Bird Lore: Brown-headed Cowbirds

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brown-headed_cowbirdRight now there are a number of young Brown-headed Cowbirds around Edmonds. Some cowbirds are year-round residents of Western Washington and some arrive in April as migrants. While cowbirds can be found in winter in any large mixed flock of blackbirds at Snohomish County fields and dairy farms, they do not arrive in Edmonds until late April or May. Most adults seem to prefer the riparian habitat around the Edmonds marsh. Young birds, as seen in LeRoy’s photo, wander away from the marsh and can be found even in suburban yards.

Historically, the Brown-headed Cowbird was a bird of the prairies that followed bison herds for the insects they raised. It moved eastward in the 1800s as forests were cleared. It is an adaptable bird that has expanded its range to include most of North America. It can be found in many habitats except dense, unbroken forest. Many consider it the pariah of the bird world because it parasitizes the nests of more vulnerable species, including a number of migratory warblers.

Seeds from grasses, weeds, and waste grains comprise about half of the summer diet of the Brown-headed Cowbird and more than 90 percent of its winter diet. The rest of its diet includes grasshoppers, beetles, caterpillars, spiders and millipedes. It forages by walking on the ground instead of hopping like sparrows or finches. It often associates with hoofed mammals such as cattle and horses because it can catch insects flushed by the grazing animals.

Brown Headed Cowbird_1The male is an iridescent greenish-black with a dull brown head and a finch-like bill, as you can see in Blair’s photo. Its courtship display includes fluffing up its body feathers, partly spreading its wings and tail, and bowing deeply while singing. The female, an overall dull gray-brown bird, does not build a nest. She lays her eggs in the nests of other birds. She will lay one egg a day for several weeks, typically up to 40 eggs in a season. She will often remove an egg from the host nest before laying her own. The male and female will have several different mates within a breeding season.

The female Brown-headed Cowbird is known to have laid eggs in the nests of over 220 other species of birds. She watches quietly for signs of other birds building nests or flutters through vegetation to try to flush birds from their nests. Over 140 species have been observed to actually raise young cowbirds. Some species have strategies for dealing with cowbirds because they recognize the egg as not their own. The Yellow Warbler, a favored host of cowbird eggs, will often build a new nest on top of the nest that contains a cowbird egg. Most species do not recognize the cowbird egg. Two of the most endangered U.S. breeding birds, the Kirtland’s Warbler and Black-capped Vireo, have sustained heavy population loss due, in part, to cowbird parasitism.

A cowbird egg hatches more quickly than do the eggs of host species. That gives the juvenile cowbird a head start in getting food from its host parents. The young cowbird develops at a much faster pace so it will sometimes destroy the eggs of its host parents. It will also toss its nest mates out of the nest or smother them in the bottom of the nest.

The oldest known Brown-headed Cowbird was a male, at least 16 years and 11 months of age when it was recaptured and re-released at a Wisconsin banding operation.

Although the Brown-headed Cowbird population sustained a 31-percent decline between 1966 and 2014, the global population is estimated at 120 million, with 77 percent spending part of the year in the U.S., 31 percent in Mexico, and 14 percent in Canada. Its conservation status is that of least concern.

You can listen to the song of a Brown-headed Cowbird here: http://www.xeno-canto.org/267970. Its chip call can be heard here: http://www.xeno-canto.org/267956.

— By Carol Riddell

Carol Riddell manages the bird education displays, on behalf of Pilchuck Audubon Society and Edmonds Parks & Recreation, at the Olympic Beach Visitor Station.

4 COMMENTS

  1. With such aggressive behavior and parasitic propagation skills, it’s surprising that the cowbird population declined 31%. Could it be from the widespread loss of natural open space and meadows?

  2. The assessment of the health of North American land bird populations is conducted by Partners in Flight, which is a cooperative venture of federal, state, provincial, and territorial agencies, industry, non-governmental organizations, researchers, and others with the common goal of conservation of North American birds. Most agencies work with limited funds so steep declines in a species population are defined as 50% or greater over the last 40 years. The cowbird decline of 31% seems significant to me, but the greater declines in migratory songbird populations, whose nests are parasitized by cowbirds, are of greater interest to researchers. In a cursory search, I could not find an article explaining the reasons for the cowbird decline. With the exception of deep forest, the cowbird is a habitat generalist. So I would think the decline in its population would be associated more with the steep decline in its host songbird species than with landscape alteration.

  3. I know all birds (as with other species) contribute some good, but it does grate when I see cowbird eggs in some other bird’s nest. When we lived in a more rural setting, we saw this often.

  4. Brown-headed Cowbirds feed on the ground in mixed-species groups of blackbirds and starlings. Males gather on lawns to strut and display for mates. Females prowl woodlands and edges in search of nests. Brown-headed Cowbirds are noisy, making a multitude of clicks, whistles and chatter-like calls in addition to a flowing, gurgling song.

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