Is the timing of bird migrations keeping up with climate change?
Consistent with observed changes in climate, birds are shifting the timing of their migrations. Yet it is unclear if the bird migration phenology has kept pace with their changing environment. Migratory birds, given their ability to move long distances, might be expected to be among the most adaptable to climate change. However, birds must anticipate climatic conditions of their distant breeding grounds from their wintering grounds, which is complicated by greater climatic change at higher latitudes. Further, while food emergence at the breeding grounds is driven by annually variable climate, birds are driven to migrate primarily instead by hormonal responses to light cues such as photoperiod, which is constant across years. As climate at the breeding grounds change, synchrony between bird arrival and the emergence of their insect prey may be lost.
We conducted the first continental scale study testing phenological asynchrony in migratory birds, presenting trends in asynchrony for 48 passerine species in North America for the 2001-2012 period. We used remotely sensed spring vegetation green-up (MODIS) dates as a proxy for resource availability and a yardstick with which to compare bird arrival, which we in turn estimated from citizen science records (eBird).
We show that asynchrony between bird arrival and green-up was significant in 47 species, with arrival lagging green-up, and significantly increased for 9 species while decreasing for 2 species. Where asynchrony increased, birds arrived earlier, but did not keep pace with rapidly advancing green-up. Asynchrony strongly increased with earlier green-up.
While asynchrony tended to increase in eastern breeding species, it decreased in western breeding species. Similarly, mean asynchrony depended on ecoregion and generally increased in the east while decreasing in the west. These patterns mirror those of green-up, which advanced in the east and unexpectedly got later in the west.
Asynchrony, where arrival was least tied to green-up, was highest in the US gulf states and parts of the west. Asynchrony was explained by climate variables and by species traits such as migration distance and clutch size.
Our results suggest that trophic levels respond to climate change at different rates and that phenological mismatches can increase as a result.