The grass was tall, the wildflowers waving in the breeze and the scene bucolic. Perfect. How could they possibly resist? I stepped out of the car and listened.
It had been years since I'd heard a bobolink sing, but I knew I'd recognize it in an instant. Bubbly, buzzy and seemingly incoherent, the song was impossible to describe but etched in my brain from a long hot summer in college years ago. I was a field assistant for one of my professors at Cornell University in upstate New York back then, and spent my days watching bobolinks breed. Well, not that part really. Bobolinks are grassland nesting songbirds that migrate up to 12,000 miles from their winter home in South America to their breeding grounds in the northern United States and Canada each year. Once they arrive, the males establish territories in open meadows and perch on tall stems and posts, singing loudly to proclaim their prowess and fend off competitors. That voice is central to successful sex. Without it, a male bobolink can't establish a good territory, advertise his appeal or attract a mate. Females would likely pass him by and flock to the raucous guy the next post over.
By the time I arrived on the scene that summer, a lot of the action had already happened. Males had established territories. Birds had paired off. Couples were building nests and laying eggs. But beneath the pastoral surface of apparent pair-bonded bliss lay a more complicated story. In reality, the grass teemed with additional liaisons – some secret, some not-so-secret. It was our job as researchers to sort them out: in other words, just who had fooled around on the side with whom?
To do this, we first had to identify individuals. Bobolinks all look pretty much the same – the males are black with two broad white bands running down their backs and an odd yellow cap on the back of their heads. Females are dull, with brown streaks running down their bodies. (Hey, they're not the ones advertising for mates).
And so we lured the hyped-up males into a net with a recording of another male's song. Rushing to defend their territories from would-be competitors, the bobolinks inevitably flew into the net. Once captured, we gave them some obvious markers: we painted their tail feathers. Each bird got a unique combination of red, yellow, blue, green or white stripes. That way, as we watched them posturing on posts and bringing food to the nest, we could easily identify each and map his territory. We marked the females the same way and figured out who they had paired with by watching them move about in the territory and visit the nest. We took blood samples from each bird and later, once the nestlings were old enough, surreptitiously collected samples from them as well.
Those blood samples would help debunk decades of simplistic assumptions about mating systems.
It was the 1980s and new genetic techniques for the first time allowed scientists to identify and match DNA from different blood samples. Until then, evolutionary biologists assumed: what you see is what you get. In other words, the vast majority of bird species are monogamous and youngsters in the nest are the offspring of the couple who build the nest and feed the young. At times, other males might force themselves onto females, but the conventional wisdom held that females usually resisted. After all, why would a female mess up a good thing with her dedicated mate? It just didn't make evolutionary sense.
Bobolinks have an obvious deviation from that rule: some males were so appealing that they attracted more than one mate. Their second, and sometimes even third, mates built nests and raised young on the males' territory. These ultra-sexy guys were deadbeat dads to their second families: the females had to feed the young by themselves. But from an evolutionary standpoint, these guys apparently were worth it. The additional females accepted their second-class status in order to gain access to good territory chock-full of food or the impressive genes that produced a particularly powerful song. We could see them raising their brood within the territories' boundaries, but the resident males never visited the nests. These were the not-so-secret liaisons – males with additional females on the side.
“The façade of avian sexual fidelity was cracking.”
But what about the females? Lab analysis of the blood samples using electrophoresis later linked each baby bird with its genetic parents. The process passes an electrical current through a gel containing blood or tissue that causes different types of DNA to migrate through the gel at different speeds. Those samples confirmed what my professor Tom Gavin had first detected the previous year: youngsters in some of the nests had more than one father. In other words, those supposedly monogamous females were hedging their genetic bets with additional partners on the side. Other biologists began getting similar results using new genetic techniques on other bird species as well. The façade of avian sexual fidelity was cracking.
Cornell Professor of Neurobiology and Behavior Paul Sherman was one of several scientists who tried to make sense of the growing body of evidence that female birds actually seek out sex on the side. After reviewing years of accumulating data, Sherman and his colleagues concluded that this behavior is actually more common in monogamous bird species than in those where males routinely have more than one mate. That's because in monogamous species the really attractive guys get chosen first, leaving all the other females with their second or third choices. These females might enjoy the benefits of pairing with a male who will help feed the young, but they'll still mix into their brood some of those fantastic genes from 'top' guy next door.
So what does all this mean for humans? The old cliché that men search out as many sexual partners as possible while women remain invested in monogamous relationships is getting re-examined. Some social scientists are exploring whether in some cases – whether for new and better genes for their offspring, access to more resources, or simply to find a better mate – it makes evolutionary sense for women, like birds, to stray.
The implications of that long-ago summer were far from my mind as I stood on a dirt road in the Canaan Valley National Wildlife Refuge in West Virginia this past month. Instead, my reflexes took over. Bobolinks don't normally nest this far south, and because I had moved away from New York state once I graduated, I hadn't seen one since. But this high elevation refuge in the Appalachian Mountains was an exception. And so, as I listened, I scanned the pasture's fence line, an irresistible perch for males to proclaim their territories. Sure enough, just two posts in from the refuge road posed an improbably clad male, his black and white tuxedo facing backwards on his body and a yellow cap perched incongruously on the back of his head. Other birds soon landed nearby. And so he did what any respectable bobolink would do: he opened his mouth and began to sing.
Audio and video courtesy of Macaulay Library of the Cornell Laboratory of Ornithology, Ithaca, NY
Animal Behaviour
Animal Behaviour
Innovation
