The Galápagos finches made famous by Charles Darwin have been under attack by parasitic flies that feed on nestlings’ blood. Now it turns out that mockingbirds on the islands may inadvertently help the flies and threaten the finches.

The parasite is the nest fly (Philornis downsi), which may have invaded the Galápagos Islands partly from mainland Ecuador. The blood-feeding fly larvae weaken and kill the nestlings of bird species such as the medium ground finch and mangrove finch. Oddly, though, Galápagos mockingbirds don’t seem as vulnerable to the parasite.

To compare the species’ responses, researchers monitored medium ground finch and mockingbird nests in a 3-by-4-kilometer site on Santa Cruz Island in the Galápagos. The team fumigated about half the nests in the study with a chemical that kills the parasites but doesn’t harm the birds. The scientists collected blood samples from the nestlings and recorded the animals’ behavior with video cameras, collecting nearly 100 hours of footage from 34 nests.

As expected, fewer finches fledged successfully in nests with more parasites. But among mockingbirds, the fledging success rate was about the same in infested and fumigated nests.

The video footage gave some clues to the reason. The mockingbird nestlings in the infested nests begged for food more than the nestlings in fumigated nests did. In response, adult mockingbirds at the infested nests spent more time feeding their offspring. So the mockingbird nestlings may have been better-equipped to survive the parasite infestation because they asked for — and got — extra food.

In contrast, the finch nestlings didn’t beg more in the infested than the fumigated nests. And the adult birds didn’t spend more time feeding their offspring in the infested nests either.

The reason might be that finches are smaller than mockingbirds and thus already spend a lot of time begging for food to meet their energy requirements. A nestling weakened by parasites might find it hard to muster the energy to beg even more. “[F]inch nestlings may experience an energetic ceiling beyond which they are simply incapable of additional begging,” the authors write.

The mockingbirds’ response could be bad news for the finches. By tolerating the infestations, the mockingbirds might act as a “reservoir” that helps the flies persist. In other words, one species’ resilience could sabotage another species’ survival. — Roberta Kwok | 17 December 2015

Source: Knutie, S.A. et al. 2015. Galápagos mockingbirds tolerate introduced parasites that affect Darwin’s finches. Ecology doi: 10.1890/15-0119.1.

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