Climate variability has idiosyncratic impacts on North American aerial insectivorous bird population trajectories

Delineating effects of climate change on biological communities is an urgent conservation challenge, as climate models predict increases in frequency and magnitude of climatic variability. Climate affects migratory animals at different times and locations throughout their annual cycles. Using long-term (50-year) population trajectories from the North American Breeding Bird Survey, we evaluated climate effects throughout the annual cycle for 37 species of aerial insectivorous birds, a group experiencing widespread population declines and exhibiting extensive within-species spatial synchrony but limited among-species concordance in population trajectories. We assessed the effects of five large-scale climatic indices at two temporal scales (current year and one-year lagged), two measures of seasonal winds, and four measures of seasonal storms (hereafter "winds and storms") to test hypotheses about the magnitude of climatic effects that may explain trend variation in population trajectories. Climate covariates - notably the Atlantic Multidecadal Oscillation - improved the fit of 77% of regional population models relative to temporal variation alone. Climate indices, particularly when lagged, had stronger effects than migratory winds and storms on population trajectories. Climate effects were strongest on swallows, diurnal migrants, northerly-wintering species, and western breeding populations, and weakest on nightjars. Such strong yet idiosyncratic climate responses reveal the full annual cycle sensitivity of migratory aerial insectivores to climate change while reinforcing recent findings that the causes of aerial insectivorous bird population declines are complex and species-specific.