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:
Thus, when fighting does occur, it may be
because the individual has either not correctly interpreted an opponent’s display, or
its sexual (aggressive) drives are too strong to be fulfilled by a display. If this is in
fact so, then an encounter of the duration and intensity described above becomes all the
more unusual.
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. |