CARRY-OVER EFFECTS OF WINTER CLIMATE ON SPRING ARRIVAL DATE AND REPRODUCTIVE SUCCESS IN AN ENDANGERED MIGRATORY BIRD, KIRTLAND'S WARBLER (SETOPHAGA KIRTLANDII)

Understanding how animals will adapt to climate change requires understanding how climate variables influence their biology year round, and how events in different seasons interact. Migratory birds may be especially vulnerable because of the wide range of geographic areas that they depend on throughout the annual cycle. We examined the potential effects of non-breeding season climate change on the breeding biology of Kirtland's Warbler (Setophaga kirtlandii, formerly Dendroica kirtlandii), an endangered songbird that breeds in the Northern Lower Peninsula of Michigan and winters in the Bahamas. Our objectives were to determine whether spring arrival dates on the breeding grounds correlate with late winter rainfall in non-breeding areas and whether this has consequences for reproductive success. We used data on spring arrival dates and number of fledglings per year, sampling many individuals across multiple years, to show that males arrive on breeding grounds later following drier winters. There was a strong male age * rainfall interaction, which indicates that first-time breeders were much more sensitive to changes in rainfall than experienced adults. Regardless of age, however, drier winters and delayed arrival and nest initiation were significantly associated with fewer offspring fledged. These results are important because the Caribbean region is currently experiencing a significant drying trend, and climate change models predict that the severity of this drought will continue to increase. Any resulting adjustments to the timing of migration could constrain spring arrival dates and limit reproductive success for the endangered Kirtland's Warbler, as well as other Neotropical migrants wintering in the Caribbean. Received 6 January 2012, accepted 30 March 2012.