Re: Warbling Vireo and brown-headed cowbird chick, Mammoth

Observers: Chris McCreedy
Email: cmccreedyatprbodotorg
Verification: MONO
Remote Name:
Date: 08/01/2007
Time: 06:30 PM -0400


This is an interesting debate. I think there are two points within; the first, that cowbirds have been brood parasites for quite a long time, and the second, whether or not they are native to California. I do not have citations in front of me, but from what I know, Brown-headed Cowbirds are native songbirds that have been brood parasites for centuries and beyond. However, with the decrease in megafauna populations in the western United States (perhaps due to humans), cowbirds appeared to have suffered a range contraction. The fossil record has them in the western United States several thousand years back, but they had disappeared from the western United States in recent centuries. With the introduction of agriculture and grazing to the deserts, human subsidization of cowbird populations enabled them to return to the eastern Sierra Nevada in the past one hundred years. It is a question of subsidization: cowbirds cannot survive in contemporary desert habitats without humans provisioning them with easy-access food supplies through agriculture, grazing, bird feeders, and urban feeding grounds such as lawns, fast food joints, etcetera. They sort of transcend the definitions of "native/exotic" because they are native to the United States and once existed here. With the loss of megafauna, for which humans are probably partly to blame, they disappeared. And because of human-caused ecological alteration, they are back. What is important is that the fragmented nature of Western habitats make it much easier for cowbirds to access many more songbird nests. Cover is less than in the east, and in cowbirds' absence, many potential host species have failed to maintain anti-cowbird behavior at the nest. In consequence, western songbirds have been heavily and negatively impacted by cowbirds in the last one hundred years.