Winter habitat use by Loggerhead Shrikes (Lanius Ludovicianus) in Mexico: separating migrants from residents using stable isotopes

Several species of migrant birds overlap in range on their wintering grounds with non-migrant conspecifics or other species that occupy a similar niche. Very little is known whether such overlap results in competition and subsequent habitat segregation since it is usually impossible to separate resident from migrant individuals. The Loggerhead Shrike (Lanius ludovicianus) is a declining grassland species in North America that winters in the southern United States and Mexico. Using stable-hydrogen isotope (δD) analysis of feathers, we identified resident and migrant shrikes wintering in northeastern Mexico based on a latitudinal gradient in precipitation and feather δD values. Indicator species analyses showed that migrants occupied areas where bare ground was less available than those occupied by residents, a pattern which held when a more restricted set of birds from the extremes of the δD distribution were considered. This provides evidence for conspecific habitat segregation. Habitat differences were also found between sites occupied by shrikes and apparently suitable but unoccupied sites. Shrikes occupied more open sites that contained shorter tall shrubs and huisache (Acacia farneasiana) and fewer tall shrubs, mesquite (Prosopsis glandulosa) and huisache than unoccupied sites. The availability of suitable winter habitat and the potential competition between migrants and residents may be factors that influence the population dynamics of migrant shrikes in North America.     

Feather isotope analysis discriminates age-classes of Western, Least, and Semipalmated sandpipers when plumage methods are unreliable

ABSTRACT Avian age‐class discrimination is typically based on the completeness of the first prebasic molt. In several calidrid sandpiper species, juvenal flight feathers grown on Arctic breeding grounds are retained through the first three migrations. Thereafter, flight feathers are grown annually at temperate migratory stopover sites during the fall or on the subtropical wintering grounds. Standard methods for distinguishing age classes of sandpipers rely on a combination of traits, including body plumage, coloration of protected inner median covert edges, and extent of flight feather wear. We tested the ability of stable hydrogen isotope ratios in flight feathers (δD_f) to distinguish young birds in their first winter through second fall from older adults in three calidrid sandpiper species, Western (Calidris mauri), Least (C. minutilla), and Semipalmated (C. pusilla) sandpipers. We compared the apparent reliability of the isotope approach to that of plumage‐based aging. The large expected differences in δD_f values of flight feathers grown at Arctic versus non‐Arctic latitudes enabled use of this technique to discriminate between age‐classes. We determined δD_f values of known Arctic‐grown feathers from juveniles that grew their flight feathers on the breeding grounds. Flight feather δD_f values of southward‐migrating adults showed bimodal distributions for all three species. Negative values overlapped with species‐specific juvenile values, identifying putative second fall birds with high‐latitude grown juvenal feathers retained from the previous year. The more positive values identified older adults who grew their feathers at mid‐ and low latitudes. Importantly, δD_f analysis successfully identified first‐winter and second‐fall birds not detected by plumage‐based aging. Flight feather wear alone was a poor basis for age classification because scores overlapped extensively between putative second fall birds and older adults. Flight feather hydrogen isotope analysis enables more definitive assignment of age classes when standard plumage methods are unreliable.     

Population structure of loggerhead shrikes in the California Channel Islands

The loggerhead shrike (Lanius ludovicianus), a songbird that hunts like a small raptor, maintains breeding populations on seven of the eight California Channel Islands. One of the two subspecies, L. l. anthonyi, was described as having breeding populations on six of the islands while a second subspecies, L. l. mearnsi, was described as being endemic to San Clemente Island. Previous genetic studies have demonstrated that the San Clemente Island loggerhead shrike is well differentiated genetically from both L. l. anthonyi and mainland populations, despite the fact that birds from outside the population are regular visitors to the island. Those studies, however, did not include a comparison between San Clemente Island shrikes and the breeding population on Santa Catalina Island, the closest island to San Clemente. Here we use mitochondrial control region sequences and nuclear microsatellites to investigate the population structure of loggerhead shrikes in the Channel Islands. We confirm the genetic distinctiveness of the San Clemente Island loggerhead shrike and, using Bayesian clustering analysis, demonstrate the presence and infer the source of the nonbreeding visitors. Our results indicate that Channel Island loggerhead shrikes comprise three distinct genetic clusters that inhabit: (i) San Clemente Island, (ii) Santa Catalina Island and (iii) the Northern Channel Islands and nearby mainland; they do not support a recent suggestion that all Channel Island loggerhead shrikes should be managed as a single entity.     

Genetic differences between the endangered San Clemente Island loggerhead shrike Lanius ludovicianus mearnsi and two neighbouring subspecies demonstrated by mtDNA control region and cytochrome b sequence variation

We investigated mtDNA sequence variation in five populations of the loggerhead shrike Lanius ludovicianus , representing four subspecies, including the San Clemente loggerhead shrike L. l. mearnsi , a critically endangered California Channel Island endemic. Variability in 200 bp of control region and 200 bp of cytochrome b was extremely low, and defined four haplotypes. Strong structure was apparent among all three southern Californian subspecies, including L. l. mearnsi , with one haplotype predominating in each subspecies. Although potential levels of gene flow between L. l. mearnsi and neighbouring populations are low, mtDNA data support field observations that some shrikes visit the island during winter but do not stay to breed, and suggest that these birds come from the mainland. The similarity in haplotypes between populations from Saskatchewan, Canada and those in southern California suggests post-glacial northern range expansion of the species. Our results confirm the evolutionary distinctiveness of L. l. mearnsi and justify continuing efforts for its conservation.     

Genetic analysis of the endemic island loggerhead shrike, <Emphasis Type="Italic">Lanius ludovicianus anthonyi</Emphasis>

Based on limited research, the island loggerhead shrike, Lanius ludovicianus anthonyi has been considered a distinct subspecies endemic to the northern California Channel Islands. We used mtDNA control region sequences and microsatellite genotyping to compare loggerhead shrikes from the southern California mainland (L. l. gambeli), San Clemente Island (L. l. mearnsi), and the northern islands (L. l. anthonyi). Habitats on the islands are recovering due to the removal of non-native ungulates on the islands, but may be transitioning to habitats less supportive of loggerhead shrikes, so this evaluation comes at a critical time. We utilized 96 museum specimens that were collected over a century to evaluate both spatial and temporal genetic patterns. Analysis of multilocus microsatellite genotypes indicated that historical specimens of loggerhead shrikes (collected between 1897 and 1986) from the two northern islands of Santa Rosa and Santa Cruz are genetically distinct from adjacent mainland and island shrikes. Birds from Santa Catalina Island showed mixed ancestry and did not cluster with the northern island birds. Historical specimens of L. l. mearnsi from San Clemente Island also showed mixed ancestry. Our study provides evidence that a genetically distinct form of loggerhead shrikes, L. l. anthonyi, occurred on the islands of Santa Rosa and Santa Cruz.     

High variation reduces the value of feather stable isotope ratios in identifying new wintering areas for aquatic warblers Acrocephalus paludicola in West Africa

Stable isotope analysis of feathers can be useful in the study of seasonal interactions and migratory connectivity in birds. For the Palaearctic–African migration system, however, the lack of isotope data from feathers of known origin in Africa renders the geographic assignment of birds captured on European breeding grounds to potential wintering areas problematic. Rectrices of the threatened aquatic warbler Acrocephalus paludicola grown in Africa were sampled across six European countries to assess whether birds in different breeding populations shared similar isotopic signatures and so were likely to have wintered in the same region in Africa. Freshly grown feathers of aquatic warblers collected at the only known wintering site in Senegal showed high variation in carbon, nitrogen, and hydrogen isotope ratios. Due to similarly high variation in isotope ratios of African‐grown feathers within all breeding populations, it was not possible to determine whether different populations wintered in different regions. However, isotope signatures of 20% of birds captured on European breeding grounds fell outside the range of those captured in Senegal, suggesting a wider wintering distribution than is currently known. We therefore assessed whether the origin of these feathers could be estimated by trying to establish isotopic gradients across sub‐Saharan West Africa. Feathers of three ecologically similar surrogate species were sampled from wetlands across a 3000 km east‐west and a 2000 km north–south transect. Within‐site variation in feather isotope ratios was frequently larger than the difference predicted by gradients across West Africa. Thus, predicting the origin of individual feathers using single‐isotope gradients was not reliable. The large within‐site variability of feather isotope ratios of a habitat specialist species like the aquatic warbler indicates that using feather isotope ratios will require large sample sizes from many locations, and may thus not be an efficient tool in identifying wintering areas of Palaearctic–African migrants.     

Using stable isotope analysis to determine the winter moult extent in migratory birds: the complex moult of Savi's warblers Locustella luscinioides

Patterns of feather wear in birds captured in spring have traditionally been analysed to describe the extent of winter moult in long-distance migrants. However, the interpretation of feather wear may be rendered extremely difficult due to long moult periods, by the progress of the season, and by the existence of complex moult patterns. Here, stable isotope analysis is used to determine the origin of the wing feather generations present in Savi's warblers Locustella luscinioides captured in Portugal. Carbon, nitrogen and hydrogen isotope ratios of feathers of known European origin differed significantly from those known to have grown in Africa. A discriminant analysis, in which 91.1% of the cross validated samples were correctly classified, was used to determine the origin of tail and wing feathers collected from birds caught when they returned to the breeding quarters. The interpretation of feather-wear generally agreed with the stable isotope analysis, but some inconsistencies were identified. The extent of winter moult in Savi's warblers is described and its moult strategy discussed.     

Using stable isotope analysis to determine the winter moult extent in migratory birds: the complex moult of Savi's warblers Locustella luscinioides

Patterns of feather wear in birds captured in spring have traditionally been analysed to describe the extent of winter moult in long‐distance migrants. However, the interpretation of feather wear may be rendered extremely difficult due to long moult periods, by the progress of the season, and by the existence of complex moult patterns. Here, stable isotope analysis is used to determine the origin of the wing feather generations present in Savi's warblers Locustella luscinioides captured in Portugal. Carbon, nitrogen and hydrogen isotope ratios of feathers of known European origin differed significantly from those known to have grown in Africa. A discriminant analysis, in which 91.1% of the cross validated samples were correctly classified, was used to determine the origin of tail and wing feathers collected from birds caught when they returned to the breeding quarters. The interpretation of feather‐wear generally agreed with the stable isotope analysis, but some inconsistencies were identified. The extent of winter moult in Savi's warblers is described and its moult strategy discussed.     

Continent-wide variation in feather colour of a migratory songbird in relation to body condition and moulting locality

Understanding the causes of variation in feather colour in free-living migratory birds has been challenging owing to our inability to track individuals during the moulting period when colours are acquired. Using stable-hydrogen isotopes to estimate moulting locality, we show that the carotenoid-based yellow-orange colour of American redstart (Setophaga ruticilla) tail feathers sampled on the wintering grounds in Central America and the Caribbean is related to the location where feathers were grown the previous season across North America. Males that moulted at southerly latitudes were more likely to grow yellowish feathers compared with males that moulted more orange-red feathers further north. Independent samples obtained on both the breeding and the wintering grounds showed that red chroma-an index of carotenoid content-was not related to the mean daily feather growth rate, suggesting that condition during moult did not influence feather colour. Thus, our results support the hypothesis that feather colour is influenced by ecological conditions at the locations where the birds moulted. We suggest that these colour signals may be influenced by geographical variation in diet related to the availability of carotenoids.     

Connecting breeding and wintering grounds of Neotropical migrant songbirds using stable hydrogen isotopes: a call for an isotopic atlas of migratory connectivity

There is an overdue and urgent need to establish patterns of migratory connectivity linking breeding grounds, stopover sites, and wintering grounds of migratory birds. Such information allows more effective application of conservation efforts by applying focused actions along movement trajectories at the population level. Stable isotope methods, especially those using stable hydrogen isotope abundance in feathers (δ²H_f) combined with Bayesian assignment techniques incorporating prior information such as relative abundance of breeding birds, now provide a fast and reliable means of establishing migratory connectivity, especially for Neotropical migrants that breed in North America and molt prior to fall migration. Here we demonstrate how opportunistic sampling of feathers of 30 species of wintering birds in Cuba, Venezuela, Guatemala, Puerto Rico, and Mexico, regions that have typically been poorly sampled for estimating migratory connectivity, can be assigned to breeding areas in North America through both advanced spatial assignment to probability surfaces and through simpler map lookup approaches. Incorporating relative abundance information from the North American Breeding Bird Survey in our Bayesian assignment models generally resulted in a reduction in potential assignment areas on breeding grounds. However, additional tools to constrain longitude such as DNA markers or other isotopes would be desirable for establishing breeding or molt origins of species with broad longitudinal distributions. The isotope approach could act as a rapid means of establishing basic patterns of migratory connectivity across numerous species and populations. We propose a large‐scale coordinated sampling effort on the wintering grounds to establish an isotopic atlas of migratory connectivity for North American Neotropical migrants and suggest that isotopic variance be considered as a valuable metric to quantify migratory connectivity. This initiative could then act as a strategic template to guide further efforts involving stable isotopes, light‐sensitive geolocators, and other technologies.     

Color expression in experimentally regrown feathers of an overwintering migratory bird: implications for signaling and seasonal interactions

Plumage coloration in birds plays a critical role in communication and can be under selection throughout the annual cycle as a sexual and social signal. However, for migratory birds, little is known about the acquisition and maintenance of colorful plumage during the nonbreeding period. Winter habitat could influence the quality of colorful plumage, ultimately carrying over to influence sexual selection and social interactions during the breeding period. In addition to the annual growth of colorful feathers, feather loss from agonistic interactions or predator avoidance could require birds to replace colorful feathers in winter or experience plumage degradation. We hypothesized that conditions on the wintering grounds of migratory birds influence the quality of colorful plumage. We predicted that the quality of American redstart (Setophaga ruticilla) tail feathers regrown after experimental removal in Jamaica, West Indies, would be positively associated with habitat quality, body condition, and testosterone. Both yearling (SY) and adult (ASY) males regrew feathers with lower red chroma, suggesting reduced carotenoid content. While we did not observe a change in hue in ASY males, SY males shifted from yellow to orange plumage resembling experimentally regrown ASY feathers. We did not observe any effects of habitat, testosterone, or mass change. Our results demonstrate that redstarts are limited in their ability to adequately replace colorful plumage, regardless of habitat, in winter. Thus, feather loss on the nonbreeding grounds can affect social signals, potentially negatively carrying over to the breeding period.     

Settling Decisions and Heterospecific Social Information Use in Shrikes

Animals often settle near competitors, a behavior known as social attraction, which belies standard habitat selection theory. Two hypotheses account for these observations: individuals obtain Allee benefits mediated by the physical presence of a competitor, or they use successfully settled individual as a source of information indicating the location of high quality habitat. We evaluated these hypotheses experimentally in two species of shrikes. These passerine birds with a raptor-like mode of life impale prey to create larders that serve as an indicator of male/habitat quality. Thus, two forms of indirect information are available in our system: a successfully settled shrike and its larder. Typically these two cues are associated with each other, however, our experimental treatment created an unnatural situation by disassociating them. We manipulated the presence of larders of great grey shrikes and examined the settling decisions of red-backed shrikes within and outside the great grey shrike territories. Male red-backed shrikes did not settle sooner on plots with great grey shrikes compared to plots that only contained artificial larders indicating that red-backed shrikes do not use the physical presence of a great grey shrike when making settling decisions which is inconsistent with the Allee effect hypothesis. In contrast, for all plots without great grey shrikes, red-backed shrikes settled, paired and laid clutches sooner on plots with larders compared to plots without larders. We conclude that red-backed shrikes use larders of great grey shrikes as a cue to rapidly assess habitat quality.     

Estimating the latitudinal origins of migratory birds using hydrogen and sulfur stable isotopes in feathers: influence of marine prey base

Hydrogen stable isotope analysis of feathers is an important tool for estimating the natal or breeding latitudes of nearctic-neotropical migratory birds. This method is based on the latitudinal variation of hydrogen stable isotope ratios in precipitation in North America (deltaD(p)) and the inheritance of this variation in newly formed feathers (deltaD(f)). We hypothesized that the typically strong relationship between deltaD(p) and deltaD(f) would be decoupled in birds that forage in marine food webs because marine waters have relatively high deltaD values compared to deltaD values for local precipitation. Birds that forage on marine prey bases should also have feathers with high delta(34)S values, since delta(34)S values for marine sulfate are generally higher than delta(34)S values in terrestrial systems. To examine this potential marine effect on feather stable isotope ratios, we measured deltaD and delta(34)S in the feathers of nine different species of raptors from both inland and coastal locations across North America. Feathers from coastal bird-eating raptors had consistently higher deltaD and delta(34)S values than feathers from inland birds. Birds that had high delta(34)S values also deviated strongly from the typical relationship between deltaD(p) and deltaD(f). We recommend measuring both sulfur and hydrogen stable isotope ratios in feathers when some members of a migrant population could potentially forage in marine habitats. We suggest using a practical cutoff of delta(34)S >10 per thousand to remove marine-foraging birds from a migrant sample when using stable isotopes of hydrogen to estimate the latitudinal origins of migrants because high deltaD(f) values for marine-foraging birds could potentially distort estimates of origins.     

Using satellite imagery to assess breeding habitat availability of the endangered loggerhead shrike in Québec

The loggerhead shrike (Lanius ludovicianus) is a grassland bird species whose preferred nesting habitat in eastern Canada is pastureland. This species has been extirpated from much of its historical range in this region, and breeding habitat loss is suspected to be an important cause of this decline. We evaluated the availability of suitable breeding habitats in Québec using satellite imagery. Because this species no longer breeds in Québec, we established habitat selection criteria from known nesting sites in the adjacent province of Ontario, from analysis of a Landsat-TM satellite image, and applied these criteria to Landsat-TM images covering southern Québec. We developed regional landscape criteria in 100 km² plots and patch indices criteria at the pasture level. Spatial analyses were conducted to characterize plots and pastures on the basis of pasture availability and spatial distribution. Pastures suitable for nesting loggerhead shrikes were those fulfilling patch criteria at the pasture level and located in plots fulfilling regional landscape criteria. Overall, 310 out of 1700 plots located in the historical breeding range of the loggerhead shrike in Québec fulfilled landscape criteria, supporting 3988 pastures that fulfilled patch criteria. More than 500 of these pastures were visited to validate their current status. The Outaouais region would be the most suitable region for nesting loggerhead shrikes in southern Québec, where suitable breeding habitat still remains because more than two-thirds of visited sites were still pastureland, hawthorns were well-distributed in the region, and pastureland fragmentation was lowest. We conclude that the availability of breeding habitat does not limit the establishment of a breeding population of loggerhead shrike in southern Québec, as we estimated that thousands of hectares of suitable habitat still remain in that province.     

Migration strategy affects avian influenza dynamics in mallards (Anas platyrhynchos)

Studies of pathogen transmission typically overlook that wildlife hosts can include both migrant and resident populations when attempting to model circulation. Through the application of stable isotopes in flight feathers, we estimated the migration strategy of mallards (Anas platyrhynchos) occurring on California wintering grounds. Our study demonstrates that mallards‐ a principal host of avian influenza virus (AIV) in nature, contribute differently to virus gene flow depending on migration strategy. No difference in AIV prevalence was detected between resident (9.6%), intermediate‐distance (9.6%) and long‐distance migrants (7.4%). Viral diversity among the three groups was also comparable, possibly owing to viral pool mixing when birds converge at wetlands during winter. However, migrants and residents contributed differently to the virus gene pool at wintering wetlands. Migrants introduced virus from northern breeding grounds (Alaska and the NW Pacific Rim) into the wintering population, facilitating gene flow at continental scales, but circulation of imported virus appeared to be limited. In contrast, resident mallards acted as AIV reservoirs facilitating year‐round circulation of limited subtypes (i.e. H5N2) at lower latitudes. This study supports a model of virus exchange in temperate regions driven by the convergence of wild birds with separate geographic origins and exposure histories.     

Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwater fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers.
2. In order to assess the extent of freshwater feeding, stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in feathers of wild cormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-ranging shags Phalacrocorax aristotelis ) and freshwater diets (juvenile goosanders Mergus merganser ).
3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormorants; each feather type therefore represented the diet over a different temporal scale.
4. Isotopic analyses of feathers indicated that, when shot, nearly all of the cormorants had been feeding entirely on freshwater prey. The mean δ¹³C value of primary feathers growing when birds were shot was –22·2‰, indicative of an entirely freshwater diet.
5. The move to freshwater habitats from coastal breeding grounds occurred over several months, but once established cormorants appear to have fed at freshwater sites throughout the autumn and winter.
6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed.
7. Although the results indicate long-term residency and feeding in freshwater systems, they do not indicate whether birds were feeding regularly at the sites at which they were shot, or the composition of the diet. It is recommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.     

Variability in stable isotopes of snowy owl feathers and contribution of marine resources to their winter diet

The snowy owl is an elusive arctic predator known for its nomadic behaviour. Satellite tracking has revealed that some adult snowy owls could make an extensive use of the marine environment during the non‐breeding season. However, the relative contribution of marine resources to their diet is unknown. Stable isotope analyses can be useful to document the diet of mobile animals during periods of the year when individuals are less accessible. This study aimed to assess variation in isotopic values (δ¹³C and δ¹⁵N) of various feather types, and the usefulness of feathers to determine the contribution of the marine environment to the winter diet of snowy owls captured in summer. We sampled feathers coming from 6 body regions of 18 breeding females at two sites in the eastern Canadian Arctic in 2013 and 2014. Prior to analyses, diet‐tissue discrimination factors of snowy owl feathers were established in captivity. Variability in isotopic values among feather types was relatively low and pairwise correlations in isotopic values between feathers on the same individual were variable and often low, which suggests differences in the diet at the time when various feathers were synthesized. Diet reconstruction models detected a contribution of marine sources to snowy owl feathers ranging from 4 to 19% among feather types. However, the marine contribution was highly variable when single feathers were examined within individuals, ranging from 3 to 71%. This indicated that no single feather type could be used alone to reliably infer the contribution of marine resources to the winter diet of owls, possibly due to a high variability in the timing and sequence of molt. For asynchronous molters like snowy owls, we recommend sampling multiple feathers from various body regions, excluding wing feathers, to investigate winter diet or habitat use.     

Linking breeding and wintering grounds of neotropical migrant songbirds using stable hydrogen isotopic analysis of feathers

 Recent studies have shown that stable hydrogen isotope ratios (δD) in the tissues of animals often correlate with δD of local precipitation. Here we examined the relationship between δD in feathers and growing season precipitation for neotropical migrant songbirds breeding over a continent-wide isotopic gradient. δD values were determined on feathers of 140 individuals of 6 species of wild insectivorous forest songbirds (Setophaga ruticilla, Empidonax minimus, Vermivora peregrinus, Catharus ustulatus, Seiurus aurocapillus, Hylocichla mustelina) taken from 14 breeding locations across North America. The δD of feathers was strongly correlated with the δD of growing season precipitation at breeding sites across North America. As feather hydrogen is metabolically inert after growth, this relationship was then used to assess the breeding origins of wintering migrants. Deuterium values of feathers from 64 individuals representing 5 species of migrants (Helmitheros vermivorus, Wilsonia citrina, Hylocichla mustelina, Dumetella carolinensis, Seirus aurocapillus) at a wintering site in Guatemala were consistent with those predicted from the known breeding ranges of these species. This study demonstrates hydrogen isotopes may become a powerful tool for linking breeding and wintering grounds of neotropical migrant songbirds, as well as other migratory species moving between isotopically distinct regions. Received: 25 August 1995 / Accepted: 28 May 1996     

Malaria infection and feather growth rate predict reproductive success in house martins

Carry-over effects take place when events occurring in one season influence individual performance in a subsequent season. Blood parasites (e.g. Plasmodium and Haemoproteus) have strong negative effects on the body condition of their hosts and could slow the rate of feather growth on the wintering grounds. In turn, these winter moult costs could reduce reproductive success in the following breeding season. In house martins Delichon urbica captured and studied at a breeding site in Europe, we used ptilochronology to measure growth rate of tail feathers moulted on the winter range in Africa, and assessed infection status of blood parasites transmitted on the wintering grounds. We found a negative association between haemosporidian parasite infection status and inferred growth rate of tail feathers. A low feather growth rate and blood parasite infections were related to a delay in laying date in their European breeding quarters. In addition, clutch size and the number of fledglings were negatively related to a delayed laying date and blood parasite infection. These results stress the importance of blood parasites and feather growth rate as potentially mechanisms driving carry-over effects to explain fitness differences in wild populations of migratory birds.     

Stable isotope ratios in winter‐grown feathers of Great Reed Warblers Acrocephalus arundinaceus,Clamorous Reed Warblers A. stentoreus and their hybrids in a sympatric breeding population in Kazakhstan

Analyses of the stable isotope composition of feathers can provide significant insight into the spatial structure of bird migration. We collected feathers from Great Reed Warblers Acrocephalus arundinaceus, Clamorous Reed Warblers A. stentoreus and a small sample of their hybrids in a sympatric breeding population in Kazakhstan to assess natural variation in stable isotope signatures and delineate wintering sites. The Great Reed Warbler is a long‐distance migrant that overwinters in sub‐Saharan Africa, whereas the Clamorous Reed Warbler performs a short‐distance migration to the Indian sub‐continent. Carbon (δ¹³C), nitrogen (δ¹⁵N) and deuterium (δD) isotope signatures were obtained from winter‐grown feathers of adult birds. There were highly significant differences in δD and less significant differences in δ¹³C between Great and Clamorous Reed Warblers. Thus, our results show that the stable isotope technique, and in particular the deuterium (δD) signal, resolves continental variation in winter distribution between these closely related Acrocephalus species with sympatric natal origin. The isotope signatures of hybrid Great × Clamorous Reed Warblers clustered with those of the Great Reed Warblers. Hence, a parsimonious suggestion is that the hybrids undergo moult in Afrotropical wintering grounds, as do the Great Reed Warblers. The observed δD values fell within the range of expected values based on available precipitation data collected at precipitation stations across the wintering continents of each species. However, the power to predict the winter origin of birds in our study system using these data was weak as the expected values ranged widely at this broad continental scale.