For the Birds Radio Program: David Bird's Kestrels
One of the surprising secrets of the natural world that ornithological research has uncovered in recent years is the fact that a lot of birds once considered monogamous actually do quite a bit of fooling around. In order to ensure that their mates won’t be tempted by neighboring males, most attached songbird males spend a lot of time attending to their mate and singing. For the most part, this keeps the females focused on their mates, but during times when a male is feeding or resting or eying the chick next door, other males often sneak in for furtive pairings.
To determine paternity of broods of young, scientists use the same DNA hybrid techniques that they use to determine human relationships in legal paternity cases. And this DNA evidence has shown that on average fully 50% of the baby Tree Swallows in a given nest have a father other than the male raising them. This species may be the most promiscuous of songbirds, but cardinals, robins, and many other species tum out to also be much more promiscuous than ornithologists ever dreamed.
DNA evidence also shows that at least one group of birds is every bit as faithful in its pair bonds as everyone thought it should be–the birds of prey. This past weekend Dr. David Bird from McGill University spoke in Duluth about hawks, and explained why they are significantly more faithful than other groups.
He’s been doing extensive research on American Kestrels during his entire career–he’s even the co-author of the kestrel entry in the prestigious Birds of North America. So naturally it was kestrels he focused on when he did his study of mate fidelity.
I’ve always thought it curious that birds have no interest whatsoever in mating except during a relatively short annual period. Kestrels limit their breeding behavior to less than 3 months each year. But Dr. Bird’s extensive field studies demonstrate that each pair of kestrels makes up for lost time by mating a good 500 times during that brief time. Of all the thousands of acts of mating Bird and his students observed, very few were outside of pair bonds.
Most of the mating acts happened on the boundaries of the territory, as if the birds were chiding outsiders to eat their heart out–this mate and property were definitely taken. When DNA tests were made on nestling Kestrels, none had different fathers from their siblings except in cases where a male had been replaced, probably after being killed. Male kestrels are responsible for most or all of the hunting while females are incubating eggs and tiny young, which gives a big window of opportunity for other males to visit while the mate is away. But the few that do cross the territorial boundaries are vigorously chased away by females whose attacks on intruders keep their mate’s bloodline clean.
So baby kestrels in a single brood have virtually a 100% probability of sharing the same father. This is clearly of benefit to mated males, because 100% of the rewards of their hunting can be focused on themselves, their mate and their direct offspring, not on the offspring of other males. And since the males are instrumental in providing food and romance for the females and food for her young, rewarding the males with faithfulness is apparently a fair trade for the females.
We humans like to use monogamous birds as examples of moral behavior, but their breeding strategies, like those of all species, are really nothing more or less than the way the survivors before them have kept their genes alive. For kestrels, monogamy lasts through the breeding season. But once the babies go off on their own, their parents’ pair bond crumbles, and the migrate and overwinter separately. Come spring, they often return to the same nesting territory again, and often end up back together. But if another bird of the opposite sex has taken over the territory, they readily accept that mate. It has nothing to do with morality, and everything to do with the struggle for existence. Long may they live.