Right now, there are a lot of baby birds calling the wrong adult “Daddy.” It turns out that infidelity among mated pairs of birds produces a family tree that looks more like a bramble bush. For some species, a nest with six eggs harbors the offspring of six different fathers. It’s a jungle out there.
About 90 percent of bird species produce young the old-fashioned way. They form pair bonds, and both parents raise the chicks. However, within those bonds, a considerable amount of straying occurs. For the species, that tends to mix up the gene pool in beneficial ways. It’s not bad for the individual female. No matter what happens, she’s certain to be passing along her genes. But what about the cuckolded male?
It’s a tradeoff. Most males expend a considerable amount of energy protecting a territory. They sing a lot. Somebody — probably an overworked grad student desperately trying to finish his thesis — actually kept count of how many times a red-eyed vireo sang in a single day. The equally overworked vireo sang more than 20,000 times.
Males also expend energy chasing intruders. Some males even follow their mates around to ensure no hanky-panky takes place. This strategy is ultimately exhausting. Most males are content to believe the majority of offspring in the nest are theirs, rather than ensuring 100 percent exclusivity but keeling over dead in the process.
Of course, it works the other way, too. A male’s pursuit of multiple females requires a lot of stamina — not to mention sneakiness. This takes energy away from raising one’s own brood. Too much philandering and too little parenting can seriously reduce the chances of passing on your genes. Thus nature settles into a balance where the gene pool gets mixed up a little, but most of the chicks being raised in any given nest belong to “Dad.”
There is another hypothesis, too — one that was actually researched in the mid-1990s. The theory suggests that females voluntarily limit their extramarital activities, because too many odd eggs in the nest may diminish their mate’s interest in raising the brood. Subsequent research six years later suggested that the more it was necessary for the father to provide food, the less likely the female would stray. To date, the only thing that has been fully proven is that you can get grant funding for any research project that involves sex.
In some species, the higher the population density, the more likely cheating will occur. Opportunity is close and quick. Some colonial nesters, such as bank swallows and cliff swallows, mix up the gene pool with reckless abandon.
The Nelson’s sparrow breeds in Maine’s salt marshes. Hidden down there in the wet grass, it’s just about impossible to prevent a mate from philandering, so the birds don’t even try. Males just wander around the grasses, singing to attract a female, coupling when they get the chance, then wandering off. With so little certainty about whose kids are whose, the males play no role in raising the brood.
The Bicknell’s thrush breeds on Maine’s tallest mountains. They also couple indiscriminately. But in this case, they form a hippie commune. Because their kids could be in any nest, all the males bring food to all the nests.
On the whole, it breaks down like this: About 2 percent of birds practice polygyny, where males mate with multiple females, but females mate with only one male. Males display all spring, hoping to bring in as many mates as possible. It is then up to individual females to raise the broods alone. Maine’s spruce grouse and ruffed grouse — often called partridge — do this. So do marsh wrens and red-winged blackbirds.
About 1 percent of birds practice polyandry, where females mate with multiple males and lay multiple clutches. The males have to raise their broods alone. Maine’s spotted sandpipers do this. So do the red and red-necked phalaropes that swarm in the Gulf of Maine in autumn.
About 6 percent are totally promiscuous. Conversely, some species appear to be totally faithful. Larger birds, especially raptors, need to collaborate to feed and nurture their young. This in turn requires strong pair bonds between the parents. It takes a lot of effort to raise an eaglet.
In the final analysis, I’d bet that many bird pairs remain faithful. For some males, cheating is not worth the stress. For females, cheating just encourages unwanted attention. After dealing with the kids all day, they just want to curl up with a glass of wine and a good book.
Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.