Diet flexibility in a declining long‐distance migrant may allow it to escape the consequences of phenological mismatch with its caterpillar food supply

Climate‐driven shifts in prey phenology may lead to asynchrony with the timing of peak resource requirements of their predators, leading to a reduction in productivity and population declines. Migrant species that cannot adjust their arrival times may be particularly at risk, especially those that breed in seasonal environments and for which a temporarily super‐abundant prey source is important, such as insectivorous passerine birds that take advantage of the seasonal flush of caterpillars to feed their young. We assess whether population declines of the trans‐Saharan migratory Wood Warbler Phylloscopus sibilatrix are likely to have been caused by phenological mismatch. We measured seasonal invertebrate biomass and various fitness parameters, including the timing of breeding and breeding success, in two time periods: 1982–1984, prior to the species’ decline in the UK, and 2009–2011, as the reduction in numbers continued. Although birds bred on average a week earlier in 2009–2011 than in 1982–1984, this was not adequate to track the more rapid advancement of peak caterpillar biomass, which advanced by 12 days and was closely correlated with spring temperatures. Moreover, although caterpillars were the dominant prey fed to nestlings, there was only limited evidence that productivity was positively related to caterpillar biomass in the environment. Considering only successful nests, synchrony with the food peak did not produce heavier nestlings and had only a small positive effect on fledging success, although there was a seasonal decline in productivity when all nests were considered. We conclude that the lack of a marked effect of the observed mismatch is due to Wood Warblers’ generalist diet, enabling them to breed successfully on prey other than caterpillars. Although other studies have demonstrated that climate‐driven asynchrony of predator and prey populations can have impacts on avian demography, this study highlights the importance of investigating the generality of those findings.