It’s not easy being a song sparrow dad. There’s the constant search for bugs to feed newly hatched young; guarding your turf against other song sparrows; the cowbirds that try to sneak their eggs into the nest. And the snakes. Oh, the snakes.
The encroachment of humans with their houses, cars and roads might seem like yet another headache. But it turns out city living suits male sparrows, pushing them to become, in the words of one scientist, more attentive “super” fathers.
“Against our expectations, we found that they visited nests more frequently and were more successful parents than rural males,” said Samuel Lane, a postdoctoral researcher at North Dakota State University.
The surprise insight isn’t just an oddity about the parenting habits of brown-and-white bird that could fit in the palm of your hand. It’s also an example of the paradoxical ways in which wildlife respond to the spread of humans across the landscape. Some species relish living in the shadow of people. Rats and cockroaches come to mind. But even something as unlikely as a little song bird can adapt and thrive.
That outcome wasn’t self-evident. When Lane and his collaborators set out to track differences in parenting behavior between urban and rural male sparrows, they predicted the city birds would neglect their nest duties in order to spend more time patrolling for competitors. Previous work had shown that male song sparrows in urban areas are more aggressive than their country relatives. Studies of some other birds have also found that more aggressive males spend less time tending to their young.
To see if their assumptions panned out, the researchers tracked the behavior of birds at six sites in southwestern Virginia ranging from urban to rural. Between 2018 and 2021 they captured 130 song sparrows, including 76 males, using delicate nets. They attached a tiny tag to one leg of each bird. At dozens of nests, the scientists placed small detectors that would register every visit by a tagged bird, a gauge of the birds’ parenting habits.
To gauge the relative ferocity of males, the researchers set up plastic song sparrow models with a recording device that played several song sparrow calls—a sign of a potential intruder for the territorial species. Then they tracked the aggressiveness of a nearby male sparrow: how close it came to the speaker, how many times they made a low-pitched “soft song”, and how much they waved their wings.
Surprisingly, while the urban male birds were more aggressive, they also made significantly more visits to the nests than the rural birds, the researchers reported in August in Frontiers in Ecology and Evolution. Females, meanwhile, visited their nests at the same rate in both landscapes.
“It turns out urban males are super males—able to defend their territories and care for their young,” said Lane.
Whether or not these added visits were a factor, the urban-hatched chicks fared better by several measures. In city nests, eggs were more likely to hatch and the chicks were more likely to grow big enough to fly away. That’s despite a larger threat from brown-headed cowbirds, which lay their eggs in the nest and fool the sparrow parents into giving food and care to the little cowbirds.
One factor favoring survival among urban chicks might have less to do with attentive parenting. There are fewer animals that prey on sparrows, such as snakes, the scientists report. That could also help explain why urban males visited their nests more often. It’s possible that rural birds minimize the number of visits to avoid alerting lurking predators to a nest’s location.
“It is often assumed that urban areas are more challenging for wild animals,” Lane said. “Our study adds to growing evidence that certain species of songbirds even benefit from living in urban environments when there is sufficient green space for them to find food and nest locations.”
Those mean city streets, it turns out, might be more friendly than the woods. At least for sparrows.
Lane, et. al. “Indirect effects of urbanization: consequences of increased aggression in an urban male songbird for mates and offspring.” Frontiers in Ecology and Evolution. Aug. 22, 2023.
Image: ©Anthropocene Magazine
