URBAN FOREST PATCHES AND STOPOVER DURATION OF MIGRATORY SWAINSON'S THRUSHES

Long-distance migration between nonbreeding and breeding grounds involves use of multiple stopover sites where birds must refuel to meet the energetic demands of migration. Migrant forest birds that encounter urban landscapes experience high levels of habitat fragmentation and often use small, isolated forest patches. We investigated factors influencing stopover duration (length of stay) and migratory departure of Swainson's Thrushes (Catharus ustulatus) using forest patches within an urban landscape in Columbus, Ohio. During May, 2004-2007, we fitted 69 Swainson's Thrushes with 0.66-g radio transmitters, experimentally relocated the birds to seven mature forest sites that varied in area and degree of urbanization and monitored each individual daily until it departed. Mean minimum stopover duration was 3.7 days (+/- 3.4 SD), ranging from 1 to 12 days. Stopover duration was negatively related to advancement of migration (Julian date) and energetic condition at capture, and departure from stopover sites was associated with low wind speed and declining barometric pressure. We found no difference in stopover duration across the seven urban sites despite large variation in forest area and degree of urbanization. Swainson's Thrushes had strong site tenacity at the five largest (11.9-38.4 ha) sites, where 100% of birds remained until migratory departure; in contrast, 28% of individuals left the smallest (0.7 and 4.5 ha) sites during stopover. Results suggest that patches of remnant forest within cities may provide Swainson's Thrushes with suitable stopover opportunities without strongly influencing their migratory schedule, but minimum area requirements may limit the thrushes' use of some patches.