[CT Birds] Bird Thugs

Craig Repasz crepasz at hotmail.com
Thu Mar 22 16:22:12 EDT 2007

Bird Thugs
>From “Science News” March 10, 2007 vol. 171 
We all know that Brown Headed Cowbirds parasitize other birds by laying their own eggs into the nests. They then fly off leaving their eggs to be incubated by the parent couple. The fledgling cowbirds usually grow faster and out feed the other chicks. This leaves the parents bird with a big demanding mouth to feed often at the expense of their own brood. One may wonder why the parent birds do not toss the foreign egg out of the nest. Researchers of the Illinois Natural History Survey in Champaign have concluded that Brown Headed Cowbirds will retaliate against a nest if their efforts to parasitize have been undermined. 
Researchers have removed the cowbird eggs from warbler nests to find that more of the warbler eggs were later smashed or carried off (56% destruction) compared to warbler nests that left cowbird eggs in place (6% destruction.). Clearly it is retaliation. 
Oddly enough the researchers did not witness the cowbirds in their destructive acts. 
Nests that had cowbird eggs removed produced 40% as many warblers as nests that kept the cowbird eggs. There is a degree of starvation of warbler chicks in nests that host an interloper, however, nesting success is still higher then nests that were destroyed by retaliating cowbirds.
Paper to be released in “Proceedings of the National Academy of Sciences.”
Personal note: The warblers are forced to take the lesser of two evils. The real problem may be forest de- fragmentation as a result of urban sprawl that creates both the grass-forest border habitat that is ideal for cowbirds and splinters the warbler’s breeding habitat. 
Your friends are close to you. Keep them that way.

More information about the CTBirds mailing list