Children’s books adore the momma bird.
She finds a cute mate in the spring and forms an exclusive bond. Together, they build a nest, produce eggs and tenderly feed the chicks until they fledge. If only it were true.
Scientists got an eyeful when they started sampling bird DNA. It turns out there’s a sexy back story. Mom has probably been sleeping around, and dad, too. Those chicks in the nest might each have a different father. Not all birds are promiscuous, but in climates with short breeding seasons, many are.
For hanky panky, look no further than the familiar black-capped chickadee. If you have a bird feeder, you’ll have noticed that chickadees fiercely enforce a pecking order. Dominant males rule. When the breeding season starts, the dominant female picks the best male, who will generally have the strongest song. He looks like just the guy who can protect the nest and provide ample food for her and her young. Pairs form down through the chickadee social order, just as they might for a high school prom.
Just because she has made her primary pick, doesn't mean she’ll settle down. Every morning, she takes note of all the males singing in the area. If she hears one with a stronger song than her beloved, she might slip out of the nest when he is off hunting or sleeping, and mate with that better singer.
Meanwhile, her mate – foraging in other trees – might be seduced by a female there. “But honey, it didn't mean a thing,” they’ll tell each other later. The primary pair usually will continue to build a nest together, although sometimes they “divorce.”
The same thing is going on deeper in our woodlands, where highly virile male Acadian flycatchers and hooded warblers quietly sneak into other males’ territories and copulate with females who already are paired. All during May, in those picture perfect hay fields of North Salem, male and female bobolinks were having furious sex with multiple partners under cover of the gently windblown grasses. In bird colonies, like those established by bank swallows and cliff swallows, there are more affairs than in reruns of Days of Our Lives.
What drives birds of some species to cheat on mates? It’s a survival strategy.
Migratory birds arrive on their breeding grounds with a lot of work to do. In just two to three months, they have to restore the energy reserves they lost in transit, establish a safe territory that provides ample food, find a mate, breed, raise a family, molt and regain enough strength to head back to their wintering grounds. Birds suffer a high mortality rate during that flight, so they need to raise as many strong chicks as possible.
Dominant males serve their species by producing the maximum number of young. Raising chicks takes a lot of effort. Ornithologists estimate a male hooded warbler can make as many as 1,000 trips to his nest to feed his chicks. A warbler that mates with many females produces not only a clutch he recognizes as his own but also chicks that will be tended to in other nests by other males with no one the wiser. This ensures a high rate of survivorship for the genetically strongest birds.
So how do the females of these species manage to lay different eggs by different fathers? It’s all in the plumbing. A female ovulates up to once a day. During that time, she might be fertile for only 30 minutes. To ensure that sperm is available at that critical time, she stores it in side tunnels, at the base of her oviduct. At the start of the breeding season, she pairs with the best bird available and then hedges her bets by mating with other, perhaps stronger, fathers. The more sperm she gets, from whatever source, the better her chance of laying a large clutch of fertile eggs. It doesn't matter which bird’s sperm ultimately gets through. Her slatternly behavior has strengthened the species by widening its gene pool.
Although it’s easy to find humor and scandal in the mating habits of our beautiful avian neighbors, the science behind these discoveries is serious and sophisticated. Studies led by researchers such as Bridget Stutchbury of York University in Toronto are opening doors that will help us understand the needs of breeding birds and help us with conservation plans to ensure their long-term survival. If you’d like to learn more about bird mating and behavior, I encourage you to buy her book, The Private Lives of Birds. It will give you a whole new outlook on what is going on at your feeder and in the trees.