By Bill Chaisson
My first brown-headed cowbirds of the year were a male and a female on a lawn in Wilmot Flat. Cowbirds are readily identifiable by two contradictory field marks: they walk like a blackbird and have a beak like a finch.
There isn’t much traffic in Wilmot Flat, so I slowed down—I was driving—and took a closer look. The light was good, and it was easy to see the chocolate-brown head of the male, who is otherwise glossy greenish-black. The sexual dimorphism is marked, as it is in some other members of Icteridae, like the red-winged blackbird. Female cowbirds are light brown and almost without fieldmarks, except for faint streaking on their sides.
The cowbird bill is singular among North American icterids; they have a high, somewhat arched culmen (the upper ridge of the beak). Several species of blackbird—bobolinks, all grackles and Brewer’s blackbird—eat seeds, but among them only the bobolink has highish culmen.
The association of beak shape and feeding ecology was studied in “Darwin’s finches.” The members of the subfamily Geospizinae are more closely related to tanagers, not finches. The wide variety of beak shapes and sizes among these closely related birds is thought to be due to specializations in diet that make it possible to divide up the natural resources of the Galapagos.
In his 1939 canonical “Darwin’s Finches,” David Lack includes a chapter called “Beak Differences and Food” that outlines the relationship between beak shape and diet among the 12 then-recognized species. The so-called “ground finch” species in this subfamily have short, conical beaks and eat seeds. The so-called warbler-finches have the least conical beaks and eat mostly insects.
In a 2017 paper in the journal Functional Ecology, Aaron M. Olsen of the University of Chicago looked at feeding ecology and bill shape in waterfowl. He found that waterfowl with beaks that are both high and wide — that is, geese — eat mostly leaves and roots. Species that have beaks that were low and wide — that is, ducks — eat mostly invertebrates. Species with beaks that were high and narrow — e.g. mergansers — eat mostly vertebrates.
As blackbirds go, the bill of the cowbird is relatively high and wide. Examinations of their stomachs find that approximately 78% of their diet is vegetable matter. Grackles and rusty blackbirds eat a lot of grains too, but their bills are not as finch-like as the cowbirds’. But the grackles in particular are considered omnivores, their diets varying widely through the seasons.
This primarily vegetarian diet may explain the changes in the abundance and distribution of the cowbird over time. Before European settlement, cowbirds followed herds of bison across the grasslands of the Midwest, and so were less common or absent in the forested East. With the clearing of the eastern woodlands and the introduction of cattle, cowbirds expanded their range. Their vernacular name is apt, as they are strongly associated with cattle and generally ignore horses, sheep, pigs and goats.
Although I have not seen it written anywhere, it seems possible—but unfortunately not testable—that the brood parasitism for which they are well known (and disliked) may be an adaptation to their attachment to bison. Bison were, after all, constantly on the move. They roamed in such large herds that it took very little time for them to deplete available resources in a given location.
In order to keep up with the peripatetic herd, it would be adaptive for a female cowbird to lay her eggs in the nests of other bird species. This scenario is a classic example of how natural selection works. Other species of birds will occasionally lay their eggs in the nests of other species. That is, the tendency is there generally. Building a nest and rearing young takes a lot of energy and resources, and failure is possible due to predation, bad weather or lack of food.
Bison and cattle trample plants and liberate the seeds, and they eat plants and excrete their seeds. They also stir up insects as they walk. The blackbirds that eat both seeds and insects, feed on the latter most heavily during the nesting season.
If cowbirds depended on bison to stir up enough insects to feed their young, and the bison moved on before a cowbird was finished raising her brood, then cowbirds would face failure more often than other bird species. Any cowbird that laid her eggs in the nest of other species and continued on with the herd would leave more descendants than one that built its own nest. Thus, the individuals inclined to brood parasitism would gradually make it the dominant behavior in the species.
This kind of judgment-free analysis is a later 20th century perspective. Edward Howe Forbush, the starchy New Englander who wrote about the cowbird in his Edwardian-era “Birds of America” believed: “Like the European cuckoo it leaves all family care to others. It might well serve as the emblem of free love.” Forbush is, of course, not referring to hippies, but an earlier generation of Bohemians typified by the Bloomsbury group in England or the wilder members of the Cornish colony in the U.S.
After describing the elaborate courting behavior of the brown-headed cowbird, Forbush wrote: “The offspring of these brief and happy unions are not nourished and cared for by the community, but are foisted on foster-mothers of other species, while the happy, care-free cowbirds, with love and song, enjoy the long summer days.” His lack of approval is painfully obvious.
The cowbird has adapted to living away from rangeland and can be found among the lawns, gardens, orchards, and other open spaces created by suburbia. But they are still most common in agricultural settings. This may explain why their numbers have steadily declined since the mid 1960s as the eastern forests returned. The Breeding Bird Survey database shows a 31% decline in its numbers between 1966 and 2014. It is, however, still quite common, with a population estimated at 120 million birds.
Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now lives and works in the town of Wilmot. Contact him at email@example.com.