Discerning the influence of earthworms on ground-nesting warblers

Background/Question/Methods

Both native and invasive species of earthworms can be found in many areas of North America, including the Eastern Shore of Maryland. Invasive earthworms have been shown to decrease leaf litter depth, reduce micro-invertebrate numbers, alter understory community composition, and decrease the nesting success of ground-nesting songbirds. We sought to determine whether the impacts of invasive earthworms on ground-nesting songbirds are due to negative effects on 1) food resources (i.e., invertebrates), 2) understory habitat, or both. Our goal was to compare the nesting success and body condition of four ground-nesting species (ovenbirds, black-and-white warblers, worm-eating warblers, and Louisiana waterthrushes) with two different feeding ecologies (i.e., ground feeding or foliage gleaning) in areas with and without earthworms. We also examined the effects of earthworms on leaf litter depth, understory vegetation, and invertebrate abundances at locations where birds were found within three field sites. With this study we attempt to determine the potential pathway through which invasive earthworms may negatively impact ground-nesting songbirds. 

Results/Conclusions

Although herbaceous cover differed among sites, this was not influenced by earthworm abundance. We found a greater abundance of native earthworms than anticipated, mainly Eisenoides lonnbergi. There was a trend for a site by earthworm interaction on the volume of invertebrates in the leaf litter, with a negative correlation between earthworm abundance and invertebrate volume at the site with no native earthworms and a positive correlation between earthworm abundance and invertebrate volume at two sites with both native and invasive earthworms. In total, we banded 73 ground-nesting birds, mostly ground-foraging ovenbirds (n = 30) and foliage gleaning worm-eating warblers (n = 33). During this first season, we found a limited number of nests, most of which were completely predated with no successful fledging.  After correcting for body size, we found that worm-eating warblers (which do not feed on earthworms) weighed more when earthworms (often native) were present; the presence or absence of earthworms did not influence ovenbird mass. We plan to continue this research to further examine likely differential impacts of native and invasive earthworm species on these warblers. Exploring the effect of earthworms on ground-nesting warblers is particularly germane as forest songbird populations are experiencing global declines.