Parasites influence mate selection and evolution of barn swallows

Barn swallows have a huge range, with breeding grounds across the Northern Hemisphere and much of the Southern Hemisphere. Many of the birds look slightly different in each region, while some are so distinctive they have diverged into separate species. 

Experts at CU Boulder have found a surprising explanation for why barn swallows are transforming in appearance. The study suggests that local parasites are changing the way barn swallows select their mates, which ultimately changes their physical traits. 

According to the researchers, parasites could be playing a major role in the creation of new species. “It’s possible we haven’t appreciated just how important parasites might be in shaping the evolution of their hosts,” said study lead author Amanda Hund.

Humans and all other living organisms have co-evolved with a unique community of parasites. Previous studies have shown that parasites influence the human immune system, pheromones, and even mate selection. 

For the current study, the team set out to examine as many parasite communities as possible in barn swallows. The researchers spent four years investigating barn swallows at sites in Colorado, the Czech Republic, and Israel. 

The experts documented the number and types of parasites present in the nests and in the blood of barn swallows. The team also recorded details about the mates chosen by females in a given breeding season – including their breast color, throat color, and tail shape – and tracked the health and the survival rate of the offspring.

Overall, the males that were found to be most attractive had fewer parasites. Somehow, the females had made informed choices about the health of potential mates based on their breast color, throat color, and tail shape.

Many birds had multiple parasites with connections to the same physical trait. In Colorado, for example, male barn swallows with darker breast color were less likely to have mites and more likely to have malaria. Nest mites are detrimental to the survival of offspring, while malaria only impacts the male bird.

“Males are investing in traits to attract females, and it looks like that comes at a cost – where they are more attractive, but also more susceptible to malaria,” said Hund. “It is a tradeoff.”

The researchers are currently investigating why local parasites are linked to certain sexual traits.

“And once you really figure that out, you can export that knowledge and our study methods to other populations and actually watch mate selection decisions and the associated reproductive consequences unfold,” said study co-author Professor Safran. “It’s like watching evolution in action.”

The study is published in the journal Evolution.

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By Chrissy Sexton, Earth.com Staff Writer