WITHIN-SEASON NESTING DISPERSAL AND MOLT DISPERSAL ARE LINKED TO HABITAT SHIFTS IN A NEOTROPICAL MIGRATORY SONGBIRD

Dispersal decisions of songbirds are relatively well understood between breeding seasons, but far less is known about dispersal between nesting attempts and during the molting period. There are several non-mutually exclusive hypotheses for why birds disperse during breeding and molt including: (1) avoiding predation, (2) because of depleted resources, or (3) seeking higher quality habitat. In this study, we evaluated the above hypotheses by radio-tracking a Neotropical migrant, the Wood Thrush (Hylocichla mustelina), during breeding and molt. During breeding, 30% (7/23) of females moved territories within-season. Females were not more likely to move off-territory following nest predation, and dispersing females were not in poorer nutritional state than non-dispersing females prior to the move. However, females occupying nest sites with large trees, high canopy height and percent canopy cover, and low percent ground cover were more likely to move off-territory for their second nest to sites with few large trees, a low canopy height and cover, and high percent ground cover. During the molting period, 61% (28/46) of birds moved off-territory to molt. Molting areas were associated with more shrubs and fruiting trees compared with breeding territories. There were no sex or age effects as to whether a bird molted on or off their breeding territory. We suggest that off-territory movements during the nesting period are to seek more suitable nesting vegetation, rather than specifically avoiding predation or because of reduced resource availability. This study has additionally shown that, without radio-telemetry, detecting off-territory movements during breeding is difficult and neglecting to include these movements may result in an underestimation of annual productivity.