Birds Delay Spring Migration As Tropical Rainfall Dips, Study Says

Declining rainfall in tropical regions can cause migratory birds to delay their departure from wintering grounds back to their northern breeding areas, according to a new study. In a five-year study of American redstarts, a species of warbler, scientists at the Smithsonian Conservation Biology Institute found that individual birds delayed their spring migration from Jamaica to North America when low rainfall produced a scarcity of insects, the birds’ primary food supply; the redstarts apparently delayed migration because of insufficient nutritional reserves. Over the last 16 years, increasingly severe and unpredictable dry seasons in Jamaica have resulted in an 11-percent decrease in rainfall. “Our results support the idea that environmental conditions on tropical non-breeding areas can influence the departure time for spring migration,” said Colin Studds, a postdoctoral fellow at the Smithsonian Conservation Biology Institute and lead author of the study published in the journal Proceedings of the Royal Society B. While it is unclear whether the delayed migration will have an adverse impact on the birds, the study said a delayed departure could ultimately affect the arrival time to breeding territory, and thus yield less time to reproduce.