Cliff swallow breeding thwarted by bird version of bedbugs

Cliff swallows in Nebraska rarely breed twice in a year when parasitic swallow bugs infest a nest, a new study finds.

February 18, 2015 at 1:15 pm

Bedbugs are no fun, even when you’re a bird.

The species that bothers humans — Cimex lectularius — only feasts on us. But other animals have to deal with their own versions of the nasty parasite. Cliff swallows (Petrochelidon pyrrhonota), for instance, are plagued by the American swallow bug (Oeciacus vicarius). When swallow bugs infest cliff swallows’ gourd-shaped nests, it’s bad news for swallow nestlings, affecting the young birds’ development and survival.

The parasites are also bad for the parents, scientists report February 17 in Royal Society Open Science. But instead of taking a toll on the parents’ bodies, the swallow bugs affect breeding.

Charles Brown of the University of Tulsa in Oklahoma and Mary Bomberger Brown of the University of Nebraska in Lincoln have been studying cliff swallows in Nebraska since 1982. There, the birds build their mud nests in places like the sides of bridges and the undersides of cliffs. A few swallows live on their own, but most live in groups of up to 6,000 nests. The birds winter in Argentina, Uruguay and Brazil, and they arrive in Nebraska in mid- to late April. By June, the birds are laying eggs, and by mid-July, the nestlings have fledged.

cliff swallow nests under a roof — Cliff swallows build mud nests attached to vertical walls. Sarah Stierch/Flickr (CC-BY 2.0)

Over the years, the researchers collected plenty of data on the birds, including body size, colony size and nesting dates. A couple of years into their study, Brown and Brown began fumigating some of the colonies to kill off any swallow bugs. Now, 30 years later, they have taken a look at what effect that fumigation effort had: It enabled a second breeding season.

There was late nesting activity (production of baby birds after the mid-July fledging) in 49 of 65 fumigated colonies and only 19 of the 1,183 colonies that were left alone. “Colonies with parasite removal via fumigation were over 45 times more likely to have a late round of nesting (and thus a nesting season approximately doubled in length) than were colonies with typical numbers of swallow bugs,” the researchers note. Colony size, when the summer nesting season began and the region’s climatic conditions in a given year were all unrelated to the occurrence of that second round of breeding.

When a person finds their home is infected with bedbugs, they often change their behavior, even moving out if the problem can’t be fixed. Cliff swallows act in a similar manner. They can’t fumigate their own homes, but if an infestation gets too bad, the birds will abandon a colony. And they don’t keep their kids around too long if their nest is infected — as soon as the nestlings fledge, the young birds and their parents leave the nest forever.

In contrast, at fumigated sites, Brown and Brown found, parents keep their young birds around for a few days and they themselves stick around even after their offspring have left. Some stay long enough to have another set of young.

The study also may explain why cliff swallows generally have only one brood a year and a similar species, the barn swallow, has multiple sets of young. The barn swallows aren’t as social, the researchers note, so they may not be as likely to share and spread parasites as the colonial cliff swallows.