Excerpts from Jim Conrad's
from the April 21, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve, QUERÉTARO, MÉXICO
A HORMONE-CRAZED BRONZED COWBIRD
For the last couple of weeks here at Reserve HQ a certain Bronzed Cowbird has been making a nuisance of himself. Early each morning he attacks his own reflection in car mirrors, even his much-distorted likeness in curved, shiny bumpers. I know it's the same bird because somehow he's lost all the feathers on one of his legs, presumably from repetitively scraping his leg against a hard, reflective surface. We've all seen birds up north do this, but I'm surprised to see a Bronzed Cowbird behaving in this manner.
For, Bronzed Cowbirds, like their northern Brown- headed Cowbird counterparts (they belong to the same genus), are nest parasites. Females lay their eggs in the nests of other species, then the other species rears the earlier-hatching cowbird's young, often to the detriment of their own less offspring. Probably you've seen those pictures of a duped parent feeding a cowbird juvenile much larger than the parent, as at http://www.birds.cornell.edu/conservation/tanager/parasitism.html.
My surprise in seeing a Bronzed Cowbird attacking his own image arises from this question: Since cowbirds don't build their own nests or feed their own young, and I see small flocks of mixed males and females flying about behaving very amicably with one another, of what need is there ever for a male to fight another male -- especially in the sustained, leg-defeathering manner I'm witnessing here?
Since Bronzed Cowbirds extend into the US only in the border states and southern Louisiana, relatively little study has been done on them. However, there's plenty of literature on Brown-headed Cowbirds, since they occur throughout most of North America. On the Internet I find reports of male Brown-headed Cowbirds attacking one another -- and causing as much head-scratching among behavior-oriented birders as my Bronzed is causing down here.
In an online edition of part of the September, 1973 edition of "The Wilson Birder" there's an article by Peter L. McLaren of the University of Toronto entitled "Physical Combat in the Brown-headed Cowbird." The species is described as territorial during part of its breeding season, but, according to the literature at that time, "Battles of any intensity are apparently unknown."
McLaren describes a hard-fought battle between two males he saw in a local park, lasting for three minutes and 15 seconds. After reviewing reasons why Brown-headed Cowbirds don't need to fight, to explain the battle he had just witnessed McLaren writes:
These are just guesses as to why male cowbirds would fight one another, but I can't come up with any better explanation. For our cowbird to have kept this up during every sunny morning for two weeks, it's even more extraordinary. It is so inexplicable that I have confirmed my identification several times, but it's hard to go wrong with this species: Red eyes, cowbird bill, inflatable ruff on the hind neck, and that raunchy, high-legged, pelvis-grinding movement he does when displaying. Couldn't be anything else. It's all a mystery and I'd appreciate hearing from others with ideas. A Bronzed Cowbird inflating his ruff is at http://www.mangoverde.com/wbg/picpages/pic204-36-2.html.
But, lots of questions surround cowbird behavior. For instance, since the young are reared by other species, how do they recognize their own kind when it comes time to reproduce?
Other questions deal with what we should do with cowbirds, if anything, since their nest parasitism has been shown to be a primary cause of lowered reproductive success of birds nesting in forest- interiors in fragmented habitats.