Monday, April 28, 2014

Aliens in the Nest!

One of my all-time favorite small birds is the song sparrow (Melospiza melodia). They’re not much to look at, though a little fancier than the average sparrow, with all their stripes, but their voices are absolutely charming.

A pair of song sparrows have made their home somewhere on our property for several years now. I have never been able to find their nest. Not too surprising as they build their nests out of grass and site them in grassy areas. It’s a bit like looking for the needle in the haystack, except the needle is hay-colored!

Despite their reticence in nesting, the birds are relatively unphased by human presence. I don’t know how many wonderful mornings I've spent on my patio being serenaded by a bold little song sparrow. They are also frequent visitors to our feeders, happy to either perch in the feeder or poke around on the ground. 

In addition to never finding their nests, I had never seen them with young until last year. Of course, I’m sure they had them, but with so many little brown birds around, I probably just missed them. Or perhaps the parents simply never brought them to our feeders. Last year, though, it was unmistakable. A pair of tiny birds started chasing the two song sparrows everywhere they went.

Whenever they landed, they would shake their little wings and utter heart-rending cries. I was ready to dig them some worms myself! The parents were on top of things, though, and were constantly ferrying some tidbit to one or the other’s gaping maws. It might have been quite touching, domestic bliss and all, if the little ingrates didn't immediately demand more.

As the season progressed, we kept a vague eye on the family but other issues were more pressing and we lost track of them until the beginning of June when we realized something was massively askew. Hubby spotted it first.

“What the hell have they been feeding those things??”

It took me a minute to figure out who he was talking about then I saw them in the feeder.

The tiny little birds that had followed their parents around so cutely were now twice as big as the parents and looked nothing like them! What the heck had happened? At first, we were afraid it was something we had done. Perhaps by putting out feeders, the parents had overfed the youngsters with disastrous results. Maybe there was something malignant in the feed? Maybe there was a radioactive barrel of toxic sludge under our property! Given our knowledge of the previous owners, this was not something we dismissed immediately.

Fortunately, the wonder of the modern age, the Internet, came to our rescue. After a quick search of song sparrows, all our questions were answered. No, we had not ruined two perfectly good chicks by providing extra food. Nor had the feed been tampered with. The parents had, in fact, been visited by an alien.

No, not that kind of alien.

The alien in question was a brown-headed cowbird (Molothrus ater), a bird a little smaller than a robin. The male has beautiful iridescent black plumage and a brown head, while the female couldn't possibly be more non-descript; a dull brown, black beak, and no particular markings.

Male Cowbird

Female Cowbird

At some point in evolutionary history, cowbirds, once also called bison birds, took to following around herds of grazing animals and living off the insects that infested them. None of these herding animals could afford to stay in one place for any length of time, simply because they ate everything available. So they were constantly on the move and the birds with them. How, then, could the birds successfully reproduce when they couldn't stay in one place long enough to build a nest, much less care for chicks?

Simple. They got someone else to do it for them.

Brood parasitism is the practice of certain birds to leave their offspring with another bird to raise as its own. The parasite bird will leave one, or several, of its own eggs in the nest of some other bird, the host. Sometimes it will also destroy the existing eggs. The cuckoo bird (scroll down a little for a truly weird picture of a cuckoo chick in a smaller bird’s nest) is one of the most well-known brood parasites, but brown-headed cowbirds also make a practice of it, parasitizing over 200 different kinds of bird.

Some birds are able to detect the difference between their own eggs and those of the interloper. Robins, catbirds and blue jays will reliably recognize and eject eggs not their own. For song sparrows, the difference between their eggs and cowbird eggs is not too great. They have similar spotting patterns and the eggs are only slightly bigger than their own, though the difference is noticeable. 

You might think the difference would be enough to boot the intruder out, yes? Are the song sparrows not bright enough to notice the difference? It turns out there may be an entirely reasonable explanation as to why the song sparrows would allow the cowbird egg to remain. You see, cowbirds watch television and apparently they have been watching The Sopranos – nice nest, there; shame if something happened to it.

A pair of researchers discovered that cowbirds will actively retaliate against birds that reject their eggs. Since song sparrows don’t reject the cowbird eggs, the scientists did it for them in experiments, removing the imposter eggs from a certain percentage of nests and leaving others alone. The scientists were also very careful to exclude any other possible predators, allowing access to the nest only to the host bird in some cases and to the host bird AND cowbirds in others.

In the cases where cowbirds were allowed access to the nests, they wrecked 56% of the nests where their eggs had been removed, while only 6% of nests left alone had any damage. In the nests where only the host bird had access there were no depredations at all. The paper also included a discussion of an activity called “farming”, where the cowbird female finds and destroys a nest with eggs then, when the parents rebuild, sneaks her egg into the new nest.

Since the cowbird chick does not destroy the host’s existing eggs or chicks, it is possible for the host to raise both their own chicks and the parasite successfully. True, parasite chicks tend to hatch sooner and grow faster than the host’s chicks but, with a little luck, the host may be able to fledge them all. Beats losing the lot, as they say.

Our pair of cowbirds apparently out-competed the rest of the song sparrow’s brood last year. We never have seen baby song sparrows. Perhaps they’ll have better luck this year. 

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