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Feathered Parasites

Updated: Jan 19, 2019

Casey McFarland

The “nesting” behavior of Cowbirds takes a distinct turn away from that of other birds. These lovely but sometimes destructive members of the Icterid family do not build their own nests, instead parasitizing those of other birds. Three species Cowbirds are found here in the US: Brown-headed are the most far-reaching, found now in every state, Bronzed occupy only the southern stretches of New Mexico, Arizona, and Texas, and Shiny are relatively recent arrivals to southern Florida.

Watchful females often locate host nests during their construction. This may be done rather sneakily from a perch or by patrolling on foot, as if hunting for the movements of nesting hosts. Nest finding may also be noisy, conspicuous affairs, flapping about from shrub to shrub to flush up nesting females of another species. Once discovered, females covertly visit and inspect a nest while the maker is away, and when the time is right she’ll lay an egg—usually only one per nest—and often tosses out or pecks one of the host’s. Several females may parasitize the same nest, resulting in a clutch of multiple cowbird eggs.

Despite an incredibly low percentage of eggs actually resulting in adults, Cowbirds have exploded in population and range. Large scale landscape alteration (courtesy of good ‘ol Homo sapiens) has created massive swaths of suitable cowbird habitat, but their success as a species is in part due to the unique physiology of female cowbirds: they are venerable egg producing machines. Long reproductive cycles and the ability to produce clutch after clutch allow a female cowbird to the exploit the innumerable nests of various host species over a remarkable period of two months—a Brown-headed cowbird can lay 40 eggs a season.

There appears to be little adherence to any one particular mating strategy, either. Across a species, cowbirds may exhibit a wandering array of sexual behavior: some may be monogamous, some polygynous, and others promiscuous. Cowbird nestlings often hatch sooner than their nest mates, are larger, grow faster, and are especially aggressive little beggars (or little bastards, some are fond of saying), all of which makes the rough road to successfully fledging even tough for a smaller species, to whom a nest "rightfully" belonged. Larger species may be unaffected by the presence of cowbird chicks, but in populations of more vulnerable birds, cowbirds have caused alarming declines.

Brown-headed Cowbirds have targeted over 220 host species, 140 of which have successfully reared young cowbirds. Next spring, keep an eye out for the dark, shiny plumage of cowbirds, and listen for their distinctive, waterdrop-like songs… chances are there’s a number of other species on the look-out for them, too.

A Brown-headed cowbird egg (upper left) placed neatly in the nest of a Lark Sparrow, Oklahoma. Photo by Nick Czaplewski

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