Brood Parasitism
I've recently noticed our local Northern Cardinals carrying nesting material into a Japanese Magnolia in our yard. This morning, Denise told me she saw a cowbird hanging around the tree. I climbed up and found a nest with three eggs, two brown-speckled cardinal eggs and the solid blue-green egg of a Bronzed Cowbird.
Cowbirds are brood parasites, laying their eggs in the nests of other species. A favorite host species is the cardinal. The common species in the U.S. is the Brown-headed Cowbird. Much less common, the Bronzed Cowbird is largely restricted to Mexico and Central America, but they occur in small numbers in the states bordering Mexico and in Louisiana.
Cowbird eggs tend to hatch ealier than the eggs of the host species, and cowbirds usually target smaller species as hosts. So, cowbird nestlings out-compete the nestlings of the host. I've been called several times by confused friends who had witnessed a cardinal feeding a larger black bird. Although similar in size to the cardinal, cowbirds also lay eggs in the nests of much smaller species, such as vireos, warblers and wrens.
While posing no threat to the abundant Northern Cardinal, cowbirds do put pressure on the populations of less common species, especially certain neotropic songbirds. Cowbirds' preferred habitat is open grasslands and fields, and Brown-headed Cowbird numbers exploded with the removal in the 19th century of the forests of the eastern U.S. Not wanting to see my local cardinals lose a brood, I removed the cowbird egg. Here it is:
I moved the egg from the cooler cloth, on which it appears above, to a warmer, more neutral poster board, and it rolled off the table and broke.
Cowbirds are brood parasites, laying their eggs in the nests of other species. A favorite host species is the cardinal. The common species in the U.S. is the Brown-headed Cowbird. Much less common, the Bronzed Cowbird is largely restricted to Mexico and Central America, but they occur in small numbers in the states bordering Mexico and in Louisiana.
Cowbird eggs tend to hatch ealier than the eggs of the host species, and cowbirds usually target smaller species as hosts. So, cowbird nestlings out-compete the nestlings of the host. I've been called several times by confused friends who had witnessed a cardinal feeding a larger black bird. Although similar in size to the cardinal, cowbirds also lay eggs in the nests of much smaller species, such as vireos, warblers and wrens.
While posing no threat to the abundant Northern Cardinal, cowbirds do put pressure on the populations of less common species, especially certain neotropic songbirds. Cowbirds' preferred habitat is open grasslands and fields, and Brown-headed Cowbird numbers exploded with the removal in the 19th century of the forests of the eastern U.S. Not wanting to see my local cardinals lose a brood, I removed the cowbird egg. Here it is:
I moved the egg from the cooler cloth, on which it appears above, to a warmer, more neutral poster board, and it rolled off the table and broke.
posted by David L'Hoste at 1:00 PM
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