"Irresponsible" cowbirds and the decline of songbirds

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By Samuel Truax

A male cowbird on a metal post. (Paul A. Johnsgard)Just like human society, avian society has many individual personalities. There are those raucous types, like crows and jays, loudly quarreling among themselves as they accomplish their duties of survival and procreation. There are those like doves, that quietly go about doing their prescribed duties. With their lovely melodies, various songbirds bring enjoyment to humans as they accomplish their tasks. And then there are those who seem to be completely irresponsible in accomplishing their life duties, without the redeeming qualities of lovely feathers or beautiful songs, like the brown-headed cowbird.

How has the brown-headed cowbird established such a reputation for irresponsibility? The book “Birding” states that “The brown-headed cowbird’s most famous, or infamous, attribute is that it is a brood parasite.” That is, “the female lays eggs in the nests of other birds and takes no part in the rearing of her young.” And the female cowbird is not even concerned about who raises her offspring. “Brown-headed cowbird eggs have been recorded in the nests of 220 species, ranging from ducks to hummingbirds.” Even more extreme, studies find that “they remove the eggs of the host bird on some occasions.”

The cowbird developed these seemingly irresponsible habits by evolving a nomadic lifestyle. Cowbirds followed the once massive buffalo herds through their migration cycles, feeding on insects and seeds stirred up by the constant movement of the buffalo herds, and so were never in one place long enough to build and tend to their own nest. In fact their name comes from the buffalo cows they associated with.

From their position in the grasslands, the female cowbird could easily spot nests for her eggs in the shrubbery and undergrowth that occur where the forest edge abuts the grassland. Normally the nests deeper into the forests were not used. The female cowbird would deposit her eggs in the selected nest and then move on with the herd. Though a seemingly irresponsible trait much criticized by humans, this was a behavior well adapted to the conditions of life in which the cowbirds evolved. The species survived, and, though some harm was done to other affected bird species, that harm was typically minimal due to the constant migration of the cowbirds as they followed the bison.

However, the habitat changes that occurred with the arrival of European settlement in America, such as the replacement of migrating buffalo with confined cattle, ex­panded the conditions suitable for the brown-headed cowbird, at the expense of the birds whose nests they used. The European settlers also cleared trees for crop fields and pastures, creating more and more forest edge suitable for host nest selection by the female cowbird. The cowbird population increased very substantially and, according to some studies, continues to increase, if not in actual numbers, then certainly in range. A study by the Audubon Society concluded that even as the cowbird’s range continues to expand, their actual population has de­clined 1 percent from 1966 to 1996. Regardless of whether the cowbird population is stable or increasing, its expanding range allows parasitism of many more species, some that are poorly adapted to avoid such behavior.

A female cowbird on a wooden fencepost. (Paul A. Johnsgard)While cowbird eggs have been recorded in the nests of 220 species of birds, and cowbirds have been fledged by 150 different species, they primarily target the nests of vireos, warblers and some sparrow species. These birds have a tendency to nest in brushy undergrowth typical of the forest edge.

The cowbird eggs have a shorter gestation period than their hosts, so they often hatch before the host species. Cowbird hatchlings are usually larger and more aggressive during feeding by the host parent, and they are often aggressive toward the young of the host species. This can cause the death of the young of the host species, and when combined with occurrences when the female cowbird actually removes the eggs of the host from the nest, the habits of the cowbird are causing a great deal of stress on the populations of some species. Interestingly, some bird species recognize and remove cowbird eggs from their nests. In some cases, engaging in what is called “mafia behavior,” the cowbird will return to the nest and destroy all of the host species eggs, sometimes even tearing the nest apart.

The black-capped vireo is on the Endangered Species List. Studies indicate that the primary cause of the endangerment of the species is nest parasitism. The second most important cause of the endangerment, after habitat destruction, of the Kirtland’s warbler, is nest parasitism. Recovery plans for the endangered golden-cheeked warbler include a cowbird-trapping program and the control of cowbird populations. Such control methods are controversial.

Studies of Bell’s vireo in South Dakota concluded that they “are very common victims of brown-headed cowbird parasitism, and are in decline in many regions.” In New Mexico it was found that Bell’s vireo “suffers from low productivity due to brood parasitism by brown-headed cowbirds.” In some years, nest failures have exceeded 90 percent in the Carlsbad population.

However, the Audubon study does not indicate that these conditions are quite as serious as other scientists believe. The study did find the black-capped vireo experienced a 90 percent nest parasitism in Texas. A study of Bell’s vireo and the yellow-breasted chat showed they had 80 percent to 90 percent of their nests parasitized. And the lazuli bunting had 50 percent of their nests parasitized in a Montana study. But these high rates of parasitism did not necessarily indicate the rates of nest failure, according to the Audubon study, because by the mid-summer season, some of the victimized birds will renest and produce a successful brood. And in studies of lazuli buntings and yellow warblers, parasitized nests were just as successful as nonparasitized nests in certain conditions. The Audubon study concludes, “Conservationists and the public tend to overestimate the significance of parasitism as a major cause of declining songbird populations.”

Cowbird control includes trapping the birds or killing them at roosting sites. The Audubon study questions the need for and the effectiveness of cowbird control. And their questioning includes the statement that “Cowbird control is expensive.” A recent study by the Northern Prairie Wildlife Research Center argues that habitat changes, more than direct control, might be a more effective solution: “Keys to discouraging cowbird parasitism or controlling populations of brown-headed cowbirds in the Great Plains are maintaining large expanses of grassland, eliminating foraging areas (e.g., feedlots) and perch sites, and reducing the extent of overgrazed pastures.”

The expense of cowbird control is only a small portion of the expense of protecting and reestablishing the endangered songbirds that are affected by the parasitism of the brown-headed cowbird. The U.S. Fish and Wildlife Service has established the 21,436-acre Balcones Canyonlands National Wildlife Refuge west of Austin, Texas, with the primary purpose of aiding the recovery of the black-capped vireo and the golden-cheeked warbler. The Leon River Restoration Project in Central Texas also targets the recovery of these two species, and the Wichita Mountains Wildlife Refuge in Oklahoma is primarily intended to assist black-capped vireo recovery. These are large expenses and do not include other projects or work being done to assist the Kirtland’s warbler and other seriously endangered species that are victimized by both cowbird and human behavior. These Wildlife Refuges are primarily paid for by excise taxes on hunting and fishing activities and benefit many species of flora and fauna beyond the primary species objectives, so these large expenses provide more good results then just offsetting the effects of cowbird parasitism.

Yes, we might consider cowbird behavior to be irresponsible. But surely we humans have been far more irresponsible with our stewardship of Mother Earth. They are unlikely to change. Are we?

 

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