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A View of Unconventional Birdlife
©June 2011, Jon Friedman

Cowbirds belong to the Troupial (TROOP-ih-al) family of birds. The name is derived from their habit of gathering in large flocks or troupes. There are 91 species within the family and all are found only in the Western Hemisphere. In North America there are 22 species, including the well-known blackbirds, orioles, grackles, meadowlarks and bobolink. The family is a remarkably diverse group, differing greatly in size (from 6½" to 21"), color, habits and behavior. Males and females are easily distinguished by color and markings. Males tend to be more colorful and females less so, as is true in many species. 

The name cowbird originated from their close association with cattle and bison which grazed over much of the Great Plains. These birds traveled with the herds and devoured insects stirred up by the large mammals. Bronzed Cowbirds, especially, rode the backs of these animals picking through their fur for ticks and other insects to eat. Trail herders and cowboys called these birds "buffalo birds".

Cowbirds, especially the Brown-headed Cowbird, during the past 150 years have expanded their range and population numbers as more forests are cleared and larger amounts of farmland are developed. The fact is that they now are numerous enough that they pose a serious threat to the continued survival of several species and sub-species of birds that it regularly parasitizes. Cowbird populations, in general, double every eight years! As a result, much research effort has recently been directed at understanding the breeding biology of Brown-headed Cowbirds, and a surprisingly complex and fascinating picture is emerging.

Bronzed Cowbird
by Richard of SearchNet Media

Obligatory nest parasites
Cowbirds are small to medium sized blackbirds that have the unique distinction of not building their own nests. Instead, they lay their eggs in nests of other birds. The foster parents raise the baby cowbirds as they would their own young. Cowbirds do not build nests or develop brood patches (used for incubation) as all other birds do. This is remarkable among birds and gives the cowbirds an unsavory reputation.

Our two cowbirds species, the Brown-headed and the Bronzed, are the only birds in North America that are completely dependent on other birds to raise their young. Therefore they are described as obligatory nest parasites. There are other nest parasites that lay their eggs in other bird's nests and incubate the eggs and raise the young themselves. But cowbirds are the only birds that abandon their eggs after laying and leave it to the host birds to completely rear the cowbird offspring as their own. That's the difference between nest parasites and obligatory nest parasites.

Unfortunately, most of the host birds are unable to distinguish their eggs from a cowbird egg in their nest. A female cowbird will lay a single egg in the host nest. She may lay between six and forty eggs in as many nests - never more than one egg per nest. However, on rare occasion, multiple female cowbirds may lay eggs in a single nest. As many as eight cowbird eggs, all from different females, have been discovered in a single host nest. After incubating the eggs in the nest, the host birds rear all the newly hatched birds.

Successful parasitism depends upon the proper timing of the laying of the female's eggs. Females must keep several nests under observation simultaneously and at the appropriate times return to each nest to lay a single egg, usually one per day. So, aside from the production of her eggs (which some ornithologists speculate is aided by watching the host bird build its own nest), a female cowbird's chief contribution to reproduction is the task of finding suitable nests, then laying in each at the proper time - neither too soon, which may cause the builder to desert - not too late, which will handicap her progeny if they hatch.

Size matters
Cowbirds usually pick nests of host birds that are smaller than themselves, giving the baby cowbirds a serious advantage in growing to adulthood. First of all, the baby cowbirds are almost always considerably larger than the host babies. Secondly, cowbirds babies out compete the host babies with their more aggressive and demanding feeding behavior, with the result being that the host babies don't survive due to malnutrition. By the time the cowbird babies reach fledging stage, they are considerably larger than the host babies and sometimes the cowbird babies can actually force the smaller, less developed host babies, out of the nest altogether.

Distinction of Eggs
While the cowbird eggs are most often larger and differently colored than the host eggs, the host adults nevertheless usually cannot distinguish between their eggs and the cowbirds eggs. Only a few of the host birds are able to detect the difference.

Among the birds that can recognize the differences are the American Robin and the Gray Catbird. These birds will immediately push the cowbirds eggs out of the nest.

Cowbird egg in Yellow Warbler nest
by Stylurus 

More often, however, another strategy is used by a larger number of host birds. These birds, as best exemplified by the Yellow Warbler, will simply build a new nest on top of the first nest, burying the eggs between the nests. Yellow Warbler nests have been discovered that are five levels high with the bottom four all containing cowbird eggs. The warblers are more insistent on having a pure brood than the cowbird is willing to repeatedly lay more eggs in the same nest.

Other species of birds employ another strategy to defeat the successful hatching of cowbirds eggs in their nest. Painted Buntings, Field Sparrows, Northern Cardinals and Yellow-breasted Chats most often abandon the nest with cowbird eggs in it and start over at another location. But, sadly, a great variety of birds accept the foreign eggs, hatch out young cowbirds that they raise as their own, usually at the price of some or all of their own offspring.

Hosts and Diminishing Populations

Bronzed Cowbird
by Richard of SearchNet Media

Bronzed Cowbirds are known for destroying the eggs of other cowbirds when they find them in host nests. Hosts for Bronzed Cowbirds frequently include flycatchers, wrens, thrushes, mockingbirds, thrashers, vireos, wood warblers, and tanagers, but chiefly a variety of sparrows and finches, and, above all, orioles and a few related birds. Brown-headed Cowbirds, by contrast, utilize a much wider variety of host species nests. Customers have shown us photographs they have taken showing large cowbird babies begging for food from "adoptive" Northern Cardinal and Pyrrhuloxia parents.

Cowbirds lay their eggs in over 220 other bird species nests. Most frequently, victimized birds are smaller than cowbirds and build their open cup-shaped nests in trees, bushes and on or near ground level. Cavity nesting birds; such as woodpeckers, bluebirds, and chickadees are rarely bothered by cowbirds.

Ornithologists now recognize that cowbirds are partly responsible for the demise and/or extinction of several species of birds, particularly some of our rarest warblers. The last remaining nests of the now extinct Bachman's Warbler were all found to have cowbird eggs in them and only the cowbird babies survived. The Kirtland Warbler is now facing extinction, with a total population of only a few hundred. Ornithologists have saved this species (at least for the time being) by physically trapping and removing cowbirds from the warblers only known breeding grounds - a now protected area in north-central Michigan, where controlled plantings and fires produce the warblers required habitat of thickets of young jack pine trees.

In California, the Bell's Vireo conservation status is increasingly threatened as a result of cowbird intervention. Black-capped Vireos throughout their range have also seen a sharp decline in their numbers as a result of cowbirds interfering with their normal breeding. Cowbirds definitely play a part in the demise of some of these endangered species but habitat destruction also plays a crucial role. These warblers traditionally spent their winters in the Caribbean Islands, but much of their winter territory has been developed into resorts and housing areas - so the cowbirds shouldn't be assigned all the blame. While cowbirds have been partly to blame in the demise of many songbird species, it should be made clear that predation (cats, hawks, snakes, etc.), physical stress of migrations, crashes into windows and buildings, habitat fragmentation, disease or illness, old age, etc. accounts for far more deaths than cowbird predation.

Fertile female cowbirds often stake out and survey smaller songbird nests. Once identified, the cowbird returns to that nest shortly after sunup once the parent bird goes off to forage. The host bird already has her eggs in the nest when the female cowbird deposits a single egg to the nest. Laying her egg in the host nest usually takes only seconds and the female quickly departs before she can be noticed. Quite often the cowbird simply devours the host's eggs, or in rare instances, pierce the egg in order to grab it and carry it off where it can eat it elsewhere and out of sight of the host bird's nest. Cowbirds are able to distinguish their eggs from the host's and only carry away or destroy the host eggs. By late summer, fledged cowbirds join up in flocks of their kind. 

Range and vocalization
The Brown-headed Cowbird is very adaptable and, as a result, is found throughout the lower 48 states and the southern half of Canada. With the demise of the great herds of buffalo on the plains the Bronzed Cowbird is considerably more limited in its range and is chiefly found in Southern Arizona, Southern Texas and the boot heel of New Mexico. Both species forage in flocks, sometime quite large flocks. Both species are usually found in their preferred habitats which include, but are not limited to, feedlots, open woodlands, farmlands, suburbs, urban parks, brushy areas, and wooded mountain canyons. Southwestern cowbirds, sometimes referred to as a dwarf species, are slightly smaller than eastern cowbirds and the Rocky Mountain and Great Basin cowbirds tend to be the largest. Both species of cowbirds have similar songs and calls. The songs are wheezy, squeaky gurgling sounds and the calls include a harsh rattle, squeaky whistles, and a harsh, guttural chuck.

Both species tend to forage on similar diets, consisting primarily of vegetative foods and insects. In summer and during the breeding season they are particularly fond of a variety of insects and larvae, especially beetles, caterpillars, bugs, grasshoppers, and spiders. At these times of the year, insects provide for more than half of their overall intake. From late fall until the following spring, the cowbirds diets are mostly seeds. Seeds of dandelion, knotweed, ambrosia, and the grass Echinochloa are staples in their diet, yet they cherish yellow foxtail grass above all else. Yellow foxtail grass has been referred to as the cowbirds "staff of life". Fruits are an important, but minor, part of their diet. They seem to prefer blackberries, huckleberries, wild cherries, grapes, and cedar berries. In farmland areas, especially where the crops grown include corn, maize (milo), wheat, oats, barley and other grains, cowbirds tend to join larger mixed flocks of other blackbirds and starlings causing the farmers considerable grief. Large flocks can strip crops and reduce significantly the total yield.

Bronzed Cowbird
by Richard of SearchNet Media

Cowbirds at our feeders
During the course of writing this article, I have observed both species of cowbirds showing up at my feeders. Each year there's a relatively short period when they become numerous. I have detoured them away from most of the feeders by providing a few feeders, out and away from the rest, in which I put only white proso millet. The cowbirds prefer this very inexpensive seed to all others, so I don't mind their presence for a short period. Mainly, I use the millet by itself in feeders to attract migrating Lazuli (and rarely Indigo) Buntings and the host of exotic winter sparrows we get here every year. I also use the white proso millet to keep house finches off my thistle feeders, a strategy many customers now also use to keep unwanted competition from the red finches away from the goldfinches.

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