The “birds and the bees” is a subject parents often don’t relish talking about with their kids. For folks in my generation, the phrase conjures up memories of junior high health class in which we learned a great deal about birds, followed by just a little bit about people.
I was happy enough to take my turn turning over the chicken eggs in our classroom incubator. But what birds get up to with each other turns out to be much more complex than our teachers told us.
Perhaps you know the phrase, “Mama, baby; Papa, maybe.” In the old days, back before DNA testing on every street corner, some babies came into this world surrounded by considerable uncertainty as regards paternity. Hence the pithy phrase.
But with birds even maternity can be unclear.
Female ruddy ducks raise their own young in their own nest but they also, from time to time, will lay an egg in the nest of a nearby ruddy duck. Professor Michael Webster at Washington State University and his students have studied this surreptitious maternal strategy.
“We’ve confirmed the female’s behavior both by observation and by getting DNA samples,” Webster said. “We call laying an egg in another’s nest a cryptic reproductive strategy.”
An individual mother bird “gains” from this system if she can get others of her species to invest the food energy needed to raise her chicks.
Cowbirds drop off their own eggs not just in the nest of others of their kind, but in the nests of birds of different species. In modern terms, cowbirds are the ultimate outsourcers. This works when other birds don’t recognize the egg as foreign and they raise the cowbird chick – sometimes even at the expense of their own, smaller offspring.
And there’s more.
Female fairy wrens lay eggs only in their own nests – just like we were taught in sixth grade – but they step out with other male birds on a surprisingly regular basis.
“Half or more of the young turn out to be offspring of a male other than the male partner in the pair,” Webster said. “There’s quite a bit of copulation that goes on outside the basic pairing.”
But it’s also true that the wrens truly form strong bonds with their partners and work together to raise young. Generally, Webster tells me, song birds engage in quite a bit of extra-pair mating. But at the same time, both partners work to raise the next generation in their own nests. Mother Nature, evidently, doesn’t see any contradiction in these facts.
In some warblers Webster has studied, the papa in the pair is not the father of any chicks at all in his nest one third of the time.
What’s driving the extra curricular activities of the females?
One issue may be that the females are trying hard to avoid inbreeding. With birds that don’t disperse widely from the nests in which they were raised, inbreeding is a real hazard and results in offspring that may not flourish or even survive. Getting around a good bit might help insure that at least some of a female’s offspring are as genetically healthy as possible.
“And we’ve found via DNA studies that it’s the females most closely related to their mate who are the most likely to go outside,” Webster said.
One of the current issues Webster and his students are now investigating with the warblers is how the birds may shift their reproductive strategies. When their customary food is in short supply, for example, warblers may change their mating habits. That could be particularly evident in the warblers if habitat quality declines due to climate change.
Some birds can arrange matters to change the ratio of the males and female chicks they bring into the world. Depending on local conditions, female fairy wrens can lay eggs that turn out to be 75 percent female rather than the usual 50-50 mix.
“The control of gender is something internal to the female bird,” Webster said. “We don’t know what, exactly is it. Science often works by answering one question in a way that raises new questions.”