Friday, June 26, 2015

The Audacity of the Cowbird

A surprising observation outside my window sent me searching through my bird books yesterday.

For several days my daughter had been pointing out a bird we've not seen at the feeder before. While I pride myself in knowing all the species in our backyard, I didn't recognize this one. It was about the size (a little smaller) and shape of a blackbird, beak and all, but was a light mousy brownish gray all over. It had a slightly darker eye-stripe and a barely discernible pattern of lines on the folded wing feathers. In general it was a dull and unremarkable bird, and might have gone unnoticed, but for its unusual behavior of quivering on the bannister for long minutes at a time with wide gaping beak, giving us a good look at its finer features.

I finally decided it was a new fledgling that hadn't yet developed its adult coloring and that it must still be confused about how to survive in the wide world. When smaller birds came to the feeder, it stepped the begging up a notch. Poor thing, I thought. I hope he finds his own parents or figures out how to get his own food before the neighbor's cat finds him. He won't last long in this world by sitting and begging.

But then, oh surprising then, his begging paid off. A tiny frail song sparrow sidled up to the insistent youngster thrice her size and, I could hardly believe my eyes, stuffed a small morsel of food into that gaping throat with great fluttering effort. Whaaat? This I had never seen, an adult of one species feeding the young of another.

After much page flipping and reading, I learned I had witnessed the unhappy result of the invasive behavior of the brown-headed cowbird. Unhappy for the song sparrow, that is---pretty happy for the cowbird, who lays and abandons its eggs in smaller passerine/perching birds' nests, leaving its young to be incubated, hatched, and cared for at the sacrifice of the host parents' young. What we had seen was a fledgling female brown-headed cowbird being fed by her foster mother, a beautiful little song sparrow, run ragged by the cowbird's insatiable appetite.

I could forgive this young cowbird its instinctual behavior if it hadn't happened at the peril of the devoted song sparrow's rightful eggs. I can't say for sure what happened to them, but I can guess. I learned that cowbird eggs hatch in only ten days, compared to the fourteen days of the chosen host's eggs, and that the first-hatched cowbird nestlings push the other eggs out of the nest so as to hoard all the food for their own greedy selves.

Today, here they are again. Now the cowbird follows the song sparrow more aggressively, and the sparrow appears to be trying to get away---but every now and then she gives in and feeds the big crybaby.

Cowbirds never build nests. They always rely on other birds to rear their young. This results in a 90% failure rate for cowbirds, yet they are replacing themselves fast enough to be expanding their own population, while reducing the populations of the more desirable passerines they victimize.

I wish I didn't know this. I can't help making the application to the human world, and the thought is alarming. dkm