Seasonal variation in arthropod prey as a driver of elevational migration by montane birds

Elevational migration is a common avian strategy to deal with seasonality in montane environments. Arthropods are an important food source for a majority of the world's birds and are also highly sensitive to temperature. In mountains, where most terrestrial bird diversity is concentrated, arthropod abundance is likely to vary with both elevation and season, potentially influencing elevational migration through resource tracking. We sampled arthropods and birds in summer and winter across a 2300 metre forested elevation gradient in the hyperdiverse eastern Himalayas. In summer, aerial and terrestrial arthropod abundance was positively correlated with elevation but declined with increasing elevation in winter. Foliage arthropod abundance declined with elevation in both seasons but with little seasonal variation in abundance. Sallying and terrestrial-gleaning insectivore abundance increased with elevation in summer and declined in winter. The seasonal movements of these species correlate with fluctuations in prey abundance, potentially indicating that they migrate elevationally to track arthropod availability. In contrast, leaf-gleaning insectivore abundance did not vary seasonally with elevation; these species are largely resident which likely feed on seasonally stable abundances of foliage arthropods, obviating the need for migration. These patterns are consistent with the hypothesis that food availability influences the movement strategies of avian insectivores.