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The relative fitness of individuals across a population can shape distributions and drive population growth rates. Migratory species often winter over large geographic ranges, and individuals in different locations experience very different environmental conditions, including different migration costs, which can potentially create fitness inequalities. Here we used energetics models to quantify the trade-offs experienced by a migratory shorebird species at locations throughout the nonbreeding range, and the associated consequences for migratory performance, survival, and breeding habitat quality. Individuals experiencing more favorable winter conditions had higher survival rates, arrived on the breeding grounds earlier, and occupied better quality breeding areas, even when migration costs are substantially higher, than individuals from locations where the energy balance on the wintering grounds was less favorable. The energy costs and benefits of occupying different winter locations can therefore create fitness inequalities which can shape the distribution and population-wide demography of migratory species.

For many migratory bird species, the latitudinal range of the winter distribution spans thousands of kilometres, thus encompassing considerable variation in individual migration distances. Pressure to winter near breeding areas is thought to be a strong driver of the evolution of migration patterns, as individuals undertaking a shorter migration are generally considered to benefit from earlier arrival on the breeding grounds. However, the influence of migration distance on timing of arrival is difficult to quantify because of the large scales over which individuals must be tracked. Using a unique dataset of individually-marked Icelandic black-tailed godwits Limosa limosa islándica tracked throughout the migratory range by a network of hundreds of volunteer observers, we quantify the consequences of migrating different distances for the use of stop-over sites and timing of arrival in Iceland. Modelling of potential flight distances and tracking of individuals from across the winter range shows that individuals wintering further from the breeding grounds must undertake a stop-over during spring migration. However, despite travelling twice the distance and undertaking a stop-over, individuals wintering furthest from the breeding grounds are able to overtake their conspecifics on spring migration and arrive earlier in Iceland.Wintering further from the breeding grounds can therefore be advantageous in migratory species, even when this requires the use of stop-over sites which lengthen the migratory journey. As early arrival on breeding sites confers advantages for breeding success, the capacity of longer distance migrants to overtake conspecifics is likely to influence the fitness consequences of individual migration strategies. Variation in the quality of wintering and stopover sites throughout the range can therefore outweigh the benefits of wintering close to the breeding grounds, and may be a primary driver of the evolution of specific migration routes and patterns.

When species occupy habitats that vary in quality, choice of habitat can be critical in determining individual fitness. In most migratory species, juveniles migrate independently of their parents and must therefore choose both breeding and winter habitats. Using a unique dataset of marked black-tailed godwits (Limosa limosa islandica) tracked throughout their migratory range, combined with analyses of stable carbon isotope ratios, we show that those individuals that occupy higher quality breeding sites also use higher quality winter sites. This seasonal matching can severely inflate inequalities in individual fitness. This population has expanded over the last century into poorer quality breeding and winter habitats and, across the whole population; individual birds tend to occupy either novel or traditional sites in both seasons. Winter and breeding season habitat selection are thus strongly linked throughout this population; these links have profound implications for a wide range of population and evolutionary processes. As adult godwits are highly philopatric, the initial choice of winter habitat by juveniles will be critical in determining future survival, timing of migration and breeding success.

The capacity of species to track changing environmental conditions is a key component of population and range changes in response to environmental change. High levels of local adaptation may constrain expansion into new locations, while the relative fitness of dispersing individuals will influence subsequent population growth. However, opportunities to explore such processes are rare, particularly at scales relevant to species-based conservation strategies. Icelandic black-tailed godwits, Limosa limosa islandica, have expanded their range throughout Iceland over the last century. We show that current male morphology varies strongly in relation to the timing of colonization across Iceland, with small males being absent from recently occupied areas. Smaller males are also proportionately more abundant on habitats and sites with higher breeding success and relative abundance of females. This population-wide spatial structuring of male morphology is most likely to result from female preferences for small males and better-quality habitats increasing both small-male fitness and the dispersal probability of larger males into poorer-quality habitats. Such eco-evolutionary feedbacks may be a key driver of rates of population growth and range expansion and contraction.

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex‐biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex‐biased impacts, we use range‐wide tracking of individually color‐ringed Icelandic black‐tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non‐breeding range to within‐site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large‐scale sex differences in distribution during the non‐breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small‐scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within‐site distribution and resource use means that local‐scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds. Migratory species are extremely mobile and often range over large geographic areas, potentially creating distinct distribution patterns for each sex, which may be linked to resource use, given sex‐specific energetic requirements. Icelandic black‐tailed godwits mostly segregate spatially at the local scale as a consequence of sex differences in prey selection. Sex differences in resource exploitation have implications for the capacity of males and females to meet their energetic requirements, and for sex differences in the response to changing environmental conditions. Photo by Graham Catley.

The major histocompatibility complex (MHC) encodes proteins that are central for antigen presentation and pathogen elimination. MHC class I (MHC-I) genes have attracted a great deal of interest among researchers in ecology and evolution and have been partly characterized in a wide range of bird species. So far, the main focus has been on species within the bird orders Galliformes and Passeriformes, while Charadriiformes remain vastly underrepresented with only two species studied to date. These two Charadriiformes species exhibit striking differences in MHC-I characteristics and MHC-I diversity. We therefore set out to study a third species within Charadriiformes, the Icelandic subspecies of black-tailed godwits ( Limosa limosa islandica ). This subspecies is normally confined to parasite-poor environments, and we hence expected low MHC diversity. MHC-I was partially characterized first using Sanger sequencing and then using high-throughput sequencing (MiSeq) in 84 individuals. We verified 47 nucleotide alleles in open reading frame with classical MHC-I characteristics, and each individual godwit had two to seven putatively classical MHC alleles. However, in contrast to previous MHC-I data within Charadriiformes, we did not find any evidence of alleles with low sequence diversity, believed to represent non-classical MHC genes. The diversity and divergence of the godwits MHC-I genes to a large extent fell between the previous estimates within Charadriiformes. However, the MHC genes of the migratory godwits had few sites subject to positive selection, and one possible explanation could be a low exposure to pathogens.

1. In migratory species, early arrival on the breeding grounds can often enhance breeding success. Timing of spring migration is therefore a key process that is likely to be influenced both by factors specific to individuals, such as the quality of winter and breeding locations and the distance between them, and by annual variation in weather conditions before and during migration. 2. The Icelandic black-tailed godwit Limosa limosa islandica population is currently increasing and, throughout Iceland, is expanding into poorer quality breeding areas. Using a unique data set of arrival times in Iceland in different years for individuals of known breeding and wintering locations, we show that individuals breeding in lower quality, recently occupied and colder areas arrive later than those from traditionally occupied areas. The population is also expanding into new wintering areas, and males from traditionally occupied winter sites also arrive earlier than those occupying novel sites. 3. Annual variation in timing of migration of individuals is influenced by large-scale weather systems (the North Atlantic Oscillation), but between-individual variation is a stronger predictor of arrival time than the NAO. Distance between winter and breeding sites does not influence arrival times. 4. Annual variation in timing of migration is therefore influenced by climatic factors, but the pattern of individual arrival is primarily related to breeding and winter habitat quality. These habitat effects on arrival patterns are likely to operate through variation in individual condition and local-scale density-dependent processes. Timing of migration thus appears to be a key component of the intricate relationship between wintering and breeding grounds in this migratory system.

Buffer effects occur when sites vary in quality and fluctuations in population size are mirrored by large changes in animal numbers in poor-quality sites but only small changes in good-quality sites. Hence, the poor sites ‘buffer’ the good sites 1 , 2 , a mechanism that can potentially drive population regulation if there are demographic costs of inhabiting poor sites. Here we show that for a migratory bird this process can apply on a country-wide scale with consequences for both survival and timing of arrival on the breeding grounds (an indicator of reproductive success 3 , 4 ). The Icelandic population of the black-tailed godwit, Limosa limosa islandica , wintering in Britain has increased fourfold since the 1970s (ref. 5 ) but rates of change within individual estuaries have varied from zero to sixfold increases. In accordance with the buffer effect, rates of increase are greater on estuaries with low initial numbers, and godwits on these sites have lower prey-intake rates, lower survival rates and arrive later in Iceland than godwits on sites with stable populations. The buffer effect can therefore be a major process influencing large-scale population regulation of migratory species.

Synchronous arrival of pairs of migratory birds at their breeding grounds is important for maintaining pair bonds and is achieved by pairs that remain together all year round. Here we show that arrival is also synchronized in paired individuals of a migratory shorebird, the black-tailed godwit (Limosa limosa islandica), even though they winter hundreds of kilometres apart and do not migrate together. The mechanisms required to achieve this synchrony and prevent 'divorce' illustrate the complexity of migratory systems.

In expanding populations, individuals may increasingly be forced to use sites of relatively low quality. This process, named the \"buffer effect,\" was previously described for the Black-tailed Godwit (Limosa limosa islandica) in its use of nonbreeding sites in Great Britain and of breeding areas in Iceland. On the basis of diet analyses from droppings and stable isotopes, we describe a new case for the expanding French wintering population of the Black-tailed Godwit, an expansion accompanied by a drastic change in feeding strategy. In the 1990s, Black-tailed Godwits started using intertidal mudflats at Ile de Re, where they eat the rhizomes of seagrass (Zostera noltii) rather than the customary shellfish (Macoma balthica) eaten at both the preferred (initial) site (Aiguillon Bay) and the area occupied last (Yves to Marennes-Oleron bays). Individually color-marked godwits appeared faithful to both diet type and site, suggesting a cost of a change of strategy. This represents a first case of rhizome-feeding in shorebirds, and it exemplifies a case of carnivorous birds occupying a new site shifting to herbivory. [PUBLICATION ABSTRACT]