Urban living comes with tradeoffs for swallows

City life seems to benefit tree swallows in Ohio, but it also poses health consequences their rural counterparts don’t face.

That’s the finding of a team of researchers who spent five years studying the birds in and around Columbus. Tree swallows in urban areas saw more reproductive success than those in protected areas, but they also had higher levels of mercury in their blood.

“I think this work leads us to think strategically about urban areas for insectivorous birds,” said Mazeika Sullivan, director of the Schiermeier Olentangy River Wetland Research Park at The Ohio State University and lead author of the study showing these findings published in Ecological Monographs. “Can we make urban environments more conducive to maximize the gains and minimize the risks?”

Recent papers have shown that North American bird populations are declining, and those declines are not restricted to rare and threatened species. Even birds considered common are losing numbers, including many that feed on flying insects. Swallows, flycatchers, swifts and nightjars are among the birds whose populations are falling, and they are particularly useful to humans by controlling insects. Meanwhile, urbanization continues to reshape the landscape in much of the country.

That prompted Sullivan and his colleagues to look at tree swallows (Tachycineta bicolor) in southern Ohio to determine their success in both urban and protected areas. What they found for tree swallows would likely apply to many other aerial insectivores, too, they figured.

“By understanding the basic science driving these declines, we may be able to put a finger on findings that will help inform very specific conservation measures,” Sullivan said.

The team put about a half-dozen nest boxes at each of seven study sites to attract the cavity-nesting birds in both urban and non-urban areas. For over five years, they collected information on the sites, including temperature, humidity, water chemistry and how much land was covered by concrete or other impervious surfaces. They also collected information on the birds, including when they started laying eggs, their clutch sizes and how many successfully hatched. And they took into account the birds’ body conditions, blood glucose levels, mercury levels and isotopes that could tell whether the birds were eating aquatic or terrestrial insects.

The birds at these sites fared differently, they found. In urban areas, swallows tended to lay their eggs earlier and have more nestlings fledge than at protected sites. Sullivan attributed that in part to temperature conditions. Urban sites had higher air and water temperatures, which “likely buffered them from cold snaps” early in the spring.

But these urban birds also had higher mercury levels in their blood than their rural counterparts — 482% higher on average. That creates a tradeoff between more reproductive success and potential harm to individuals’ health, Sullivan said.

Sullivan wonders how long the benefits will last, though. While the urban landscapes’ warmer temperatures benefited the birds, they also had higher temperature extremes. As the climate continues to warm, he said, those heat extremes during the breeding season — which the team found correlated with lower body weight — could become a bigger problem for swallows.

The work shows the importance of thinking strategically about what urban areas are helpful to insectivorous birds, Sullivan said, and of reducing water contaminants that detract from the advantages that cities can offer.

“Ultimately, we need to think about rewilding cities in safe ways maximizes the benefits and minimizes those risks,” he said.