The study’s authors say it’s the first of its kind to cover the Western Hemisphere during the year-long life cycle of North American migratory birds that feed on vegetation, seeds, nectar, insects, or meat. The findings were published in the Journal of Animal Ecology.
Migration synchronization with vegetation greenness is strongest for birds that eat vegetation, seeds, or both, during spring and autumn migration, but especially during spring. Birds follow the green vegitation north in the spring and then follow it in reverse during the fall.
Carnivores, such as hawks and eagles, in the West do not follow this trend during either migration period. The evidence is also weak for synchronization among insect-eating birds during spring migration in eastern and central portions of the United States. Birds that rely on nectar — hummingbirds primarily — also showed looser ties to vegetation greenness in the West. The reason for the lack of synchronization for insect-eating birds in the East is a massive geographical barrier to migration: the Gulf of Mexico. Birds wintering in Central and South America cannot detect vegetation changes on the U.S. side of the Gulf in spring and vice versa in the fall.
Scientists used data from satellites to estimate the greenness of vegetation year round and cross-referenced that data with eBird observations for 230 North American migratory bird species from 2006 through 2018. eBird is the Lab’s worldwide bird observation reporting platform.
Vegetation green-up in the spring is controlled by changes in temperature and precipitation; die-back of vegetation in the autumn is controlled by temperature and hours of daylight — all factors important in timing of migrations.
Funding for this study comes from the Wolf Creek Charitable Foundation and the National Science Foundation.
Photo of Indigo Bunting by Bob Howdeshell.