Some birds have been able to adapt to global warming trends by nesting earlier or moving their nesting areas toward the poles. But for the American redstart, this may not be an option. Although redstarts breed in North America they must winter in tropical areas where their insect prey can be found in the colder months. To get to the breeding grounds earlier, or to nest further to the north, they would have to leave their wintering grounds earlier.
But studies in Jamaica, part of the redstarts' wintering area, show that global warming is also changing rainfall patterns. Winters are drier in Jamaica than in the recent past and long-term predictions show that they will be even drier in the future.
Rain in Jamaica means more bugs. And more bugs means that redstarts can fatten up before spring migration. Rainfall in March is especially important to redstarts because this is the time just before they migrate.
Lush, green habitat on left is mangrove;
drier, more open habitat on right is scrub.
During 3 out of 4 years of the study (2002 to 2005) redstarts that wintered in the mangrove habitat, which tends to remain fairly wet even in late winter, migrated earlier, and in better condition, than their brethren that wintered in adjacent dry scrub habitat. In 2004, when rainfall in March was plentiful, birds in both habitats migrated early and in good condition.
Female redstarts may be more affected by drying conditions because more of them winter in the dry scrub habitat. Males take the best territories in the mangroves.
This article summarizes the information in this publication:
Studds, Colin E. and Marra, Peter P. 2007. Linking fluctuations in rainfall to nonbreeding season performance in a long-distance migratory bird, Setophaga ruticilla. Climate Research, 35: 115-122.
Research on long-distance migratory birds has yielded some of the strongest evidence that shifts in climate are changing ecosystem processes. Much of this work has focused on understanding
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