A few weeks ago Ron and I were having lunch with some friends in the California Academy of Sciences’ quasi-outdoor snack area, which is cheaper and has a significantly lower decibel level than the indoor food court. As is not unusual in al fresco cafes in San Francisco, we were surrounded by Brewer’s blackbirds waiting for a crumb to drop. There was also a male brown-headed cowbird, who seemed more interested in attracting a mate than grabbing a bite. He walked around, and sometimes under, our table, spreading his wings and singing: a kind of musical gurgle which has been verbalized as “glug glug glee” and “bublowcomseeee.” Alas, no female cowbirds showed up, and the blackbirds ignored him.
I don’t think the cowbird was displaying at us. I have been displayed at a couple of times, an unsettling experience. Once in central Florida a pair of sandhill cranes followed me along a lakeshore, bugling at me. I realized later that they might have seen my red baseball cap and gray sweatshirt as crane colors and pegged me as an intruder who needed to be escorted out of their territory. I’ve also been propositioned by more than one captive parrot.
Brown-headed cowbirds perform pretty much the same song and dance—the topple-over display, so called because the bird finishes by pitching forward as if about to fall on his face--to warn off male rivals and to lure females. The intensity level varies, though. Males confronting other males perform longer than those soliciting females, bow deeper, and spread their wings wider. This difference allowed UC Santa Barbara researchers Adrian L. O’Loghlen and Stephen I. Rothstein to set up an elegant experiment designed to identify what female cowbirds are looking for in a mate.
O’Loghlen and Rothstein played video recordings of both male cowbirds displaying to males and males displaying to females to a captive audience of female cowbirds. Some of the videos were silent; others included the gurgling song. Female interest and/or arousal was measured by the strength of the copulation solicitation display, or CSD (which sounds like one of those forensic cop shows), in which the bird tips her body forward, points her bill up, extends her wings, and elevates her tail.
It might have been predicted that the female cowbirds would be more turned on by the more intense male displays. One of the features of Darwin’s model of sexual selection is that, in bird displays at least, nothing succeeds like excess, to borrow one from Oscar Wilde. The gaudy plumage and acrobatic performances of birds of paradise and pheasants are believed to have evolved through a runaway selection process, with both spectacular male traits and female preference for such traits increasing in the population. That also holds for auditory stimuli like the song repertoire of mockingbirds and marsh wrens. The Australian lyrebirds combine vocal and visual displays.
In fact, the reverse was true in the experiments: females responded to both kinds of displays, but more strongly to the lower-intensity male-to-female displays. Is male aggression a turn-off? Or is something else going on?
A female bird contemplating a prospective mate may evaluate him based on his apparent health and vigor, or on his genetic endowment. For a cowbird, the male’s health is irrelevant. Cowbirds, like Old World cuckoos and a few other birds, are brood parasites. Females lay their eggs in the nests of other species and move on, leaving their offspring to be reared by the hosts. Neither sex is involved with nest construction, childcare, or territory defense. What would be the advantage of choosing a strong healthy male?
The authors point to another set of criteria indicating the male’s age as a surrogate for genetic quality. They suggest that females are actually looking at the color of his underwing covert feathers, which are only exposed during the spread-wing display. These feathers are brown in yearling males, black in mature males. Females also appear to judge males by the complexity of their song repertoires. That may in fact be more important than feather color; O’Loghlen and Rothstein report that females respond more reliably to the playback of songs without a visual component than to silent videos of displaying males. But, since a few yearlings have black covert feathers, song repertoire may be a more reliable clue to age.
What male cowbirds look for is unknown. Even though they’re not required to be providers, males may form seasonal or multi-year bonds with one or more females. (Cowbirds can be monogamous, polygynous, polyandrous, or promiscuous.) Since females are prodigious egg-layers, pumping out up to 40 a year, males may stick around to ensure paternity of as many offspring as possible.