Last time, I took a quick tour of the various sorts of nests birds build, again showing how many ways there are to be a bird. The brown-headed cowbird not only skips nest building, it doesn’t even raise its own young. If you have teenagers, you might find this idea to be appealing.
The brown-headed cowbird is not alone. Other species of cowbirds in the genus Molothrus — screaming, shiny, bronzed and giant — do the same thing. They lay their eggs in the nests of other species and then walk away. Well, fly away. These species are called brood parasites because they parasitize other species’ broods. Like other parasites, they take, and they do not give.
There are other groups of brood parasites, including indigobirds, whydahs and honeyguides in the Old World. Several species of cuckoos in both hemispheres are brood parasites, and this evolutionary adaptation is also found in some insects and fish. But as with many other bird topics, let’s start here in Idaho with our own species.
First, let’s look at the mechanics of this process in cowbirds. Females begin to search for the nests of other species even before they are ready to lay eggs. When they are ready, they simply drop an egg (sometimes more) in the nest of the host, say a yellow warbler, and leave.
This cowbird has been recorded as attempting to parasitize 220 other species of birds. I say “attempting” because some species recognize that the cowbird egg is not their egg, and they reject it. This can be done by tossing out the odd egg, abandoning the nest or even building a new nest on top of their original nest. But 144 other species do not see the cowbird egg as an outsider and go on to treat it as their own.
Steve Rothstein, an ornithologist at UC Santa Barbara, spent many years researching the behavior and evolution of brood parasitism by the brown-headed cowbird. One of the techniques he perfected was creating artificial cowbird eggs and then placing those eggs in the nests of various species to see how the owners would react. Those owners are called “hosts” in this little drama, although they don’t want to be hosts, in the evolutionary sense. They “want” to raise their own young.
I’d been studying sagebrush birds in the early ‘80s around Shoshone, Idaho, and Steve sent me some artificial eggs to place in nests of sagebrush sparrow, Brewer’s sparrow and sage thrasher to discover how they would react. The sparrows I found did not recognize the artificial eggs (one per nest) and continued to incubate as if nothing were amiss. After several days, I went back to those nests and retrieved the artificial eggs to use in other nests.
But the thrashers were not fooled. They would remove the artificial egg within a day or so. I think it’s likely they removed the eggs as soon as they saw them. But I was not able to watch a given nest continually to discover how quickly they responded. I found one of those eggs, by pure luck, about 30 feet away from the thrashers’ nest. One advantage the sage thrasher has over these two sparrows is a relatively large beak. It can more easily pick up a foreign object and carry it off.
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Cowbirds have other adaptations that give them an advantage in the nests of other species. They have a shorter incubation period than many species so cowbird young hatch before the hosts’ own young. This immediately gives the young cowbirds an edge in getting fed by the adults. Bigger young have a major advantage in other species as well, e.g., owls and hawks. They outcompete their smaller siblings. When food is limited, those youngest birds don’t make it.
Young cowbirds also work to eject their nest mates from the nest. They back into both eggs and young and flip them over the rim of the nest cup. When this succeeds, they are the only young bird left in the nest and so receive all the food adults deliver. Once the young cowbird has fledged, you can still see the host adults feeding it. Some hosts, like warblers, are much smaller than the young cowbird and seeing them feeding a giant, begging baby is both funny and sad.
The driving force for some evolutionary adaptations is obvious. For example, osprey evolved long, sharp talons and a mobile toe to better carry slippery fish. The advantage to birds who could better carry fish over the millennia is obvious. But leaving your unborn young in the care of strangers seems pretty risky. How could that pay off?
One simple factor is that a female can lay many more eggs. Female cowbirds can lay 30-40 eggs per year during the breeding season. This is obviously huge in comparison to the three to five eggs females of most species of small birds can lay in their own nests.
Of course, that’s the easy part (although laying 30-40 eggs doesn’t strike me as especially easy). The tough part is relying on a bunch of strangers of many species to raise your kids. But in evolution, uncountable numbers of long shots have succeeded. We have eyes and peacock tails after all, both seeming long shots from the primordial soup.
On the flip side, we see the vast majority of bird species lay eggs in their own nests and raise their own young. So, this has been the most common evolutionary strategy overall. But brood parasitism has evolved in several bird groups, mentioned above, and in various geographic locations and habitats. I wish I could have been in the head of the first female of species X who thought, “Hmmm …. maybe I’ll just drop an egg in this little cup and see what happens.”
Of course, those birds did not “see what happens” in the day-to-day sense humans have. But evolution will always “see what happens” and will reward the better choices, as it has always done. If your behavior leads to increased survival of your young — no matter who raises them — your genes win and carry on into the future.
Brown-headed cowbirds are seen as villains by some. But they are actually clever survivors. They’ve exploited a narrow niche and succeeded. But wait. There’s more. With so much at stake, why don’t all host species just get rid of the foreign eggs? Why invest in raising young that aren’t yours? Why haven’t all bird species evolved long ago to get rid of the bogus egg? Stay tuned.