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Walking across the UC campus in mid-April, I noticed a couple of cliff swallows orbiting Hertz Hall and spotted a jug-shaped mud nest under the building’s eaves. I seem to recall a long-running battle between the swallows and the university’s maintenance crews which involved blasting the nests away with hoses. But the persistent birds keep coming back. 

There are places in the Coast Range, along Del Puerto Canyon Road and Corral Hollow Road, where you can still find cliff swallows nesting on cliffs. But, like barn swallows and chimney swifts, these birds have adapted readily to human structures: buildings, bridges, freeway underpasses, culverts. They’re a colonial species; some cliff swallow nest sites contain up to 3,700 pairs. 

Biologists have argued for years about the propensity of some birds to nest in large groups. Some claimed it was a matter of safety in numbers: Group size dilutes the risk of predation. This ties in with a phenomenon called the Fraser Darling Effect, after British ornithologist Sir Frank Fraser Darling, in which mating, egg-laying, and hatching is synchronized among birds in a colony. The timing is supposed to overwhelm potential predators with a flush of eggs and young and improve the odds that any given nesting pair will succeed in raising their family. In some birds, like yellow-billed magpies, breeding seems to be limited by a minimum colony size. 

A couple of decades back, the Israeli ecologist Amotz Zahavi countered this notion with his “information center” hypothesis: the idea that birds nest colonially to take advantage of their neighbors’ discovery of food sources. That seems a more likely explanation of coloniality in large birds like Old World vultures that are not vulnerable to predation. 

For the past 20 years, Charles and Mary Brown of the University of Tulsa have been testing these and other models of colonial behavior with culvert-nesting cliff swallows along the Platte River in Nebraska. As recounted in Charles Brown’s Swallow Summer, an engaging diary of field work, they’ve learned remarkable things about the costs and benefits of being social, and the strategies that the birds use to maximize their reproductive output.  

The Browns are convinced that cliff swallow colonies do function as information centers. The birds pay attention to what their neighbors bring back to the nest. A swallow coming home with a mouthful of insects has clearly hit pay dirt, and it’s a good idea to follow him or her on the next foray. They can also tell from flight behavior whether a bird is heading back to an insect swarm. 

But there’s a downside to colonial living. Cliff swallows have to cope with a host of ectoparasites, the worst being Oeciacus vicarius, the swallow bug, a relative of the bedbug that plagues humans. Over a thousand of these little bloodsuckers have been found in a single swallow nest. The larger the swallow colony, the worse the infestation. Swallow bugs and other pests clearly depress the birds’ reproductive success; the Browns found that experimentally fumigated colonies boomed as more swallows moved to the parasite-free location. 

That’s only one kind of parasitism, though. Cliff swallows are not the best of neighbors: “These little birds do rotten things to each other,” Brown says. Extra-pair copulations are frequent; males will attempt to mate with strange females as they gather mud for nest construction. Swallows will steal a neighbor’s nest material, both the wet mud that forms the nest and the grass that lines it. Intruders will even enter another pair’s nest and toss out the eggs. Nestlings plant themselves in a strange nest and intercept the owners’ food deliveries to their own chicks. And one of the first things the Browns discovered was that some swallows are brood parasites, like the infamous cuckoos and cowbirds. Early on they found two freshly laid eggs in the same nest on the same day, evidence that someone had been egg-dumping. They eventually figured that up to a quarter of the nests in the larger colonies were parasitized.  

Successful brood parasitism requires speed and stealth. One female managed to lay an egg in 15 seconds while the homeowners were fending off another intruder. Parasitic females time their visits to reduce the likelihood of bumping into the host female.  

Remarkably, the Browns found that some swallows picked up their own eggs in their beaks and moved them to a neighboring nest. They verified this by marking eggs and seeing which turned up in new nests, and eventually catching females in the act.  

Unlike cuckoos, parasitic female swallows also raise their own young in their own nests. By parasitizing their neighbors, they avoid putting all their eggs in one basket, hedging their bets against losses from predation, an excess of swallow bugs, or the collapse of the nest. They also seem able to assess the quality of the nests where they sneak in their eggs, choosing those with fewer bugs. The Browns found that annual and lifetime reproductive success—fitness, in the Darwinian sense of leaving the greatest number of descendants—was higher for parasitic females than for either hosts or nonparasitized birds. 

So swallows nesting in a large colony increase their risk of raising someone else’s offspring. The Browns’ most recent research shows that a cliff swallow’s choice of colony size has a genetic basis. They moved nestlings from small colonies to foster homes in large colonies and vice versa; when they matured, the birds followed the colony-size preference of their biological parents, not their foster parents. The sum of all those tradeoffs—information about food, insurance against predators, vulnerability to parasitic insects and to their own kind—is somehow encoded in a swallow’s genome, along with instructions for building their nests, migration routes between California (or Nebraska) and South America, and a taste for flying insects.