As a group, birds tend to be monogamous.
There are exceptions to be sure. Pheasants, grouse and turkeys, but not quail, are polygamous, with males courting several females. Ostriches and emus also are polygamous. Some shorebirds have still other ideas when it comes to the breeding season.
In monogamous species such as robins, a pair forms a social bond to rear young. The male sings a serenade each morning and evening and sometimes during the day when a new nest is under way. The female builds the nest, which will soon hold two to four blue eggs. Together the male and female feed the young, and if they escape the attention of a western scrub-jay or western screech-owl, there will soon be speckle-bellied young in the yard begging for still another mouthful of worms and insects.
If only it was that simple. There is a lot more going on in the hedgerows than we ever suspected.
There are new tools that let researchers identify individual birds. With DNA testing, it is now a relatively simple matter to determine parentage of the young in a nest. A drop of blood or a plucked feather from the parents and young contain enough DNA to reveal the family history. And what researchers are learning has been eye-opening. Birds aren't as monogamous as we once thought.
A great many birds fool around — a lot. OK, birds that lay one or two eggs, such as penguins, petrels and eagles, tend to be faithful. But songbirds? It's quite a different game. Cliff swallows, for example, live in colonies. These are the swallows that construct gourd-shaped mud nests on cliffs and freeway overpasses. The communal life apparently fosters a closeness of other kinds, as well. I have seen figures that show on average 40 percent of the eggs in a nest are fathered by males other than the mate.
This isn't restricted to colonial species either. Sparrows, wrens, warblers and many others around the world all fool around. I'm not sure about robins, but it wouldn't surprise me if they played this game too. The pattern among small birds is that 20 to 40 percent of the young in any nest are fathered by different males.
This brings to question the definition of monogamy. Many scientists have begun to use the term "social monogamy" for species that form a pair bond and the term "genetic monogamy" for those that stay faithful to the pair bond.
Why all this infidelity? There are a couple of potential reasons. Just how good is the male from a genetic standpoint? By mating with more than one male, the female buys an insurance policy, if you will. A diversity of genes ensures that at least some of the young have the right stuff.
Another possible explanation is that a female may not be paired with the most desirable male in the neighborhood, but she can still get the benefit of his genes. Not that this is all on the females. Males are playing the same game at the same time.
The dawn chorus in spring takes on a whole different meaning when you are aware of all the intrigue that is playing out in forest and field.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.