PASSERINES USE NOCTURNAL FLIGHTS FOR LANDSCAPE-SCALE MOVEMENTS DURING MIGRATION STOPOVER

Knowledge of stopovers made by migratory birds comes mostly from studies at relatively fine spatial scales. While this focus yields important information about processes at those scales, it ignores possible processes acting at broader spatial scales. We established an array of three automated radio-telemetry receiver towers housing ten antennas arranged to sample a landscape and its associated airspace at Lake Erie in southern Canada. We used digitally coded tags to monitor the behavior of multiple Swainson's and Hermit Thrushes (Catharus ustulatus and C. guttatus) simultaneously during fall 2008. The towers registered flight activity of 86% of the 69 radio-tagged individuals, whose flights occurred predominantly shortly after the end of evening civil twilight. We recorded 15 nocturnal flights that were not departure flights, indicating that during stopover passerines make nocturnal flights for purposes other than the continuation of migration. The flights we recorded were distributed throughout the night, and in eight cases they resulted in individuals moving many kilometers. These multiple instances of nocturnal landscape-scale movements represent an ecological process that is not detectable when the focus of sampling is too small. We suggest that in most studies of passerines' stopover ecology, researchers need to consider temporal and spatial scales more carefully.