MOVEMENT PATTERNS AND STOPOVER ECOLOGY OF WILSON'S WARBLERS DURING SPRING MIGRATION ON THE LOWER COLORADO RIVER IN SOUTHWESTERN ARIZONA

We used radio-telemetry to examine fine-scale movement patterns of Wilson's Warblers (Wilsonia pusilla) at a stopover site on the lower Colorado River during spring migration in 2005 and 2006. The overall movements of Wilson's Warblers were restricted to small, localized areas, with an average linear displacement of only 332 m. Warblers exhibited exploratory behavior characterized by fast, long-distance, directed linear movements during the first and second day after presumed arrival at the stopover site. However, exploration was limited within the overall landscape (<2 km diameter), suggesting a cost to extended exploration. As individuals gained more information about the distribution of resources at the stopover site through exploratory behavior and direct sampling of the habitat, movements became more aggregated within a localized area, suggesting that Wilson's Warblers settled within a microsite at the stopover site to replenish fuel Supplies. The overall movement patterns exhibited by warblers during stopover were influenced by the age of the individual and changes in resources, both within a season and between years. Movement patterns of Wilson's Warblers documented during this study provide essential information about the temporal distribution of a warbler's locations within a stopover site, indicating how birds search for and acquire food resources throughout their stopover.