See Cowbird Biology with information on nesting behavior. Photo below by Bet Zimmerman of CT
LEFT: Photo by Bet Zimmerman of cowbird egg in Black-capped Chickadee nest in a Gilwood box. This nest was abandoned.
Photo of Titmouse nest with cowbird egg by Jay Brindo of Ohio.
A cowbird egg (top) in this open cup nest (probably a Song Sparrow) in a holly bush. Photo by Jodie Tolbert of Hendersonville, North Carolina.
Cowbirds seem to prefer open cup nests, and the nests of other birds that also lay speckled eggs.
A cowbird that hatched in a Black-capped Chickadee nest in another Gilwood box. I don't know how many eggs there were total. When I checked the box again, the chickadee eggs were gone.
Egg Description: Eggs are oval with variable shape, with a moderately glossy, granulated shell, white or grayish white, evenly dotted with brown/ reddish brown/gray, sometimes with heavier markings at the large end. Granulated, moderately glossy. Similar to a House Sparrow egg, but larger (size varies). The eggs of the Bronzed cowbird are pale bluish-green and have no markings.
Photo on left: Cowbird nestling (same as photo directly above). Two days later when I checked this box, the nestling was dead. I assume it was abandoned. Photo by Bet Zimmerman.
Cowbirds mostly eat seeds (75%) and arthropods.
Photo below by Jay Brindo of Ohio, probably one cowbird egg (the larger one) in a House Sparrow nest.
Nest Description: Cowbirds do not build their own nest - they lay eggs in the nests of other birds, depending on the host to incubate and raise their young.
Since cowbird young tend to hatch earlier, their eyes open soon, and their mouths are bigger than the host's young, the rest of the nestlings often die.
Some nestbox monitors remove cowbird eggs when they are found. Technically, in most states this is illegal, as cowbirds are native birds. In some states (e.g., TX) where cowbirds threaten certain declining bird species (e.g., Kirtland's Warbler) permission may be obtained to remove cowbird eggs and to trap adults.
Cowbirds are short distance migrants. They often flock and roost with Red-Winged Blackbirds, Common Grackles, European Starlings and American Robins, making up 1-3% of those mixed flocks. (BNA.)
The student of Nature wonders the more and is astonished the less, the more conversant he becomes with her operations; but of all the perennial miracles she offers to his inspection, perhaps the most worthy of admiration is the development of a plant or of an animal from its embryo.
-Thomas Henry Huxley, British biologist and educator. Reflection #54, Aphorisms and Reflections, selected by Henrietta A. Huxley, Macmillan, 1907.
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