Vagrancy, where individuals occur outside of known population distributions, is a poorly understood ecological phenomenon. It can however be a key driver of site colonisation and range expansion. Evidence is emerging that presumed vagrant Siberian passerines in Western Europe, e.g. Richard’s Pipits Anthus ricardii, are colonists, with geolocator-tracked individuals returning to breed in Siberia after wintering in Western Europe. As such, ‘vagrancy’ patterns in these taxa could provide a model system to understand large-scale range shifts. For example, determining the origins of vagrant individuals and linking these to morphology and arrival date could help to identify the potential drivers of range dynamics. Here, we investigate the origins of vagrant Yellow-browed Warblers Phylloscopus inornatus (a migratory Siberian breeding passerine) in Western Europe by analysing stable hydrogen isotopes, morphology and phenology. We measure the isotopic patterns of feathers grown on the breeding grounds and their relation to those from two sub-species of Common Chiffchaff Phylloscopus collybita. We found that Yellow-browed Warblers have similar hydrogen isotopic signatures (δ2H) to the Siberian sub-species of Common Chiffchaff Phylloscopus collybita tristis and δ2H values did not overlap with those from the European nominate race of Common Chiffchaff Phylloscopus collybita collybita. There was weak evidence that variation in δ2H values was linked to differences in migratory distances in sampled Yellow-browed Warblers. The variation in δ2H values for Yellow-browed Warblers was similar to Chiffchaffs of the collybita and tristis sub-species. This suggests that Yellow-browed Warblers in Western Europe may originate from a relatively broad-front and not exclusively from an expanding western breeding range margin. It is unclear if vagrant Yellow-browed Warblers in Western Europe make viable return migrations to Siberia. If they are, the subset of individuals that become colonists could help us understand how vagrancy drives biogeographic processes, such as the establishment of novel migration routes.

1. Researchers generally ascribe demographic drivers in a single or few sub-populations and presume they are representative. With this information, practitioners implement blanket conservation measures across metapopulations to reverse declines. However, such approaches may not be appropriate in circumstances where sub-populations are spatiotemporally segregated and exposed to different environmental variation. 2. The Greenland White-fronted Goose Anser albifrons flavirostris is an Arctic-nesting migrant that largely comprises two sub-populations (delineated by northerly and southerly breeding areas in west Greenland). The metapopulation has declined since 1999 but this trend is only mirrored in one sub-population and the causes of this disparity are unclear. Here we compare the drivers and trends of productivity in both sub-populations using population- and individual-level analysis. 3. We examined how temperature and precipitation influenced population-level reproductive success and whether there was a change in the relationship when metapopulation decline commenced. In addition we used biologging devices to reconstruct incubation events and modelled how phenology and environmental conditions influenced individual-level nest survival. 4. Correlations between reproductive success and temperature/precipitation on the breeding grounds have weakened for both sub-populations. This has resulted in lower reproductive success for the northerly, but not southerly breeding sub-population, which at the individual-level appears to be driven by lower in nest survival. Earlier breeding ground arrival and less precipitation during incubation increased nest survival in the northerly breeding population, while no factors examined were important for the southerly breeding sub-population. This suggests reproductive success is now driven by different factor(s) in the two sub-populations. 5. Demographic rates and their environmental drivers differ between the sub-populations examined here and consequently we encourage further decomposition of demography within metapopulations. This is important for conservation practitioners to consider as bespoke conservation strategies, targeting different limiting factors, may be required for different sub-population.

Climate change has been driving long-distance migratory birds to alter their schedules under the threat of being mismatched with their food peak at the breeding grounds. It is important to study the relative contribution of environmental, genetic and ontogenetic components in various spring timing traits in the wild in order to predict the true potential for migratory birds to adapt to the changing environment. We aimed to detect if heritable and ontogenetic components can explain variation in the timing of spring migration and breeding in pied flycatchers (Ficedula hypoleuca). Geolocator tracks of 44 locally hatched birds deployed during 2016-2019 in the Netherlands and the United Kingdom were used to investigate the role of early-life traits in the pre-fledging phase, as well as parental timings, in contributing to individual differences in the timing of spring migration and breeding in adulthood. We found a positive relationship between an individuals’ birth date and spring departure date from Africa in adulthood, but not for breeding arrival or laying date. Variation in spring departure date could not be explained by any other early-life trait in the pre-fledging phase, yet was well explained by the arrival dates of its parents in its birth year. This suggests that under natural conditions, individual differences in spring departure timing have a strong heritable component (in the broad sense), but that environmental conditions experienced along the migratory route and at breeding sites are partially masking this expression in arrival and laying schedules in these early breeding populations. Such environmental masking may reduce heritability in the timing of arrival and laying, thereby slowing down climatic adaptation towards earlier time schedules in pied flycatchers.