Nesting density of Hermit Thrushes in a remnant invasive earthworm-free portion of a Wisconsin hardwood forest

I observed an exceptionally high density of Hermit Thrush (Catharus guttatus) nests (3.1 nests/ha) over two breeding seasons in an isolated 1.3-ha portion of an earthworm-free study site in the Chequamegon-Nicolet National Forest, Wisconsin. This density was much greater than the 0.1 to 0.6 nests/ha observed over the rest of the study area and exceeds by an order of magnitude most previously reported estimates for this species. The mean distance among Hermit Thrash nests in earthworm-free sites (215 m; 95% CI = 180-250 m) was lower than in invaded sites (250 m; 95% CI = 236-264 m); this difference was not statistically significant. Nest density did not differ significantly between categories. An abundance of suitable nest sites in a favored nesting substrate (clubmoss; Lycopodium spp.) could have contributed to the exceptionally high density of Hermit Thrush nests observed. High Hermit Thrush nest densities may occur in association with forest floor conditions that are characteristic of earthworm-free areas. Received 3 August 2011. Accepted 16 January 2012.
Invasive European earthworms (Lumbricus spp.) are spreading through north temperate hardwood forests of North America that were previously glaciated and historically earthworm-free (James 2004). Lumbricus earthworms alter soil structure and [...]