THE INFLUENCE OF BODY CONDITION ON LOCAL APPARENT SURVIVAL OF SPRING MIGRANT SANDERLINGS IN COASTAL NORTH CAROLINA
Many shorebirds are long-distance migrants, and they depend upon widely spaced stopover sites for refueling during their annual migrations. Two competing hypotheses attempt to explain stopover duration: one predicts departure based on time minimization (timeselection hypothesis) and the other predicts departure on the basis of maximum energy gain (energy-selection hypothesis). During spring 1993, we tested these hypotheses during a markresight study of migrant Sanderlings (Calidris alba) at
... lidris alba) at Portsmouth Island, North Carolina. We individually color-banded 204 Sanderlings and used capture-recapture methods to estimate local apparent survival during 10 five-day intervals from late April to mid-June. We found that survival rates were best modeled as a decreasing quadratic time trend; a linear time trend and period-specific survival rates received little support. We found no evidence that either body mass or a simple body condition index better explained survival rates. Our estimates of the 5-day local survival rates of Sanderlings at Portsmouth Island remained relatively high ((O; > 0.80) through late May, after which they dropped rapidly as birds left the area. The periodspecific conditional resighting probability was 0.39 (SE = 0.03). Our results do not provide strong support for either the time-or energy-selection hypotheses and hint that studies of migratory behavior need to be conducted even more intensively and across much larger regions in order to better understand the underlying factors.