From ANIMAL PEOPLE, October 1999:

Bird habitat
The U.S. Fish and Wildlife Service credits killing
thousands of nest-parasitizing cowbirds since 1991 with bringing
the least Bell’s vireo up from just 268 known pairs in 1991
to more than 2,000 in 1999. “Just as important,” explained Los
Angeles Times reporter Gary Polakovic, “the vireo’s comeback
may prove that habitat along streams in Southern
California is recovering––a critical indicator of environmental
health in a state that has lost 97% of its riparian woodlands,
more than any other.” As Illinois Natural History Survey
scientist Scott Robinson observed in 1995, after examining the
relationship between vanishing songbirds and cowbirds,
“Small nature preserves, which work fine for preserving plants,
don’t work for migratory birds,” whose nesting sites become
vulnerable to cowbirds when deforestation removes their cover.
“The [British] Royal Society for the Protection of
Birds are completely barking,” Game Conservancy Trust
head of grouse research David Baines recently told Daily
Telegraph environment editor Charles Clover, because after
five years of intensively killing crows and foxes to protect a
rare grouse called the capercaillie, the RSPB has experimented
since 1995 with not killing predators. The capercaillie population
is down from 2,200 in 1995 to about 1,000. But the RSPB
says the main reasons for the drop have been bad weather at
nesting season and, wrote Clover, “the death of up to a third of
its capercallie by flying into deer fences put up to allow the
regeneration of native pines.”

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