MASS-LOSS AND PROBABILITY OF STOPOVER BY MIGRANT WARBLERS DURING SPRING AND FALL MIGRATION

Although lean migrants are more likely to be recaptured than fatter on es at many stopover sites, the effects of mass loss on the probability of recapture have not been investigated experimentally. During fall m ist-netting on Appledore Island, Maine, when two Northern Waterthrushe s (Seiurus noveboracensis) or American Redstarts (Setophaga ruticilla) were captured simultaneously they were divided randomly into control and experimental categories. In the spring, pairs of American Redstart s or Magnolia Warblers (Dendroica magnolia) were treated similarly. Ex perimental individuals were held without food for up to 6 h before rel ease to investigate the effects of mass loss on the probability of rec apture, while control birds were banded and immediately released. Expe rimental birds significantly decreased in mass during holding. During the fail, experimental individuals were more likely to be recaptured a t least 1 d after initial capture than control individuals. However, d uring the spring, I did nor find a difference in recaptures between co ntrol and experimental categories, although recapture rates were quite low. These results indicate that mass loss, either directly or as a r esult of lost foraging time, does lead to extended stopover by migrant s during the fall. However, differences in priorities apparently exist between spring and fall migration. During the spring, mass loss may n ot lead to extended stopovers if migrants have enough energy to contin ue migration, especially when migrants are close to their final destin ation.