Forest area and habitat quality for nesting Wood Thrushes

Abstract

For Neotropical migrants, rates of nest survival and brood parasitism are thought to decline with the area of the forest fragment in which the birds nest. Using the Wood Thrush (Hylocichla mustelina), we conducted the first test of the 'area' hypothesis based on direct measures of annual production and related demographics of marked individuals. We predicted that productivity per female and per ha, proportion of females nesting successfully each year, and return rates of adults would decrease and that rates of nest predation and brood parasitism and the ratio of yearlings:older adults would increase with a decrease in area of forest. Intensive season-long banding, nest finding, and nest monitoring to determine the annual production of all residents yielded data for two annual (1991 and 1992) tests in which we compared mean results from 14 small fragments (≤2.1 ha) with those from a 15-ha site, the University of Delaware Woods (UDW). Fledglings per female and per ha, and percent of females fledging ≥2 young per year, were lower, and percent of nests lost to predation and percent of nests parasitized were higher, in small fragments than in the larger tract at UDW in both years. The ratio of yearlings:older adults was significantly higher in small fragments for males and tended to be so for females in 1991, but neither result occurred in 1992. Return rate for each sex was lower in small fragments in the only year tested. The 17.5 ha of small fragments collectively produced many fewer young per female and per ha than the 15-ha UDW. Three analyses indicated that the small fragments were population sinks, whereas UDW probably was a source of recruits. Although a few small fragments equaled the UDW in some measures of quality, our data support the area hypothesis in relation to quality of breeding habitat.