Linking fluctuations in rainfall to nonbreeding season performance in a long-distance migratory bird, Setophaga ruticilla

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 whether rising temperatures on temperate breeding grounds are advancing migration phenology and limiting reproductive success. However, conditions on tropical nonbreeding quarters can also shape these processes, yet few studies have directly measured bird responses to climate during this part of the annual cycle. We tested the hypothesis that variation in winter rainfall can influence food availability and the nonbreeding season performance of American redstarts Setophaga ruticilla occupying 2 contrasting habitats: wet, mangrove forest and dry, second-growth scrub. From 2002 to 2005, food availability, body mass, and spring departure schedules of birds in both habitats were highly dependent on rainfall. Food availability in mangrove forest was higher than in second-growth scrub in 3 out of 4 yr, allowing birds in this habitat to maintain better physical condition through the winter and depart earlier on spring migration. However, abundant rainfall in the spring of 2004 led to abnormally high food availability in scrub and early departure of birds in both habitats. These results suggest that rainfall on tropical wintering areas can have major effects on the nonbreeding season performance of migratory birds, and that the timing of rainfall within the dry season, not just the absolute amount, may be critical for orchestrating migratory departure schedules. Because rainfall in tropical regions is projected to decline drastically over the next 50 yr, migratory birds could face increasingly severe food shortages prior to spring migration. Such conditions could force departure schedules to become delayed and constrain adaptation to selection for earlier breeding in response to rising temperatures at breeding areas.