SPATIAL FORAGING DIFFERENCES IN AMERICAN REDSTARTS ALONG THE SHORELINE OF NORTHERN LAKE HURON DURING SPRING MIGRATION

Abstract

Lowland coniferous forests adjacent to northern Lake Huron provide important stopover habitat for landbirds during spring migration. Large numbers of aquatic insects emerging from nearshore waters of northern Lake Huron appear to be an important food source. In this study we compared the foraging behavior of a long-distance landbird migrant, the American Redstart (Setophaga ruticilla), in areas with high densities of emergent aquatic insects to areas with few or no emergent aquatic insects to assess the significance of these arthropods as an early spring food source. Redstarts foraged differently in shoreline habitats relative to inland habitats of similar vegetation composition. Both males and females gleaned significantly more in shoreline habitats as compared to inland areas of similar vegetation composition, and inland birds performed more sally strikes than birds at the shoreline. Both sexes also varied the use of tree species in which they foraged. Redstarts used northern white cedar (Thuja occidentalis) more at shoreline than inland, while inland redstarts foraged in deciduous trees more than at the shoreline. We suggest that differences in foraging between shoreline and inland locations were responses to differences in prey types and abundance, most notably the presence of emergent aquatic insects (Diptera: Chironomidae) in shoreline habitat. Our results complement those of previous work, suggesting that midges provide a critical early season resource for landbirds migrating through Michigan's eastern Upper Peninsula during spring. Received 30 September 2003, accepted 26 March 2004. During spring and early summer in temper- ate North America, large numbers of aquatic insects that have metamorphosed into sexually mature adults often amass in terrestrial habi- tats adjacent to riparian and lacustrine systems (Armitage 1995, McCafferty 1998). These in- vertebrates are relatively weak fliers (e.g., Ko- vats et al. 1996) and tend to be restricted to nearshore terrestrial habitats. In Michigan's eastern Upper Peninsula, midges (Diptera: Chironomidae) are the predominant aquatic arthropods when migratory landbirds stop during spring migration. These invertebrates swarm profusely in shoreline areas while be- ing virtually nonexistent inland (DNE unpubl. data). Recent evidence suggests that lowland co- niferous forests adjacent to northern Lake Hu- ron provide important stopover habitat for spring migrants. More landbirds are found in 1