Causes and Consequences of Long-distance Dispersal in a Migratory Bird, the American redstart (Setophaga ruticilla)

Abstract

Long-distance dispersal, the movement of individuals beyond the boundaries of their population for the purpose of breeding, is a central process in ecology and evolution. Unfortunately, the causes and consequences of long-distance dispersal are poorly understood, especially in migratory species due to the difficulty of tracking individuals throughout their annual cycle. Furthermore, although events experienced during one period of the annual cycle can influence the costs of dispersal in subsequent periods, a review of existing literature on dispersal in migratory species indicated that these seasonal interactions have not been widely incorporated into dispersal research. To advance this subject, I used observational and experimental approaches to quantify the causes and consequences of long-distance dispersal in a migratory bird, the American redstart (Setophaga ruticilla). Stable hydrogen isotopes from feathers (d2H) indicated that yearlings and adults were more likely to disperse north in years with early breeding-season phenology and that yearlings were also more likely to disperse north following winters with poor habitat conditions in the Caribbean. These results are consistent with the hypotheses that individuals use conditions experienced during migration as a cue for selecting breeding areas. Experimental simulation of social cues further demonstrated that redstarts use both the presence of conspecifics and habitat features to select breeding sites and d2H values from individuals that responded to playback treatments indicated that long-distance dispersers rely more heavily on social cues than local individuals. Reproductive success was not influenced by long-distance dispersal in either sex but male reproductive success was positively correlated with non-breeding territory quality. For adult males, non-breeding territory quality directly influenced the number of young produced. For yearling males, in contrast, high-quality non-breeding territories were associated with higher mating and nesting success but once these differences were accounted for, non-breeding territory quality had no further influence on reproductive success. Collectively, these results suggest that long-distance dispersal may be an effective strategy for responding to advances in breeding-season phenology driven by temperate climate change but that migratory birds may ultimately be limited by the drying in tropical non-breeding areas.