With the return of songbird nesting season, it’s time for cowbirds to get busy! But not everybody will be happy about that. Cowbirds are a native species, but their reputation is as invaders who displace other more “desirable” birds. The journal Birding even published an article (Vol. 26, pp. 254–257) entitled “An open solicitation for cowbird recipes.” Why are they so disliked?
Cowbirds are in the family Icteridae, related to blackbirds and orioles. Our local species, the brown-headed cowbird, Molothrus ater, occurs across the U.S. and is the only cowbird in Ohio. Brown-headed cowbirds are rather drab; the male sports a brown head and glossy black body, while females are brown all over. The name “cowbird” derives from their historical habit of following large mammals—first bison, then horses and cattle—to find their seed and insect foods, but they now occur widely in open habitat, and their numbers have increased as humans have cleared and fragmented forests.
The cowbird trait that disenchants bird lovers, but that raises many fascinating questions, is their mode of reproduction. That is, cowbirds are among a small number of birds, and the only ones in North America, that lay eggs in the nests of other species rather than build their own nests. Those host parents then do all the work of raising the cowbird nestlings. Other so-called “brood parasites” include some cuckoos, African honeyguides, and a few other species.
Several aspects of cowbird nest parasitism contribute to their success. Among all nest parasitic birds, cowbirds are the least selective; they have been recorded laying eggs in nests of more than 200 species. They also are prolific egg layers. Most songbirds lay a clutch of four or five eggs, and they may repeat that two or, perhaps, three times per season. In contrast, cowbirds lay nearly an egg a day throughout their breeding season, totaling 30 or more eggs per year. Those eggs hatch slightly ahead of the host eggs and the chicks often are larger than those of the host. (Many cowbird host species are small, like sparrows, warblers, and wrens.) The host parents are highly stimulated to feed the large gaping cowbird mouths, and smaller host chicks may suffer food deprivation as a result.
For nest parasitism to work, the cowbird needs to find a host nest and lay its egg at just the right time. How does it do that? The answer is rather simple but impressive: cowbirds have great powers of observation. When breeding season arrives, females start surveying the landscape, often from elevated perches. They are experts at spotting nests and interpreting host behavior. They wait until the host is just beginning to lay eggs, then usually lay their own eggs in early morning, when dim light makes them less conspicuous. The female often removes a host egg in the process. Those exceptional “puzzle-solving” skills are supported by the hippocampus, a brain structure critical to memory formation, which is larger in female cowbirds than in males.
Why don’t hosts reject the cowbird egg? In most cases, that egg does not match the appearance of the host eggs. Despite that, most North American host species tolerate cowbird eggs. (In a few exceptions, such as robins, catbirds, and warbling vireos, the host does resist by puncturing, ejecting, or burying the invader egg.) There certainly are costs to accepting the cowbird; fledging success of host chicks can be reduced by 50% or more in parasitized nests. But there may be costs of rejecting the cowbird egg, too; for example, cowbirds may return to destroy the host eggs in nests where their own egg has been rejected. If the host can re-nest later in the season, there’s a lower risk of cowbird parasitism, which wanes through the summer. Continuing evolution may lead to more selectivity by cowbirds and greater rejection by hosts, as occurs in European cuckoos.
Like other birds, cowbirds have distinct, recognizable vocalizations. (It’s a stretch to call the male’s milky whistling a “song”!) But the chicks grow up mostly hearing their hosts, not their blood relatives. How do they learn to “speak cowbird”? Studies of brain structures important for singing suggest that the window for song learning stays open longer for cowbirds than for many other species. So, cowbird chicks can learn their correct vocalizations even after leaving the nest. Cowbird chicks also may sneak away (and return) occasionally as they become able to fly, so that they start spending time with other cowbirds even before they leave the care of their host parents.
Cowbird chicks also face another challenge. That is, they are exposed to the pests—lice and bacteria, for example—of whatever host nest they parasitize. As such, they must be able to fend off infestation of more diverse pathogens than a typical songbird. To assist with that, cowbirds have more robust immune systems than other songbirds. In particular, their innate immunity—the systems that provide non-specific, fast-acting defenses against diverse pathogens—is unusually strong, even compared with related species like red-winged blackbirds (which are not nest parasites) or bronzed cowbirds (which are more selective nest parasites).
So, it is true that cowbirds invade other species’ nests, and that they have abandoned their duties as parents. Moreover, cowbird parasitism can, sometimes, endanger the survival of birds with small populations. For example, trapping and removal of cowbirds has been critical to sustaining Kirtland’s warblers nesting in the jackpines of Michigan. More often, though, evidence that cowbirds cause declines of host populations is scant. Cowbirds may not be the most beautiful or melodious of songbirds, and they may have habits that give us pause. But they do have some remarkable special talents, and really, they are worthy at least of tolerance, if not of grudging respect.
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.