Migratory bird species that breed in the U.S. and Canada often winter in the Caribbean, Central America, and South America. A Smithsonian study of American Redstarts on their wintering grounds in Jamaica found that rainfall changes disrupted the birds' migration schedule in the spring.
Precipitation in Jamaica is highly seasonal, with consistent rainfall from September to November and a pronounced dry season from January to March. The scientists observed the redstarts in their non-breeding territories for five years during the dry season. They paid special attention to the annual variation in dry season rainfall. The correlation between the amount of insects in a bird's territory and the timing of its departure suggested to the team that annual variation in food availability was an important determining factor in the timing of spring migration. Had the redstarts relied on internal cues alone to schedule their spring departure, they would have all left their winter territories at the same time each year.The result has obvious implications for climate change, since warmer average temperatures are likely to reduce rainfall in many areas. A major question is how birds will cope with the disrupted migration schedule. In many cases, birds time their migrations to arrive on their breeding grounds just as certain food sources are becoming available, whether those are insects or seeds or some other source.
"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's Migratory Bird Center and lead author of the study. "We found that the same birds changed their spring departure from one year to the next in relation to the amount of rainfall and food in March."
During the past 16 years, the dry season in Jamaica has become both increasingly severe and unpredictable, leading to an 11 percent drop in total rainfall during the three-month annual drought. Making the future even more dire, climate models predict not only increased warming on temperate breeding areas but also continued drying in the Caribbean.