Introduced
Species Summary Project
Brown-headed Cowbird (Molothrus
ater)
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Common
Name: Brown-headed Cowbird
Scientific Name: Molothrus ater
Classification:
Phylum or
Division: Chordata
Class: Aves
Order: Passeriformes
Family: Icteridae
Identification: On the
small side for blackbirds – 6 to 8 inches – Brown-headed Cowbirds are sexually
dimorphic. The males are a shiny black with brown heads while the females are
more brownish gray all over. The birds
have somewhat rounded tails and pointed, long wings.
According to The Sibley Guide
to Birds, there are a couple of similar looking species: the Shiny Cowbird
(Moluthrus bonariensis) is similar in
appearance, but with a longer tail and more rounded wings, and the Bronzed
Cowbird (Moluthrus aeneus) is
stockier, with a longer, heavier bill. Neither the Shiny Cowbird nor the
Bronzed Cowbird have the kind of expansive range of the Brown-headed Cowbird.
Original Distribution: The Great Plains of North America
Current Distribution: The
Brown-headed Cowbird can now be found across North America. They’re considered partially migratory,
spending winters in the more southern parts of their broad breeding range
Site and
Date of Introduction: These birds are a classic example of homegrown
exotics. They weren’t introduced from another country or region of the world,
nor is there one site of introduction. Instead, as Europeans spread out across
the North American continent, so did the cowbirds.
Mode(s) of Introduction: Native to
the United States, Brown-headed Cowbirds aren’t native to many of the regions
in which they now live: including habitats as varied as the Northeast and the
Sierra Nevada. Their introduction into these areas followed the eradication of
forests and the spread of livestock in these regions, and happened over a
number of years as farming, clearcutting, suburban sprawl and other man-made
habitat changes occurred across the country. The birds used to follow herds of
bison across the Great Plains, eating the insects that the bison’s hooves
turned up and leaving their eggs in the nests of other birds so that the
cowbirds could move on with the bison and still have chicks raised to adulthood
(by the nest-building birds).
Reason(s) Why it has Become Established:
Brown-headed Cowbirds seem to be remarkably hearty and adaptable birds. They
were able to adapt from frequenting bison habitat to locating near domestic
livestock and spread with livestock across the country, practicing brood
parasitism wherever they went.
Ecological Role: Brood parasites do not
build their own nests. Instead, the females lay eggs in the nests of other bird
species, and then leave those eggs for the host birds to raise as their own.
This style of parasitism served them well when they inhabited only their native
Great Plains, as it apparently enabled them to follow along with the roaming
bison herds, leaving their potential offspring under the care of others. And
the parasitism seems to serve them well in their newer, invasive role. The
Cornell Ornithology Laboratory estimates that, while over 100 different species
have been host to cowbird parasitism, Yellow Warblers, Song Sparrows, Red-eyed
Vireos, Chipping Sparrows, Eastern Phoebes, Eastern Towhees, Ovenbirds, and
Common Yellowthroats are the most commonly targeted host species.
Benefit(s): I’m not really sure about potential benefits
from Brown-headed Cowbirds. Perhaps
there’s an argument that a certain amount of brood parasitism helps keep any
one songbird population from skyrocketing too high, but that’s just speculation. The cowbirds have spread with spread of
grain storage facilities and the like, but one probably should not argue that
they help keep excess grain under control!
Threat(s): While some have blamed the
Brown-headed Cowbirds for a national decline in the number of songbirds, more
legitimate concerns about the birds’ negative impact on songbird biodiversity
seem to focus on regional issues. In Texas there is concern about cowbird
parasitism affecting the endangered Black-capped Vireo, and in the Sierra
Nevada there is concern about cowbird parasitism affecting populations of
certain species of flycatchers and vireos.
Control Level Diagnosis: The
scientific and birdwatching community is somewhat divided on Brown-headed
Cowbirds and their impact – some argue that their nest parasitism poses a major
threat to other songbirds, particularly endangered species such as Black-capped
Vireos. Others, like the authors of a 1997 Audubon paper on Brown-headed
Cowbirds, argue that reports of their threat to local species are greatly
exaggerated, and even that Brown-headed Cowbirds actually saw a population drop
in the mid-1990s. Others point out that
brood parasites have such a bad reputation that it is difficult to assess their
actual impact.
Control Method: Attempts to control cowbird populations have
centered on shooting adult birds at roosting sites or otherwise
trapping/killing the adult birds and removing eggs from the nests they’ve
parasitized.
Author: Joanna
Cagan
Last Edited: March 1, 2002
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Project Editor: James A. Danoff-Burg, Columbia University