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Brown-headed Cowbird

Brown-headed Cowbird: Brood Parasite

One has to admire a creature who has managed to eliminate the laboriousness of raising its offspring.  Brown-headed Cowbirds, renowned brood parasites, have done just that.  These birds do not build nests; females lay up to 40 eggs a summer in the nests of more than 220 species of birds which raise their young for them.  Cowbird eggs are generally larger than the host bird’s and hatch in fewer days, thereby putting Cowbird chicks at a distinct advantage over the host’s chicks when it comes to parental attention.

In this photo a Brown-headed Cowbird has deposited three eggs in the nest of an Eastern Phoebe (which has constructed its nest inside an abandoned American Robin nest). Unlike some songbirds, Phoebes do not recognize and remove the Cowbird’s eggs. Neither do they build a new nest on top of the old one, as some smaller songbirds (i.e. Yellow Warblers) are known to do.

Cowbird chicks develop faster than the chicks of the host bird, thereby often getting the first crack at the food parents bring to the nestlings.  Not only are the host species’ chicks often at a disadvantage when it comes to parental care, but they are at the mercy of the Cowbird chicks which often remove both the eggs and chicks of the host. (Thanks to friends in Thetford, VT for the use of their photograph of this parasitized Eastern Phoebe nest. The three larger, speckled eggs are Brown-headed Cowbird eggs; the four smaller white eggs are Eastern Phoebe eggs.)

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Black-and-White Warblers Raising Young

6-14-19 black-and-white warbler nest 0U1A0066One of the first warblers to return to its northern nesting grounds, the (female) Black-and-White Warbler has already built a nest, laid and incubated 4-6 speckled eggs and is now brooding and feeding nestlings.

Finding a Black-and-White Warbler nest can be challenging – these bark-foraging insect eaters usually build their well-hidden nest on the ground at the base of a tree, rock, stump or fallen log. Tucked into vegetation with the rim often at ground level, their 5-inch diameter nest blends into its surroundings. Bark strips, grass, dry leaves and pine needles are used to construct the nest which is lined with moss, horsehair and dried grasses.

The male helps the female feed the nestlings and defend the nest. Female Black-and-White Warblers have been observed performing “rodent run” distraction displays, in which the bird assumes a hunched posture and drags its tail, luring potential predators away from the nest.

The Brown-headed Cowbird is a major threat to Black-and-White Warblers as it frequently parasitizes their nests. One documented Black-and-White Warbler nest in Michigan contained 10 eggs, eight of which were laid by cowbirds. (Thanks to Susan and Dean Greenberg for photo opportunity.)

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