Recent advances in spring arrival dates have been reported in many migratory species but the mechanism driving these advances is unknown. As population declines are most widely reported in species that are not advancing migration, there is an urgent need to identify the mechanisms facilitating and constraining these advances. Individual plasticity in timing of migration in response to changing climatic conditions is commonly proposed to drive these advances but plasticity in individual migratory timings is rarely observed. For a shorebird population that has significantly advanced migration in recent decades, we show that individual arrival dates are highly consistent between years, but that the arrival dates of new recruits to the population are significantly earlier now than in previous years. Several mechanisms could drive advances in recruit arrival, none of which require individual plasticity or rapid evolution of migration timings. In particular, advances in nest-laying dates could result in advanced recruit arrival, if benefits of early hatching facilitate early subsequent spring migration. This mechanism could also explain why arrival dates of short-distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants.

Abstract. 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.

Summary 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.

Summary 1.Buffer effects occur when increases in population size result in an increasing proportion of a population inhabiting poor quality sites. When there are fitness costs to inhabiting poor sites, buffer effects can potentially regulate population sizes. In migratory populations, the regulatory capacity of buffer effects will clearly be influenced by their role in both the breeding and nonbreeding seasons, but previous studies have been restricted to one season only. 2. Icelandic black-tailed godwits Limosa limosa islandica are currently increasing in number and previous studies have revealed a large-scale buffer effect operating on the wintering grounds. 3. Here, we reconstruct the pattern of population expansion and colonization of new breeding grounds, in relation to breeding habitat quality, to investigate whether a buffer effect is also operating during the breeding season. 4. Godwit breeding success is higher in marsh habitats than in dwarf-birch bogs. Prey densities are also higher in marsh habitats, and breeding success increases with the density of shallow pools, which are more common on marsh sites. Large lowland basins with higher marsh coverage were colonized earlier than small ones with low marsh coverage. Recent colonizations have been into basins that are closer to occupied sites and have higher cover of dwarf-birch bog. 5. Thus godwits appear to be expanding into poorer quality breeding habitat as well as poorer quality winter habitat. The large spatial scale of these analyses and the fitness costs of occupying poor quality sites suggest that this double buffer effect is likely to play a key role in regulating this expanding population. In most migratory populations, some level of density dependence is likely to operate at both ends of the range. Double buffer effects may therefore be a common phenomenon and an important mechanism regulating migratory populations.

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