Impacts of cowbird parasitism on wood thrushes and other neotropical migrants in suburban Maryland forests

During 1988-1993, we monitored nests of neotropical migrant birds in seven suburban Maryland forests to compare parasitism and predation rates in forests of different areas. Of 1,122 nests monitored, 672 were of Wood Thrush, the most commonly found nesting species. Study sites were forests that ranged in size from 21 ha to more than 1,300 ha in the Piedmont and Coastal Plain regions of Maryland within 50 km of Washington, D.C. Parasitism rates of Wood Thrush nests varied greatly among sites, ranging from 0% (29 nests in 1990-1992) in a site in extensive forest to 68% (31 nests 1992-1993) in a 21-ha, selectively logged old-growth forest. A sudden increase in parasitism from 9% (102 nests 1990-1991) to 35% (125 nests 1992-1993) in a 23-ha old-growth forest was noteworthy. The surrounding environment at this site is changing from rural to residential. Wood Thrush parasitism rates dropped as the breeding season progressed, but peaks of parasitism coincided with peaks of nesting activity. Parasitism rates for Hooded Warblers (88% of 17 nests-all sites) were most alarming. High predation rates were a much greater factor in low productivity for Wood Thrushes than parasitism.