Nocturnal Activity Levels and Stopover Behavior of Spring Neotropical Migrants Along the Southern Lake Ontario Shoreline

Abstract

The energetic condition of individual birds is thought to influence the selection and use of stopover sites during migration. Through analysis of nocturnal restlessness and stopover patterns, I investigated the relationship between physiological state (distinguished by the amount of stored fat reserves) and spring stopover behaviors of selected species of Neotropical and temperate migrants at a site along the southern shoreline of Lake Ontario. During spring 1999, there was no significant difference between the amount of nighttime locomotor activity in lean and fat Catharus thrushes or Dendroica warblers held overnight in activity cages. During spring 2000, there was no significant difference in nocturnal activity between captive lean and fat Swainson's Thrushes (Catharus ustulatus) or White-throated Sparrows (Zonotrichia albicollis). Very few Swainson's Thrushes or White-throated Sparrows were recaptured in either 1999 or 2000, and analysis of initial capture data for these species revealed that most individuals arrived lean with depleted fat stores. There was no significant relationship between the hour after sunrise (time of day of initial capture) and mass or condition index for either species in 1999, and only White-throated Sparrows showed significant positive relationships between hour after sunrise and mass or condition index in 2000. Habitat selectivity was determined by examining the proportion of individuals captured in the three major habitat types (field edge, early secondary, mature secondary) at this site. During spring 1999 and spring 2000, there was no significant relationship between mean energetic condition and habitat type for both species. For all initial captures combined, there was no indication of habitat selectivity for Swainson's Thrushes in 1999 or 2000. However, there were significantly more White-throated Sparrows captured in the mature secondary habitat than in early secondary or field edge habitats in 1999, and significantly more in the early secondary habitat than in mature secondary or field edge habitats in 2000. Flying insects were sampled in each of the three habitat types and were present throughout the spring migratory season in both 1999 and 2000. There were no significant differences in insect abundance among habitat types in 1999. Insect abundance differed significantly among habitat types in the first three weeks of sampling during spring 2000. Based upon the results of my study, I conclude that the behavior of birds stopping at this site is not strongly influenced by their energetic condition upon arrival. Many birds at this site are near the end of their spring migration, and factors related to time constraints and pressure for early arrival at their breeding grounds may influence their behavior during stopover, resulting in a pattern of short stopover lengths, little weight gain during stopover, and no obvious difference in migratory activity between individuals in different physiological or energetic states. Differences between the results of this study and previous studies conducted along the Gulf Coast suggest that stopover behaviors and requirements are different between sites located early along the spring migratory route and sites located near the Great Lakes shorelines. However more work is needed on a broader range of longer-distance and insectivorous species in order to assess the quality and availability of stopover habitats along Great Lakes shorelines, as well as the stopover requirements and behaviors of birds near the end of spring migration.