Brown-headed cowbirds (Molothrus ater) have long been considered the "bad boys and girls of the avian world," says Cornell University doctoral student Mark Hauber. Cowbirds are brood parasites: They lay their eggs in other birds' nests, leaving the hosts responsible for feeding and nurturing their young, often to the disadvantage of a host bird's own offspring.
However despicable, cowbirds' parasitic behavior inspires many interesting questions. How, for one, does a cowbird chick know that it is a cowbird? The nestlings are fostered by unrelated parents, whether sparrows or warblers, and it isn't until five or six weeks of age, when they have become fully independent of their host parents, that the juveniles seek out other cowbirds. Recognition of one's own species might be a genetically determined trait, or perhaps it also involves learning. The fledgling cowbirds' unusual situation, Hauber and his colleagues thought, might offer a chance to answer this question.
So it was that Hauber, along with his advisor, behavioral ecologist Paul Sherman, and Dóra Paprika of Albert Szent-Györgyi Medical University in Hungary, began painting young cowbirds.
In a set of creative experiments, the scientists dyed the feathers of 14 nestlings over a three-year period, using a nontoxic, permanent, black Sharpie pen. The gray-streaked feathers of the control nestlings remained unchanged. (To control for nonvisual variables related to the painting, such as scent and texture, the undersides of their wings were painted.) The nestlings were removed from the nests of song sparrows and eastern phoebes shortly after hatching and reared separately in visual isolation from all birds.
Just before molting (at approximately two months of age), the nestlings were introduced to two adult female cowbirds, one painted and one normal. These encounters were the young cowbirds' first visual exposure to other birds. Females were chosen because their plumage is most similar to that of pre-molt juveniles. The team noticed that as the trials progressed, the cowbird nestlings' behavior became more and more "birdlike." And they found that, as the birds became more social, the painted young cowbirds approached the painted adults with significantly more frequency than they approached the nonpainted birds. The young nestlings, that is, tended to choose as social partners those birds most similar to themselves in appearance.
What this means, Hauber says, is that the young cowbirds incorporated the wing-feather modification into what he calls their "recognition template." In other words, the cowbird nestlings demonstrated the ability to inspect themselves, memorize certain characteristics and seek out individuals exhibiting those same characteristics to be social partners. Hauber and his colleagues consider this study, published in late 2000 in Animal Cognition, to be "the first evidence of self-referent phenotype matching through experimental manipulation of a recognition cue."
Phenotype matching is the process by which individuals inspect certain characteristics of their relatives or social partners and use those characteristics as a recognition guide when encountering unknown individuals. The idea that members of a given species might recognize each other by inspecting themselves (self-referent phenotype matching) was dubbed the "armpit effect" by Oxford biologist Richard Dawkins in 1982. Dawkins thought that if individuals were given the wrong cue (say, by a foster mother or host nestling), they might inspect themselves, and in people, the closest analogy would involve sniffing our armpits to determine our own smell.
The first conclusive evidence of the armpit effect was provided in April of 2000 by Jill Mateo and Robert Johnston at Cornell. By cross-fostering golden hamsters, they concluded that the hamsters were able to recognize family members they had never met by sniffing them and comparing the unfamiliar animal's odor with their own. A shared genetic composition should be reflected in a related hamster's odors, the scientists reasoned. What their results didn't show, however, was whether the hamster's familial recognition had any kind of learned component or was largely genetically determined. Uncovering a learning mechanism requires trying to "fool" an individual by experimentally manipulating its phenotype and documenting any resulting recognition errors, say Hauber and Sherman.
Hence the painted cowbirds. And the next step, according to Hauber, is to consider other aspects of self-referent phenotype matching. He is investigating whether vocal cues might provide another characteristic pertinent to conspecific recognition. Perhaps the juvenile cowbirds listen, hear a cowbird-specific call, approach and then visually discriminate in order to identify conspecific social partners.
Finding birds of one's feather doesn't necessarily require any psychological awareness on the part of the cowbird. Nevertheless, learning is a key part of the process. Undoubtedly, says Hauber, both the armpit effect and genetic determination are at work as a fledgling cowbird begins its search for conspecific companions.
The juvenile cowbirds are not unlike the ugly duckling from Hans Christian Andersen's tale. As Andersen says, "to be born in a duck's nest, in a farmyard, is of no consequence to a bird, if it is hatched from a swan's egg." Alas, no matter how one paints a cowbird, it will never be a swan.—Rebecca Sloan Slotnick