SHORT COMMUNICATIONS 
375 
The Wilson Journal of Ornithology 124(2):375—379, 2012 
Nesting Density of Hermit Thrushes in a Remnant Invasive Earthworm-free 
Portion of a Wisconsin Hardwood Forest 
Scott R. Loss 1 - 2 
ABSTRACT—I observed an exceptionally high 
density of Hermit Thrush (CiUharits 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 ncsts/ha 
observed over the rest of the study area and exceeds hv 
an order of magnitude most previously reported 
estimates for this species. The mean distance among 
Hermit Thrush nests in earthworm-free sites (215 m: 
95% Cl = 180-250 m) was lower than in invaded sites 
(250 m; 95% Cl = 236-264 ml: this difference was 
not statistically significant. Nest density did not differ 
significantly between categories. An abundance ol 
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 Hour conditions that are 
characteristic of earthworm-free areas. Received 3 
August 2011. Accepted 1 6 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 the seed bed. and consume the 
leaf litter layer (Hale et al. 2005). These changes 
result in reduced cover and diversity of herba¬ 
ceous plants and tree seedlings, and a preponder¬ 
ance of disturbance-adapted sedges and grasses 
(Hale et al. 2006. Holdsworth et al. 2007a). 
Ground-nesting passerines are also affected by 
earthworm-caused habitat changes. Density ot 
singing male Hermit Thrushes (Cathurus gallants) 
and Ovenbirds (Seiums aurocapilla) is reduced by 
Lumbricus invasions at the scale of 10 to 20-ha 
forest stands, and Ovenbird nest success is 
reduced in relation to decreased litter depth and 
increased sedge cover caused by Lumbricus in 
'Conservation Biology Graduate Program, University ot 
Minnesota, 1980 Folwcll Avenue, St. Paul, MN 55108, USA. 
'Current address. Smithsonian Migratory Bird Center. 
National Zoological Park. P. O. Box 37012 MRC 5503, 
Washington, D.C. 20013, USA: e-mail: LossS@si.edu 
hardwood forests ot Wisconsin (Loss and Blair 
2011). Relationships between invasive earth¬ 
worms and nest success and nest density of other 
ground-nesting species, including Hermit Thrush¬ 
es, are unknown. 
Density of Hermit Thrush breeding territories is 
highly variable across the species' range (Jones 
and Donovan 1996; Table I). Density estimates 
front most Hermit Thrush studies are based on 
indirect observation of breeding activity (e.g., spot 
mapping surveys), and few studies report density 
based on direct nest observations. Habitat factors 
influencing variation in Hermit Thrush density are 
poorly understood, but nest locations are likely 
selected based on concealment from predators 
(Flaspohler et al. 2000), abundance of suitable 
nest sites in potential nest patches (Martin and 
Roper 1988), and habitat in the surrounding 
landscape (Flaspohler et al. 2001). The above 
factors are thought to interact with predator 
communities to influence nest predation rates 
(Martin 1993). 
Lumbricus invasions substantially alter forest 
floor habitats, and it is possible they influence 
Hermit Thrush nesting locations and territory 
densities. My objectives were to: (1) report 
observations of exceptionally high nesting density 
of Hermit Thrushes in an isolated earthworm-free 
forest stand in the Chequamegon-Nicolet National 
Forest in northern Wisconsin, and (2) compare 
Hermit Thrush nest density and nest success 
between Lum^r/cus-invaded and Lumbricus-ixte 
forest stands. 
METHODS 
I conducted Held research at six study sites in 
2009 and 2010 in the Chequamegon-Nicolet 
National Forest in northern Wisconsin (46 N, 
91 W) that were a subset of 10 sites used for an 
analysis of Ovenbird nesting success relative to 
earthworm invasions (Loss and Blair 2011). The 
sites were selected based on status of earth¬ 
worm invasion ascertained during previous work 
(Holdsworth et al. 2007a); three sites were 
