Bird Watchers Digest - January/February 2012

years, everyone from European colonists to avid gardeners and anglers has helped European earthworms invade localities where the worms did not previously exist. These worms are known to reduce the amount of “forest litter” and even to alter soil structure and plant communities.

Scott Loss and Robert Blair of
the University of Minnesota in St.
Paul recently noted in Conservation
Biology that little is actually known
about how these worms might influ-
ence our ground-nesting birds. They
looked at how worm populations
might be affecting two species of
ground-dwelling birds—ovenbird
and hermit thrush—in the hardwood
forests of northern Wisconsin. At 12
study sites up to about 50 acres in
size, they surveyed worm, bird, and
plant populations and found “strong
evidence” that invasive earthworms
are affecting bird populations.
Ovenbird and hermit thrush num-
bers went down as worm numbers
went up. This might be because the
invasive European worms reduce
litter depth and encourage the
growth of grasses and sedges—both
factors that appear to decrease how
well ground-placed nests are hidden
from potential predators. a

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