We investigated selection of stopover habitat by juvenile Swainson's Thrushes (Catharus ustulatus) during fall migration at a site along the northern California coast. The study site vegetation consisted mainly of coniferous forest (pine [Pinus] and spruce [Picea]), with interspersed patches of broadleaf forest (willow [Salix] and alder [Alnus]) in poorly drained swales. For 26 birds captured and radio-tracked in 2002 and 2003, the average minimum stopover duration was 8.9 ± 1.0 days. For 20 of these birds with a sufficient number of locations, the average home range size was 1.9 ± 0.3 ha. Thrushes showed no overall pattern of selection for forest type within the study area or for forest type used inside their home range. Fat and lean birds selected forest types similarly within the study area and their home ranges. However, locations occupied by lean birds had twice as much huckleberry (Vaccinium ovatum) shrub cover and were 1.3 times more concealed by vegetation than locations occupied by fat birds. There were 2.5 times more huckleberries at occupied than random locations, and locations occupied by lean birds had 2.1 times more berries overall than those frequented by fat birds. Fecal analyses confirmed that huckleberries were a commonly consumed food (70% of sampled thrushes), but also revealed that thrushes ate arthropods (87%) and wax myrtle (Myrica californica) bracteoles (43%). The overall lack of forest type selection coupled with differences between fat and lean birds in selection for cover and fruit abundance suggests that fat level may influence microsite selection.
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Vol. 109 • No. 4