Marriages can break up for a variety of reasons—financial issues, the stress of raising a family, or simply growing apart with age, to name just a few. But for songbirds, the problem is often more prosaic: pesky humans who encroach on prime avian territory and drive bird families out. Urban development can force even the most devoted monogamous pairs to split up in search of new mates and better lives.
Now researchers have found that for some, that decision can have unfortunate consequences down the road.
“When we come in and develop an area we’re oftentimes causing [songbirds] to fail. We’re causing them to move somewhere, and when they move somewhere they have to find a mate,” says David Oleyar, a senior scientist at HawkWatch International and one of the coauthors of a new study published in the peer-reviewed open-access journal PLOS ONE. “That’s potentially a lost year of breeding. For a bird that doesn’t live that long, that can have a huge impact.”
Urban birds face many direct threats from their manmade environments. A report by the New York Times noted that strikes against building windows alone could account for upwards of 97 million bird deaths a year, while cars could kill another 60 million. But it’s more challenging for researchers to suss out how changing habitats affect bird populations in long-term ways, such as lower breeding success or poorer survival rates in fledglings.
For this paper, they focused on six species of songbirds that they split into three different categories: those that tend to avoid human development (avoiders), those that can adapt to it (adapters) and those that exploit it (exploiters). While adapters and exploiters did fairly well both in developed areas and transitioning areas, they found, avoider species faced serious challenges in urban landscapes. “You see that actual bump in success in the adapter group in the sparrows and the flexible species, but you don’t see that in the avoider species,” Oleyar says.
It’s worth noting that, from the researchers’ perspective, success in songbirds doesn’t have to do with achieving lifelong fulfillment or satisfaction: It was measured solely by whether or not a bird couple raised and fledged at least one offspring, and therefore passed their genes on to the next generation.
And by that metric, avoider species didn’t do so well. These birds were often forced to move twice as far to find mates as more adaptive bird species—about the length of one and a half football fields on average, according to the study—and once they got there, their new nests were less successful. The reason: “Divorce” puts obstacles in front of future mating success, since birds have to worry about setting up in a new territory with new risks and new competitors.
“You’ve got to figure out the new rules in the new spot,” Oleyar says. “If you miss a year or two because you’re trying to find a territory and you’re trying to have mate, that can have a huge impact.” Since we’re talking about birds that only live 5 to 8 years on average, a missed year or two can greatly affect numbers in a given area.
Amanda Rodewald, an ornithologogist at Cornell University, this research helps reveal a more nuanced story about the challenges urban birds face. “It’s not just that predators are eating you, or that you are knocking into windows and dying, or that there’s just not enough food,” says Rodewald, who was not involved in Oleyar’s study. “The consequences of urbanization can be much more subtle.”
Rodewald studied the ecology of birds in urban areas for 13 years and found that another avoider species, the Acadian flycatcher, also had a hard time with transitions. The birds were more reticent to settle in urban areas; even in areas of so-called green development, where only around 10 percent of the forest cover is lost, these avoider species still suffered. “Those practices, although they might be good for some species, those aren’t going to be sufficient to avoid negative outcomes for some sensitive species,” she says.
But before you write divorce off as a viable (avian) option, remember that there are a lot of different kinds of birds.
In some seabirds, researchers are finding that sticking blindly to monogamy can be just as wrongheaded. A recent study on a colony of common terns off the coast of Germany found that these terns actually lower their success by chaining themselves to unproductive partners. At first, that strategy seems to makes sense: The terns fly thousands of miles across empty oceans every year, meaning they don’t have much opportunity to bump into potential new mates.
“The choice of partners is not driven by ‘I’ll do better with this guy or this girl.’ It’s simply based on availability,” says Fernando Colchero, one of the coauthors of the recent study published in the Proceedings of the Royal Society B. “Whoever is available: ‘Let’s get together and remain together as long as we can.’”
Colchero and a team of researchers studied around 1,300 birds in the Heligoland Bird Observatory, an ornithological science station set up on German islands in the North Sea. Using special towers set up on the island, researchers were able to track the activities of individual birds through implanted microchips.
“By reading the microchip, they can see who’s sitting on which egg. They know which hatchling belongs to which couple and that’s how they can construct this whole genealogy,” says Colchero, an associate professor at the University of Southern Denmark who studies aging as part of the Max Planck Odense Center.
Through these observations, the researchers found that most common terns are monogamous to a fault. Humans might interpret this behavior as loyal: The birds stick by their mate, even when their partner is incapable of producing offspring. “This notion that we have that we’re going to try to always optimize, well, it’s not always the case,” Colchero says.
More studies need to be done to figure out if there are ulterior motives that justify terns sticking with a single mate, he adds. For instance, the energy it takes to woo a new lover could be a factor in making it more favorable to stay committed on the off chance that your partner finally comes around and produces children.
But in Colchero’s eyes, at least, the finding is “very romantic.” It shows that these terns stick by their mate, no matter how much their genetic legacy suffers for it.