Stopover of Neotropical and Temperate Landbird Migrants on the South Shore of Lake Ontario during Spring Migration

Abstract

Selected Neotropical and long-distance migratory landbirds were studied during spring migration at a known stopover site on the southern shore of Lake Ontario. A total of 1277 individuals were captured by mist net from 1 May through 1 June 1995, and 22 April through 1 June 1996, for banding and measurement of morphological characteristics. Poor energetic condition was typical of the long-distance migratory landbirds captured at this site during this study (80.6% fell in the lowest 3 out of 6 fat classifications). Few birds were recaptured after the day of their initial captures. No significant differences in initial condition index or fat class were found in recaptured birds, compared to those captured only once, in three species with adequate sample sizes. Individuals were found to lose, maintain, or gain weight during their stay, and showed no significant correlation between length of stay and change in mass. A total of 775 individuals representing 14 species were examined for outward physiological indication of male breeding readiness and grouped latitudinally into two categories based on relative distance from the study site to the southernmost limit of the species' geographic nesting range. Two individuals from the near-nesting group exhibited a cloacal protuberance. No individuals of the far-nesting group exhibited protuberance. No significant difference in the proportion of males exhibiting a cloacal protuberance was found between the two groups. Cloacal lavage samples were taken from 22 Whitethroated Sparrows (Zonotrichia albicollis) and 26 Swainson's Thrushes (Catharus minimus). None of the migrating individuals lavaged exhibited presence of cloacal sperm regardless of species, sex, or date of capture. Flying insects (Diptera) were abundant throughout the two study periods while Lepidopteran larvae were not present until the final week of May in both 1995 and 1996. Numbers of Diptera were significantly higher at nearshore sampling sites during the final weeks of each study period. Results of my two year study suggest that: 1. Most Neotropical migrants reaching Braddock Bay continue migrating after stopping only one day. 2. Neotropical migrants reaching Braddock Bay are in relatively poor condition, as indicated by predominance of low fat class scores. 3. But most migrants seem to be able to improve condition adequately for another night's flight during a single day. 4. There is no obvious relationship between body condition, as indicated by fat class score or condition index at initial capture, and length of stay of migrants reaching the stopover site at Braddock Bay. 5. Neotropical migrants may concentrate near Lake Ontario not only due to the geographic barrier imposed but also to take advantage of abundant food resources and safe cover. 6. Depleted energetic stores may explain the lack of early sperm production or release in migrants at this stopover site.