This article first appeared in the May 2004 issue of the San Antonio
Each winter, enormous flocks of Brown-headed Cowbirds (Molothrus ater) congregate in Texas, uncounted hordes of cowbirds leaving their communal roosts and forming snaking rivers of birds flowing across the sky. At such times one gets an inkling of what it must have been like on this continent in centuries past, when observers recorded flocks of many different kinds of birds on a scale almost beyond comprehension today. Looking at those cowbird flocks one feels a sense of awe, mingled with the awful realization that each individual cowbird in the flock likely brought about the untimely death of at least a few songbird nestlings.
Across the Americas, five species of cowbirds are obligate brood parasites, entirely dependent upon the efforts of other birds to raise their young. A sixth species, the Bay-winged Cowbird (Agelaioides badius) of South America, is the only cowbird species that habitually raises its own young. Ironically, the Bay-winged is normally the sole host of the dramatically-named Screaming Cowbird (M. rufaxillaris). The tropical Giant Cowbird (Scaphidura orizivora) also displays a degree of host specialization, parasitizing the hanging nests of seven species of Oropendulas and Caciques.
Three cowbird species parasitize the nests of a great many potential host species. The Bronzed Cowbird (M. aenius) of Central America and Mexico has been observed to target the nests of 82 different species, favored hosts being other members of the family Icteridae. The Shiny Cowbird (M. bonariensis) of South America and the Caribbean has been observed targeting the nests of some 200 potential host species. Here in North America, Brown-Headed Cowbird eggs have been found in the nests of 220 different species, 144 of which have been observed raising cowbird young.
Brood parasites that target but a few host species often have eggs and young that mimic those of the host, minimizing the chance of rejection. Such host mimicry is at best incidental in the three cowbirds species mentioned above. In these species, losses of eggs and young due to host rejection and other causes are high, this deficit being made up by dint of sheer productivity on the part of the cowbirds. A female Brown-headed Cowbird may lay more than forty eggs each year, most being left in separate nests.
The presence of a cowbird nestling is frequently fatal for the other members of the brood. The young of smaller species may starve in competition with the cowbird. In addition, young cowbirds typically beg more insistently than do most nestlings, monopolizing the attention of the host parents at the potential cost of attracting predators to the nest.
Some host species can recognize and reject cowbird eggs, but such rejection may still involve considerable cost to the host. Larger species such as Catbirds may simply eject the eggs of cowbirds from their nests, losing any of their own eggs that may have been pecked or removed by the cowbird. Smaller host species may be physically unable to eject a cowbird egg, their only option being the abandonment of their own eggs along with that of the cowbird. Worse, female cowbirds have been observed ejecting nestlings from unparasitized nests, a behavior which may function in inducing potential hosts to attempt a second nest subject to parasitism by the cowbird.
All three of these cowbird species have benefited greatly from human activity, increasing in numbers and expanding their range far beyond what was probably the case in centuries past. The Shiny Cowbird, originally limited to South America, has since colonized the West Indies. Scattered individuals turn up regularly across the southern states and a breeding population may currently exist in Florida. Bronzed Cowbirds were once considered a South Texas rarity but now occur commonly across much of the southwest. The Brown-headed Cowbird, historically most common on the Great Plains, has now become a common breeding bird across most of North America.
Given the large number of nests that a single cowbird female may parasitize or disrupt each year, it is not surprising that these expanding cowbird populations have been implicated in causing the extirpation of some songbird species from many areas, and the near extinction of others.
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