Recent changes in the timing of spring migration associated with climate change are occurring in hundreds of species of Northern Hemisphere birds. Much less is known about effects on autumn migration, especially for birds of prey. I studied simultaneous changes in spring and autumn phenology using data from 14 raptor species at two bird observatories on the shore of Lake Superior, North America. Median migration date advanced by 0.13 days·year-1 in spring and was delayed by 0.23 days·year-1 in autumn, with significant heterogeneity among species. Long-distance migrants were observed late in spring and early in autumn and showed less phenological change during autumn than short-distance migrants. The migratory period has become more extended, especially for short-distance migrants. Opposite responses during the two seasons had the effect of extending time spent to the north of the study area, by up to 30 days in some species since the early 1970s. These phenological shifts—potentially related to climate change— are causing dramatic changes in the annual cycle of North American raptors; whether these are beneficial or detrimental is unknown.
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Vol. 129 • No. 4