Wood thrush population sinks and implications for the scale of regional conservation strategies

I measured the season-long nesting productivity of 60 pairs of color-banded Wood Thrushes (Hylocichla mustelina) in three of the largest forest tracts (1100-2200 ha) in southern Illinois (U.S.A.) in the Shawnee National Forest from 1990 to 1992. The purpose was to determine if these tracts are likely population sources - if they produce a surplus of young that would be available to colonize hypothesized population sinks in smaller, more fragmented tracts where productivity is unlikely to be sufficient to compensate for mortality. Levels of parasitism by Brown-headed Cowbirds (Molothrus ater) were high (75-95% of nests were parasitized; 2.0-2.7 cowbird eggs/parasitized nest), as were nest predation rates (typically 50-80% of the nests failed). In spite of the Wood Thrush's capacity to raise two or three broods a season and raise mixed broods of hosts and cowbirds, nesting productivity was low (0.3-2.1 fledglings/female/year). Using a range of estimates for adult and juvenile survival, I estimated that populations in all three sites were population sinks in most years. Cowbird parasitism and nest predation contributed roughly equally to the low productivity; some sites would have been sinks even without parasitism, whereas other sites would continue to be sinks even with substantially reduced nest predation rates when cowbird parasitism levels remain unchanged. Maintaining Wood Thrush populations in the U.S. Midwest might require much larger (>2500 ha) reserves than populations in the East, where even a small woodlot (<20 ha) has been shown to be a consistent population source. Impacts of habitat fragmentation may have greater regional variation that has previously been acknowledged.