Speaking of Nature

Speaking of Nature: The ‘Mafia behavior’ of cowbirds

Sunday morning was surprisingly bitter. There was a stiff wind, there was no sun and the thermometer read 16 degrees. The warm weather of the preceding week had knocked the depth of the snow down quite a bit and in places the grass was plainly visible, but the scene provided only an illusion of warmth. Spring is, after all, another four weeks away.

But when I stepped out onto the porch, I found my senses confused. The cold wind cut through my shirt and had almost chased me back into the house for a jacket when my ears picked up the mellifluous sound of bird song. Somewhere down in the thicket at the bottom of the hill a male cardinal had been overwhelmed by the promise of spring’s return and decided to herald her coming with his sweetly whistled serenade. At least that’s how a poet might explain it.

The biologist might offer an explanation of the effects of day length on the brains of birds. Glands once dormant are awaken and they begin to secrete hormones into the bloodstream that will revive the reproductive organs from their regularly scheduled seasonal atrophy. Genetic programming kicks into gear and the winter male that is reasonably concerned with survival becomes the breeding male that is unreasonably concerned with mating. Every song is a plea for attention, but predators might be out there just as easily as females.

Cardinals were not the only birds to behave so giddily that morning. Woodpeckers were drumming, house finches were singing and even a male mourning dove offered a brief performance of his melancholy cooing. To hear such a mournful sound coming from such an elegant bird is really unexpected.

So the sings were all there that spring was on her way. But just in case I hadn’t paid attention to the sounds I was hearing, I was provided with a more definitive visual clue just a shortly thereafter. While marking the sounds of bird song in my journal, I happened to look up from my writing in time to catch a glimpse of a very dull and very drab visitor. Her presence overwhelmed the imagination where her plumage may have underwhelmed the eye. It was none other than a female brown-headed cowbird.

A species often reviled by birders, the brown-headed cowbird (Molothros ater) is native species that has managed to take advantage of the human reorganization of the landscape. Prior to the arrival of European colonists, the brown-headed cowbird was a species of the Great Plains. Bison in the millions migrated in a pattern similar to those followed today by the great herds of Africa. And as the bison migrated, so did the cowbirds.

Because of this relationship, the cowbirds must have found it difficult to put down roots in one place for the purpose of nesting. The bison created an ideal situation for the cowbirds by clearing the tall vegetation and kicking up insects in the process. Buffalo chips must have also attracted a multitude of different insects, all of which represented a bounty for the birds.

So, the cowbirds dabbled with the idea of letting others raise their young. This is not a particularly uncommon strategy in the world of birds and it is employed by species from many different families. Why spend too much energy raising your own offspring if someone else might do it for free?

Eventually the cowbird became a nest-parasitism specialist. A balance between the parasites (the cowbirds) and the hosts (the unwitting parents) was struck and the strategy worked well enough for all parties concerned. Then humans arrived and changed the balance.

Starting in the East and moving steadily to the West, the growing nation that became the United States saw a great transformation unfold. Vast forests that had once covered the eastern half of the continent were leveled for building material, fuel and to make room for people. Those people had to eat something, so more forest was cleared to make room for farms. Humans like milk and they seem to love beef, so many of those farms had cows and cattle. The cowbirds noticed the change.

While Europeans expanded westward, the cowbirds expanded eastward. New territories also provided them with new hosts and an entire ecological community of birds that had never had to contend with the likes of cowbirds before. This meant easy pickings for the cowbirds and a long downward decline in the populations of the species so recently exposed to the threat. The cowbird is booming, while the host species are going bust.

I am ambivalent toward cowbirds. I see them as wild birds, native to this continent, that are simply taking advantage of a new situation. I also see them as a serious threat to other species of native birds that have been exposed to danger at the hands of human meddling. Unlike starlings and house sparrows, cowbirds are protected by law and rightly so. In only a few situations, is it reasonable to remove the cowbird from the equation. In the case of the endangered Kirtland’s warbler, the species might go extinct if cowbirds were not vigilantly removed with extreme prejudice.

There is even evidence to suggest that the cowbirds, which have kept their parasitic strategy even though they have become more sedentary in their movements, might be taking a more active role as parents. Since they are no longer required to follow the bison and leave their eggs behind, female cowbirds apparently check in on the nests that they have parasitized from time to time. If a host bird has noticed the addition of an unfamiliar egg and has removed it, there is even evidence to suggest that the cowbird might take revenge.

Birds that have rejected cowbird eggs may return to their nests to find that the entire nest and its clutch of eggs has been destroyed. A very clear message of, “you kill my kid, I’ll kill your whole family!” is thus sent. A study done by the Florida Museum of Natural History described this as “Mafia behavior.”

In this case, it is the females that are responsible for the majority of the trouble. Male brown-headed cowbirds are almost completely distracted with the process of courtship and mating. In pursuit of these endeavors, the males have evolved a gorgeous plumage consisting of a chocolate-brown head and iridescent black feathers covering the rest of the body. When trying to impress a female, a male will puff up his feathers, spread his wings and emit a song that sounds like bubbles of mud boiling up and releasing squeaky steam.

When trying to impress each other, the males will stand toe-to-toe and point their beaks straight up in the air. This is a form of ritualized combat that prevents unnecessary fighting between unevenly matched males. I always image the males are daring each other with comments like “You wanna go? Huh? You ready? You wanna?” If no obvious winner can be predicted, they might actually go for it.

So, as the wind continues to howl at your door and the temperatures continue to discourage your imagination, I want you to consider two things. First is this: the days are getting longer and at a rate of two minutes a day we will have an extra hour of daylight in just a month. Second we have this: some of the birds are already returning. Spring cannot be far off now!

Bill Danielson has worked as a naturalist for 16 years. In that time, he has been a national park ranger, a wildlife biologist and a field researcher. He currently works as a high school chemistry and biology teacher. To contact Bill, or to learn more about his writing, visit
www.speakingofnature.com

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