We investigated the relationship between migratory restlessness and stored energy reserves in two species of landbird migrants at Braddock Bay, Lake Ontario during spring 1999 and spring 2000. There was no significant difference in the amount of nighttime locomotor activity between lean and fat Swainson's Thrushes (Catharus ustulatus) or White-throated Sparrows (Zonotrichia albicollis) held overnight in activity cages. We found no significant relationships between nocturnal activity level and energetic condition (mass/wing chord) or date of capture. In addition, there were no significant differences in nocturnal activity levels between age groups in Swainson's Thrushes. These results suggest that the migratory restlessness of spring migrants stopping at this site is not strongly influenced by their energetic condition upon arrival, regardless of age or date of passage. Birds nearing the end of their spring migratory route that are very close to their breeding grounds may show decreased levels of migratory restlessness and short stopover durations, independent of energy reserves. When stopping at northerly sites, spring migrants may therefore display distinct differences in stopover behavior and requirements compared to sites used earlier along the spring migratory route.
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Vol. 76 • No. 3