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The brown-headed cowbird is an obligate brood parasite; it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors.
A shiny cowbird chick (left) being fed by a rufous-collared sparrow Eastern phoebe nest with one brown-headed cowbird egg (at bottom left) Shiny cowbird parasiting masked water tyrant in Brazil. Brood parasitism is a subclass of parasitism and phenomenon and behavioural pattern of animals that rely on others to raise their young.
The female cowbird may continue to observe this nest after laying eggs. Some bird species have evolved the ability to detect such parasitic eggs, and may reject them by pushing them out of their nests, but the female cowbird has been observed to attack and destroy the remaining eggs of such birds as a consequence, dissuading further removals.
Song sparrows' nests are parasitized by the brown-headed cowbird. The cowbirds' eggs closely resemble song sparrows' eggs, although the cowbirds' eggs are slightly larger. Song sparrows recognize cowbirds as a threat and attack the cowbirds when they are near the nest. There is some evidence that this behavior is learned rather than instinctual ...
Brown headed cowbird egg in an eastern phoebe nest There are several species that will increase their offspring's chance of survival through a means that is slightly different than egg tossing, which is brood parasitism .
Two of the five Brown-headed Cowbird eggs produced fledglings. The study did not determine if there was a difference in nest success between parasitized and nonparasitized nests. [51] In West Virginia, only 3 of 41 eastern towhee nests were parasitized by the Brown-headed Cowbird.
Shiny cowbird: Molothrus bonariensis (Gmelin, JF, 1789) At least 102 species [7] Brown-headed cowbird: Molothrus ater (Boddaert, 1783) At least 174 species [7] Screaming cowbird: Molothrus rufoaxillaris Cassin, 1866: Most commonly Agelaioides badius, occasionally four other species [d] [8] Giant cowbird: Molothrus oryzivorus (Gmelin, JF, 1788)
One study found that 9% of nests had brown-headed cowbird eggs in them. [32] It likely evades parasitism partly because of its late breeding season. [33] The American goldfinch also makes a very poor host for brood parasites, with studies showing low hatching rates of brown-headed cowbird eggs and no fledging success.