Yesterday, the distinctive, gurgling call of a male, brown-headed cowbird rang through our neighborhood. Soon, his first mate of the season will be checking out nests in the area, waiting for the opportunity to deposit one or two of her own eggs in another bird's clutch; when doing so, she often removes one of the adoptive parent's eggs. Since her eggs tend to be larger, the adoptive parent may discard or destroy them; if not, the cowbird's chicks usually hatch first and, aggressive by nature, get most of the food that the attentive parents bring to the nest. Meanwhile, the female cowbird is off looking for another nest to parasitize and may deposit up to forty eggs (fertilized by a number of males) in various nests over the course of spring and summer; studies have shown that less than 5% of these cowbird chicks will reach adulthood.
Native to the plains of North America, brown-headed cowbirds moved into the eastern States as forest was cleared for farming and suburban sprawl. Though a given female may favor the nests of a specific host, a wide variety of songbirds fall victim to their ploy, threatening the survival of some uncommon species.
Since we tend to view the behavior of animals from a human perspective, many birders despise brown-headed cowbirds, loathing their parental irresponsibility and their impact on innocent avian victims. But nature is rife with parasites and their behavior is purely instinctual; furthermore, such species are essential to the natural balance of life on Earth. Nature, herself, is neither fair nor judgmental.
