"Because American redstarts return to the same site to breed each year, arriving later may make it harder for them remain to remain in synch with their breeding cycle," Studds said.

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.

Volunteers from New York Audubon identified American Redstarts and Yellow Warblers. Wood Thrushes, Bicknell's Thrushes, Baltimore Orioles and various species of Tanager may also have been trapped.