Extreme variations in spring temperature affect ecosystem regulating services provided by birds during migration

Extreme weather is becoming more pronounced, making phenological patterns less predictable. Among the potential consequences, extreme weather may alter relationships of migratory birds with their seasonal food resources and thus impact valuable ecosystem regulating services (e.g., bird predation of herbivorous insects). Our goal was to quantify the effect of an extremely warm spring on these relationships in a U.S Midwest oak savanna. Average regional temperatures in the spring of 2009 coupled with record highs in 2010 (8°C warmer) were the basis of a natural experiment for addressing our goal. In both springs we documented tree flowering and leaf‐out phenology, related these to arrival and foraging behavior of the three most abundant migratory wood‐warbler species ( Parulidae ), and quantified the effects of migratory bird foraging on insect density, size, and herbivory using a branch exclosure experiment. In 2009, the dominant tree species at our study site, eastern black oak ( Quercus velutina ), flowered in mid‐May and the wood warblers foraged heavily in the savanna during this time. Branches from which birds were excluded exhibited a trend toward higher insect density, larger Lepidopterans, and greater flower damage than control branches. In 2010, tree phenology was four weeks earlier than in 2009 and the wood warblers were nearly absent from the savanna (83% fewer), likely because peak food availability preceded their arrival in mid‐May. Insect density was 83% greater in 2009 than 2010. However, in 2010, 81% of sampled leaves exhibited substantial damage (>25% of leaf‐area removed) compared with 27% of leaves in 2009, presumably due to a lack of herbivorous insect regulation by birds. Our results suggest that the extremely warm spring of 2010 altered linkages between migratory birds and their invertebrate prey that are typical during years of average weather, which likely affected habitat use and the delivery of ecosystem regulating services.