Declining Rainfall in Tropics Affects Migratory Birds

American Redstart by Darlene Friedman
Scientists have discovered that American redstarts vary departure dates from a wintering area in Jamaica depending on how much rain falls from January to March. Photo by Darlene Friedman.

Scientists have long believed that changes in day length are the trigger that signals migratory birds wintering in the Tropics to begin flying north for the breeding season. But now that long-held assumption is being challenged by new research focusing on the American redstart, a colorful warbler that breeds across much of North America.

For the past five years, researchers from the Smithsonian Conservation Biology Institute’s Migratory Bird Center have studied a group of redstarts at a wintering site in Jamaica. To tell one bird from another, each individual was fitted with a unique combination of colored leg bands. Because American redstarts occupy exclusive winter territories to which they return year after year, they “were a perfect species for this study,” says Pete Marra, a research ecologist at the center and coauthor of the study. “These behaviors made it relatively easy to keep track of individual birds over multiple years and document changing spring departures.”

Less Rain Means Less Food

And spring departures did indeed change from year to year. In a paper published this week in the journal Proceedings of the Royal Society B, the researchers report that the amount of rainfall during the island’s January to March dry season strongly influenced when a redstart took off on its flight north. The less rain, the later a bird departed. The explanation, say the scientists, is that less rain means fewer insects—which the birds need to fuel their long migrations.

“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,” says Colin Studds, a postdoctoral fellow at the Smithsonian center and lead author of the study.

The Climate Change Connection

Because global warming is causing declining and more variable rainfall in many parts of the Tropics, the results are worrisome—not just for American redstarts, but for scores of other bird species that breed in North America and winter in the Caribbean, Mexico, Central America or South America. Delayed departure from wintering grounds may mean these birds have less time to successfully reproduce. Late arrival in North America also “may make it harder for them to remain in sync with their breeding cycle,” says Studds.

Previous research suggests that migratory birds may be suffering other ill effects from global warming. In the current issue of National Wildlife magazine, Stanley Temple, professor emeritus of wildlife ecology at the University of Wisconsin-Madison reports on a comparison he made between springtime bird arrivals during two time periods: 1935 to 1948 and 1976 to the present.

As global temperatures have risen steadily in central Wisconsin since the 1930s, he’s discovered that short-distance migrants such as robins, sparrows and blackbirds have advanced their arrival times by a week or more. But long-distance migrants such as fly-catchers, thrushes and warblers have not been arriving earlier. This may put long-distance migrants at a disadvantage “on breeding grounds where competition for scarce resources such as nesting sites can be fierce,” Temple says.

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Find out which birds are on the move throughout the spring migration season.

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Make your yard hospitable to migratory birds by turning it into an NWF Certified Wildlife Habitat.