Climate change can affect the distribution, abundance, and phenology of organisms globally. Variations in the timing of passage during autumn and spring migration can have consequences at individual and population levels. We assessed whether global climatic indexes and increasing air temperature over a 28 yr period were concurrent with shifts in the autumn migration phenology of 16 eastern North American raptor species. We used count data from 7 eastern North American raptor-migration watch sites and examined whether key species-specific traits such as migration strategy (complete vs. partial and trans-equatorial vs. not), diet specialization, body mass, flight strategy (soaring vs. flapping), and latitude of the northern limit of breeding distribution were associated with a shift in the timing of autumn migration. Our results suggest an overall delay across species in autumn migration passage date of ∼1 day decade−1, which coincided with an increase in temperature across eastern North America. This shift in average autumn passage date was more pronounced in short-distance migrants ( 1.03 days decade−1); no shift was detected in trans-equatorial migrants. Although we did not detect clear links between annual climatic indexes and the other life-history traits studied, the results nonetheless indicate that the autumn migration phenology of eastern North American raptors may be delayed by ongoing climate change. However, the amplitude of these effects varies on a species-by-species basis. Our results—combined with new evidence of an earlier passage during spring migration for the same species in the same area—suggest that, since 1985, most raptors spent ∼2 additional days decade−1 north of our study's migration sites. Such an increase in time spent at northern latitudes in a large number of terrestrial avian predators over a wide geographic area may have profound impacts on population and ecosystem dynamics. Unraveling such impacts will require linking interspecific and intraspecific variations in phenological adjustments to ongoing climate change.
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Vol. 134 • No. 4