Brown-headed Cowbird - A Different Kind of Parasite
The cowbird is known as a parasitic breeder, because instead of building a nest itself for its eggs, it spies on other bird species, and before they start sitting on their own eggs, cowbirds sneak in and lay their egg in the host bird's nest -often more than once. In general, female birds do not usually lay all eggs at once. She may lay one a day for several days, and not start incubating the set until the last one is laid. So she does not have to stay in the nest until then.
Later, the cowbird eggs usually hatch first, and their babies are typically bigger that the host bird who built the nest. Cowbird babies often get food from the adult birds first, and get the most food, as they are more aggressive than the other young birds that belong in the nest. In this picture of a House Finch nest taken by Locke Nunez, the large, darker egg is from a cowbird, as is the already-hatched chick, while the two other remaining eggs are from the house finch. There probably were more house finch eggs, but cowbirds are known to push some of the host bird's eggs out of the nest, to give an advantage to their eggs. And since cowbirds do not help feed or raise any of the young birds in any nest they use, they can lay eggs in several other nests as well, yet another advantage to their species. Some host birds have ways to try to combat the cowbird behavior. Some will abandon their nest and eggs start over in a new nest; some will cover their own eggs and the cowbird eggs in their nest with a layer of nest materials and will start a new set of eggs on the 'a second floor' of their nest; some will push the cowbird eggs out of their nest to protect their own set of eggs. Peterson Field Guide to Eastern Birds' Nests |
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