The avian Order Passeriformes (meaning "sparrowlike;" the group is often described as the "perching birds") is the largest and most diverse organizational group of birds—over half of all the approximately 10,000 species of birds fit this category (some 60 Families). Its Suborder Oscines (singing birds) contains the Family Icteridae. It means "yellow" or " jaundiced" and refers to the many species that have—or partially have—yellow or orange feathers (despite that it's generally considered the blackbird Family). These are New World birds and include blackbirds, bobolinks, meadowlarks, grackles, orioles, and cowbirds.
Cowbirds are generally broken into six species. Five in the genus Molothrus ("intruder," this will make sense shortly): bay-winged cowbird (M. badius), bronzed cowbird (M. aeneus), brown-headed cowbird (M. ater), screaming cowbird (M. rufoaxillaris), and the shiny cowbird (M. bonariensis). The giant cowbird (Scaphidura oryzivora) has a separate genus. More recent research has found that the bay-winged cowbird is likely not a true cowbird but it has an interesting place in the group (see below).
The birds are brown to black and have longish, pointed wings and a conical beak. The diet consists of about 75% grains and seeds and 25% insects. It was the bird's habit of following cattle and bison and feeding on the insects that were "flushed out" from the animals trampling over the grass that gave it its name. Other variations of the name have included buffalo bird, cowbunting, cow blackbird, and lazy bird.
Originally, the only cowbird in North America (excluding Mexico) was the brown-headed cowbird (much of the information below if for this species, though generally applicable). It is a bird that lives near the forest edges of open areas and plains. Human clearing of the land has allowed the bird to expand its territory to most of the United States and a good deal of Canada. The bronzed cowbird can be found in southern Texas and Arizona (a native of Mexico) and in recent years the shiny cowbird, which was a South American species (as are the remaining cowbirds), has expanded through the Caribbean into Florida.
The most interesting aspect of the cowbird is its nesting behavior—or lack of it: the cowbird does not make a nest (it does lay its eggs in one—just not its own). This behavior is known as "brood parasitism." Similar behavior can be found in the cuckoo. Differences include the cowbird's eggs not mimicking the host's eggs and the chicks lack of pushing their nestmates out of the nest. The young are then reared by the host, often out-competing the other chicks for food (sometimes, but not always, leading to fewer surviving young for the host). An exception to this is found with the bay-winged cowbird, which raises its young—only in other birds' nests. The most interesting part of this is that the bay-winged cowbird, in turn, is often parasitized by the screaming cowbird.
The cowbird has managed to evolve into a rapid egg-laying machine. She can lay an egg in about 10-30 seconds (some cuckoos have been reported to manage it in as little as three but more typically birds spend between 30-90 minutes) which helps her as she swoops in near dawn to lay the egg in the host nest while the parents are away feeding (occasionally removing or disposing of the host eggs). Nests of the host are found with a combination of visually searching and listening for the song of the birds. The eggs usually hatch sooner than the host species and the young tend to grow faster, helping ensure success.
Over 140 species of hosts have been recorded. Common "victims" are yellow warblers, sparrows, and vireos. While there is generally no significant longterm population declines due to the cowbird, there are a few species that are endangered, requiring control of the cowbird if they are to survive.
As is common in biology and evolution, many birds have developed defenses against the parasitism (though some have not in what is termed "evolutionary lag" due to the relatively rapid expansion of the species to areas it previously did not inhabit). Some birds hide their nests (which may have stimulated the "listening" behavior noted above), some abandon the nest if the eggs appear. Certain species, the yellow warbler for example, build a "floor" over the cowbird eggs to keep them insulated from incubation (nests of up to six "stories" have been found). Other birds will push the eggs out or pick the egg up and fly off, dumping it as far as 50 m (164 feet) away. Still others (usually smaller ones) will stab the egg with their beak and toss the pieces from the nest.
While that sounds bad for the cowbird—in fact, studies show that as few as 3% of cowbird eggs result in adult birds—the "egg laying machine" lays between 30-40 eggs each season and will fly as far as 8-10 km (4.9-6.2 miles) to look for nests.
Cowbirds tend to be disliked or even hated due to what ( anthropomorphizing) people see as its deceptive and perhaps "cowardly" behavior. Of course, nature has no such qualms, nor does it attempt to conform to people's expectations of how it should work. Astutely noted by zoologist Stephen Rothstein of the University of California, Santa Barbara: "people hate cowbirds, yet love hawks. Hawks catch adult birds and rip them to pieces. Maybe people hate cowbirds because they're tricky" (www.blarg.net).
But all is not lost. In one instance the cowbird's parasitism actually benefits the host to some extent (making it more like mutualism). The South American giant cowbird tends to lay eggs in the nest of the chestnut-headed oropendola (Psarocolius wagleri, interestingly another icterid). It was found that the cowbird eggs in some of the nests more closely resembled the host's eggs than others. Further investigation found that certain oropendolas are more likely to "eject" the eggs than others. It was easy to conclude that those more likely to eject the eggs were the nests where the cowbird eggs more resembled the host's. Evolutionary pressure would tend to favor those that look more like the host's, while in nests where they are not rejected there is no pressure and the eggs are less likely to resemble those of the host.
While a bit interesting in itself, the further one looks, the complex the relationship. The first big question is why would the cowbird eggs not be ejected from some of the nests when a lack of success for the host's brood would be certain—especially since other oropendolas are able to discriminate (and unlike in parts of North America, there is no evolutionary lag, in this case)? Even stranger was the discovery that the "non-discriminator" nests with cowbird chicks actually were more successful at raising chicks. This seemed counter-intuitive.
More study determined the reason. The answer was found in the world of insects. In some areas, the botfly is a pest and a threat to the young of oropendolas, which are born blind and lacking feathers as well as being slow to develop. The more quickly developing cowbird chicks, in addition to a healthy appetite for, among other things, insects, are also supplied with dense down that makes it difficult for the botflies to do harm. In fact, the cowbird chicks have even been found to groom this "food" from nestmates. This way, being less discriminatory benefits the bird (despite the burden of having fewer resources to spread around) by an increase in the possibility of the chicks surviving into adulthood.
But that still leaves a small question. If the botfly is the answer to the "nondiscrimination," why do the other oropendolas eject the eggs—even though botflies cover the same range as both sets of birds? Again: insects. There are species of stingless bees and certain wasps that live in proximity to those nests. These insects keep the botflies in check, making raising a cowbird chick a burden on resources and giving no survival benefit to the host's own young. In this case, it pays to discriminate.
(Sources: http://aniamldiversity.ummz.umich.edu/accounts/molothrusm._ater$narrative.html, http://birds.cornell.edu/BOW/BNHCOW, www.blarg.net/~critter/AvianFamily/cowbird_1.htm, http://members.aol.com/askdrjay2/oropend.htm, www.montereybay.com/creagrus/icterids.html)