A question came in to my blog in the last few days and someone asked if she should pull a cowbird egg from a nest. This is an interesting question. For people not familiar with brown-headed cowbirds, they are nest parasites. A female cowbird will lay her egg in another bird's nest. Often the adult bird raising the cowbird is half the size of the young cowbird. Sometimes other nestlings will not survive and sometimes they will, depends on the species. The cowbird can contribute to the decline of species on the brink, like Kirtland's warblers by having the adults "waste time" by raising a cowbird and not another Kirtland's warbler.

For years it was believed that cowbirds evolved this pattern because they followed herds of bison eating insects. That nomadic lifestyle left little time for raising young. As bison disappeared, cowbirds started following humans...who are rather sedentary and now a female cowbird who would lay 60 eggs in a summer over several miles is now laying those 60 eggs in one neighborhood. Makes sense...that is until Alvaro stepped in.

Alvaro Jaramillo who wrote the book about blackbirds (literally, he wrote the book on blackbirds) has this theory:

“This cannot be true. If you look at the evolution of the cowbirds (the entire genus Molothrus) you will find that they most likely arose in South America where there are no bison. As well, the Shiny Cowbird and Brown-headed Cowbird appear (based on the phylogeny developed by studying variations in mitochondrial DNA sequence) to be the most recently derived species in the cowbird clan. Thus, by the time that the Brown-headed Cowbird arose as a species it was already part of a lineage of brood parasitic birds, it was not the first. Cowbirds were ‘pre-adapted’ to a nomadic lifestyle due to their brood parasitism, but this behavior did not evolve due to a ‘need’ to follow Bison.”

Arrrrrrrg!

Officially, the answer is that it is illegal to remove a brown-headed cowbird egg from a nest. They are a native species and therefore protected under the Migratory Bird Treaty--unless you have a depredation permit from the federal government to remove them as in the case of Kirtland's warblers.

Do one remove it regardless of the legality? Is removing one cowbird egg helping an overall bird population? I've posted the same question on Facebook and people have opinions. How about you?