Late arrival on the breeding grounds has been associated with reduced fitness in migratory birds. Because migratory stopover accounts for the majority of time spent on migration, how individuals respond to both exogenous and endogenous factors associated with stopover will influence the pace of migration and arrival timing on the breeding grounds. Among the most important endogenous factors, body condition (i.e. fat stores) is expected to influence both movement behavior and departure decisions of individual migrants during stopover. In a novel experiment, we manipulated the condition of after-second-year (adult) male American Redstarts (Setophaga ruticilla) captured during spring migratory stopover and radio-tracked all individuals after release using a passive automated telemetry array and conventional hand tracking. After accounting for the positive effect of tailwind and capture date on departure probability, we demonstrated that body condition influenced both the movement behavior of individuals and stopover duration. Individual movement decreased throughout the stopover period at a rate of 0.7 km day−1, supporting the notion that birds progressively limit their movements as stopover progresses. However, treatment birds whose condition was experimentally reduced (i.e. food-restricted) exhibited ∼3.4 times greater daily movement rates, and control birds were 4 times more likely to depart on a given day than treatment birds. These results underscore the importance of energetic condition in mediating the movement and stopover dynamics of migratory birds. Further, despite their capacity to make behavioral adjustments, our results suggest migratory songbirds that experience decreases in body condition during migration (which could result from inclement weather or low-quality stopover habitat) are likely to be delayed in their arrival on the breeding grounds, which could lower reproductive performance and ultimately have population-level ramifications.
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Vol. 135 • No. 1