A male red-winged blackbird may mate with as many of 15 females in a breeding season. Photo by Bryan Pfeiffer.
Blackbirds do it. Chickadees do it. Even educated emus do it.
Some birds are cheaters. Their trysts, dalliances, one-morning stands, and other infidelities would constitute a racy script for a wildlife soap opera.
But first, the faithful: In the vast majority of bird species, one male and one female unite for the purpose of raising young. In this classic form of monogamy, or “single marriage,” the pair stays together either for a single breeding season (the case among most songbirds) or for as long as they both shall live (in geese, gulls, and swans, for example).
Among the rest (or the restless, as the case may be), polygamy, or “many marriages,” ranges from casual to calculating.
A male red-winged blackbird mates with a number of females, and each tends her own nest. This most common form of infidelity is called polygyny, “many women.” The general hypothesis is that females prefer to mate with the male that can defend the best territory (size and quality matters) or exhibits some other evidence of having better genes. The superior redwing’s harem averages five females, but he may mate with as many as 15.
It might seem that a female and her hungry young would suffer if the male in her life was preoccupied with a dozen or more partners. But polygyny sometimes occurs with conditions favorable to a neglected mother: an abundance of food, for example, or precocial young more able to fend for themselves, which is the case for the occasionally polygynous ruffed grouse.
The point here is that polygyny doesn’t only benefit males. Out in the marsh, as male redwings call honk-a-REE, the female is calculating. She figures that joining the harem of a male with upscale territory is better than monogamy on inferior turf. Her offspring prosper if she chooses a king’s palace over a loser’s hovel.
But in matters of avian romance, turnabout is fair play. Spotted sandpipers display classic polyandry, or “many men” – a single female mating with multiple males. Oh, sure, she finds a mate. She might even help him incubate eggs and raise their young. But while the male does most of the parenting, she’s on the prowl.
A female spotted sandpiper often leaves her first mate to breed with a second. She might help rear young from both nests at the same time (simultaneous polyandry). But sometimes she abandons her first family and devotes her parental energies only to her second (serial polyandry).
Paternal care of young is rare in birds, but those devoted dads may have been customary earlier in avian evolution. One paleontologist studying fossilized dinosaurs positioned over eggs concluded that incubating adults were predominantly male. And the eggs in their care seemed too numerous for the lone adult, suggesting that dad was tending to a collection of eggs from more than one female.
We still have dinosaurs exhibiting this behavior: the emu and some other ground-dwelling birds. (Remember, only non-avian dinosaurs went extinct.) A male emu is often the lone parent to an oversized brood from several females. Also noteworthy is that the emu is among our most primitive birds, relatively close in lineage to its therapod dinosaur ancestors.
Much further along from emu evolution is the cheating chickadee. Male and female black-capped chickadees usually pair up as their winter flocks begin to disperse in early spring. They are monogamous. Well, almost.
Even as she’s laying eggs fertilized by her mate, a female chickadee sometimes sneaks off for copulations with another male, only to return to her nest to lay extramarital eggs. So, in our inventory of mating strategies – monogamy, polygamy, polyandry – it turns out that chickadees are simply promiscuous.
In all these examples of feathered infidelity, the research varies, and I advise readers to be wary of complete or convenient explanations. Avian affairs are no different than our own. In other words, when it comes to relationships, as you might expect, “it’s complicated.”
Bryan Pfeiffer is an author, wildlife photographer, guide, and consulting naturalist who specializes in birds and insects. He lives in Montpelier, Vermont.