Birds
From ANIMAL PEOPLE, November 1993:
Cowbirds, native to the
midwest, invaded California 10 to 15
years ago and are now blamed for extir-
pating at least four threatened or endan-
gered songbirds from key parts of their
range. Female cowbirds indirectly kill
as many as 48 young songbirds apiece
per nesting season by laying their eggs
in songbirds’ nests. The songbird par-
ents then raise the fast-hatching cowbird
offspring ––who push the songbirds’
own eggs out before they hatch.
Songbird species such as the Bell’s
vireo, willow flycatcher, yellow-breast-
ed chat, and white crown sparrow are
believed capable of withstanding losses
of 10% of their eggs, but decline quick-
ly when the losses exceed 20%. Studies
of white crown sparrow nests in San
Francisco’s Golden Gate Park indicate
losses to cowbirds may exceed 50%.
Hopes for the eventual recovery of the
highly endangered Bell’s vireo were
raised this year when one or two pairs
reportedly nested along the Ventura
River, near Santa Barbara, for the first
time since 1908––but a lone Bell’s vireo
seen in Monterey County, a former
stronghold of the species, failed to find
a mate. Bell’s vireos apparently haven’t
nested successfully there since the cow-
birds arrived, circa 1983.