. De definitieve rapportage heeft geruime tijd op zich laten wachten, onder andere omdat lange tijd onduidelijk was welke parameters er in het kader van de rapportage over de habitatrichtlijn er voor de Europese Commissie nodig waren. Naast de Nederlandse gegevens worden in deze rapportage ook die van Denemarken, Zweden, Verenigd Koninkrijk en Duitsland gegeven
Individual feeding specialisation in shorebirds is reviewed, and the possilble mechanisms involved in such specialisations. Any specialisation can he seen as an individual strategy, and the optimum strategy for any given individual will be conditional upon its specific priorities and constraints. Some specialisations are related to social status and some to individual skills. Some are also probably frequency-dependent. However, most shorebird specialisations are constrained to a large extent by individual morphology, particularly bill morphology. For example, larger birds are able to handle larger prey, and birds with longer bills are able to feed on more deeply buried prey. Sex differences in bill length are uncommon in the Charardriidae, which are surface peckers, but are common in the Scolopacidae, which feed by probing in soft substrates. Sex differences in bill morphology are frequently associated with sex differences in feeding specialisation. There is evidence that different feeding specialisations are associated with different payoffs, in which case the probability of failing to reproduce or of dying will not be distributed equally throughout the population. I consider the population consequences of such feeding specialisations, particularly the different risks and benefits associated with different habitats or diets. I also consider the way in which individuals may differ in their response to habitat loss or change. I suggest that population models designed to predict the effect of habitat loss or change on shorebirds should have the ability to investigate the differential response of certain sections of the population, particularly different ages or sexes, that specialise in different diets or feeding methods.
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