If you've been a regular reader of this column, I assume you have reserved judgment that cowbird reproductive behavior is an unsavory topic and are willing to hear me out for a few paragraphs.
In order to do this I suspect one somehow avoids applying morality to cowbird egg laying. It is nevertheless true that cowbirds have been considered immoral: They lay their eggs in nests of other kinds of birds, which then incubate the eggs to hatching and feed the nestlings to independence. If they are not immoral, are they simply irresponsible?
Call it what you will, brood parasitism is not really a macabre lifestyle, but a remarkably satisfactory way for cowbirds to solve the problem of reproducing their genes.
Cowbirds developed the behavior over evolutionary time, although humans have recently become important players in the drama. But the entire story is a good one that we may open by examining some of the features of cowbird biology.
Dozens of eggs
Cowbirds form no permanent pair bond. Females mate with one or more males in the female's home range; she will lay from 20 to 40 eggs per season. These develop in periodic ``clutches'' of around five eggs, which are deposited in nests of other birds whose activities have been monitored. The normally smaller hosts that incubate eggs and rear nestlings put an appreciable part of their energy into these young cowbirds.
Female cowbirds do not develop a brood patch, conserving energy that otherwise would go into its development. Laying females tend to remove an egg of the host when they deposit their own; this maintains the host's clutch-size. Cowbird eggs have a short incubation period, so they may well hatch before those of the host. Being fed by the hosts before their own eggs hatch gives a cowbird hatchling an energetic advantage over later host nestlings. Cowbird young also tend to have a more rapid posthatch development than host young.
Unwilling foster parents
One of the reasons cowbird behavior is successful is that resources otherwise unavailable -- those of the foster parents -- are used to bring new cowbirds into existence. Hosts are drawn from a great range of possible foster parents. More than 200 host species have been recorded in North America, and more than 50 of these are known to be used by cowbirds in Kansas. Some are parasitized regularly at high frequency -- red-eyed vireos and song sparrows -- and others rarely -- Eastern kingbirds and cedar waxwings. Others, such as red-winged blackbirds, may be untouched in some areas -- central California -- and parasitized in others -- 54 percent of nests in Nebraska.
In Kansas, if young cowbirds are fed by as many as 50 species, then 50 different food bases are used to support new cowbirds. Also, nest predation may be spread over 50 species so that a predator using a search-image for red-eyed vireo nests will reduce cowbirds in red-eyed vireo nests, but not of those in nests of Northern cardinals or field sparrows.
Additionally, since spring and summer weather is unpredictable in Kansas, another advantage accrues for cowbirds. Assume that cool-wet years favor the successful reproduction of one set of bird species, and that hot-dry years favor a different set. Cowbirds use so many different hosts that their own reproduction will not be seriously affected by normal or even unusual variation in temperature and precipitation. When dickcissel reproduction is poor owing to drought, Northern cardinals may do well, and cowbirds, having hedged their reproductive bets by laying eggs in nests of both species, will do better than if they had specialized on dickcissels alone.
Some hosts are seriously affected by such parasitism. A few, such as the Kirtland's warbler and California populations of the Bell's vireo, have been so heavily parasitized that their existence has been at risk. The impact of cowbirds on these birds is partly a result of human modifications to the wild landscape. And, for the record, the significance of cowbirds to Kirtland's warbler also has been reduced by human intervention.
Until most of the other birds that cowbirds exploit learn to avoid being used, cowbirds will proceed to do just as they have been doing. In North America, robins and brown thrashers reject cowbird eggs, and other species, such as yellow warblers and field sparrows, may desert their parasitized nests and build new ones. Eventually, more of the current hosts will remove cowbird eggs or desert their nests, and this will select cowbirds for mimicry of eggshell color and markings. Such, at least, has been the history of the European cuckoo, an older and now more seriously evolved brood parasite. Cuckoos show fine mimicry of eggshell size, color and markings of a relatively few hosts, making it difficult for hosts to recognize the parasitic eggs.
Meanwhile, cowbirds have the freedom to pursue the alternative to the cuckoo's precision approach, namely to produce an enormous number of eggs each year, and broadcast them in nests of any species they are able to detect at the right stage in the nesting cycle. They lose only a few eggs to those species that are currently able to recognize cowbird eggs as foreign, and there is little pressure for cowbirds to be any more precise in their egg-laying behavior than they are now.
The original adaptations of cowbirds were to prairie grassland habitats. We may envision cowbirds as closely associated with bison herds in midcontinental North America, following them in order to exploit the excellent feeding opportunities the moving herds created.
European humans later reduced the numbers of bison to nearly zero and introduced nonmigratory cattle, and at the same time reduced woodlands and forests to agricultural fields. Cowbirds had lost their herd animals, but gained a reasonable substitute plus an enormous amount of new open-country habitat. They also found that living without migratory herds also was possible.
Additionally, since cowbirds have always used woodland edges to a depth of perhaps a quarter-mile when looking for nests to parasitize, most of the scraps of woodland now left in eastern North America are thoroughly explored by cowbirds. Consequently, many woodland species that may once have been exempt from cowbird attentions are now regularly used as hosts.
A dramatic example of this is available for an especially sensitive species, the Kirtland's warbler. Kirtland's warbler was on the verge of extinction in its limited breeding range in Michigan, partly because of loss of habitat but also because cowbird parasitism was causing ineffective breeding in the remaining warbler population.
A program was initiated to save the warblers. This consisted of creating a good amount of 5- to 6-year-old stands of jack-pine woodland -- the warblers are extreme habitat specialists -- plus the local removal of cowbirds. It is clear that at the time when only a few dozen pair of Kirtland's warbler existed, both of these measures were necessary and that otherwise the warbler would today be extinct. Such aggresssive control measures by people may well become more and more necessary, because today's cowbirds are extremely good at brood parasitism.
-- Richard F. Johnston, is professor emeritus at the Kansas University Natural History Museum and Biodiversity Research Center and a member of the Kansas Ornithological Society.