Bronzed Cowbird, MOLOTHRUS AENEUS

Excerpts from Jim Conrad's
Naturalist Newsletter

from the April 30, 2017 Newsletter issued from Rancho Regenesis in the woods ±4kms west of Ek Balam Ruins; elevation ~40m (~130 ft), N20.876°, W88.170°; north-central Yucatán, MÉXICO
BRONZED COWBIRDS
One morning this week a small flock winged into the rancho, resulting in the picture below. Below, you can see a male with his neck feathers fluffed out, attended by a female who apparently finds it alluring:

from the April 11 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
COWBIRD LEWDNESS
The other day I stepped from my door and right above me in a leafless Cedro tree a male bird was gesturing toward two nearby females in a way that most people would interpret as lewd. You can see what I mean below:

The birds were Bronzed Cowbirds, sometimes called Red-eyed Cowbirds, MOLOTHRUS AENEUS, distributed from the southwestern US south to central Panama. They belong to the same genus, Molothrus, as the North's Brown-headed Cowbird, and probably you know that Brown-headed Cowbirds are "brood parasites." That means that female cowbirds lay their eggs in the nests of other bird species, causing the other species to do all the work raising the cowbird young. The female cowbird frequently pierces and thus kills the host's eggs already in the nest.

Bronzed Cowbirds also are nest parasites. So far 82 bird species are known to be parasitized by Bronzed Cowbirds. Thus if any bird species should be guilty of lewdness, we might expect it to be this one, even if we know how anthropomorphic the whole concept of lewdness is.

In the picture the male at the top, left is performing his "bow display." Cornell Lab of Ornithology's Bronzed Cowbird page describes the display as "Directed towards female by male. He first lifts the feathers on the back of the neck, followed by those of the upperparts, and finally those of the underparts. The tail is brought forward and under the body while the wings are arched slightly and head is bent."

If you visualize the above description -- the tail brought forward and under the body while the wings are arched slightly and the head bent -- you realize that here we're basically talking about a fully executed pelvic thrust.

But, birds don't have real pelvises and, if the truth be known, the vast majority of bird males don't even have penises (chickens and turkeys notably excepted). In typical bird-sex, males introduce sperm into female bodies by pressing their sexual openings against the female's sexual opening, kiss-like. Usually this is accomplished with the male mounted atop the female, tottering and flapping his wings to keep from slipping off.

You can imagine that the Bronzed Cowbird's bow display is rooted in the interspecific male urge to present himself as big and powerful, and ready to boogie, as well as the interspecific vulnerability of many females to such highly suggestive, macho approaches.

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 https://nestwatch.org/learn/general-bird-nest-info/brown-headed-cowbirds/.

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.

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.