For small landbirds, long-distance migration between wintering and breeding grounds involves the use of multiple stopover sites where birds must refuel to meet high energetic demands. Yet in many regions habitat fragmentation has isolated remnant forest patches for migrating birds. Urbanization may pose a considerable challenge to a migrant bird’s ability to select quality habitats and regain energy reserves due to high levels of fragmentation and altered ecological processes.
I investigated habitat use and behavioral decisions of migrating Swainson’s Thrushes (Catharus ustulatus) within an urbanizing landscape. Specifically, I examined whether differences in stopover location, energetic condition, and weather patterns influenced stopover duration. I examined both intrinsic and extrinsic influences on short-term movement rates of thrushes and total distance moved during stopover. Finally, I developed models of optimal avian migration to gain insight into how migration events influence arrival in breeding areas.
During May of 2004-2007, I caught 103 Swainson’s Thrushes in a small woodlot within the metropolitan area of Columbus, Ohio and fitted each bird with a 0.66 g radio transmitter (less than 2.5% of Swainson’s Thrush body weight). To simulate arrival at a stopover location, I experimentally relocated thrushes to one of seven urban forest patches. Birds were monitored daily to quantify stopover duration (days) and fine-scale movement patterns used during foraging and site exploration. Birds showed high site tenacity with 93% remaining at the release site until migratory flight was initiated; all five birds that left release sites were located at the two smallest sites (< 4.5 ha). Mean stopover duration was 3.7 days (±3.4 SD; range 1-12 days). Stopover duration was negatively related to the advancement of the migratory season and energetic condition at capture. Although the influence of stopover site was uninformative in explaining duration, I detected a positive relationship between distance moved by an individual and area of the stopover site. On average, birds moved 313.5 m (± 182.75 SD) during the first three days of stopover, but total distance moved was more limited in smaller patches. Modeling results highlight how migrating birds may increase their chances of reaching the breeding grounds by maintaining energetic reserves early in the migratory period while still conforming to a migratory schedule. The importance of energetic reserves and migratory timing are patterns consistent with the field data. These results indicate that Swainson’s Thrush, and possibly forest interior birds, may show a degree of area sensitivity in isolated urban forest remnants, but these stopover areas in many cases can provide suitable stopover habitat and opportunities for forest interior landbirds to forage and rebuild energy stores.