Bird-eating snakes ravage nests in forest reserve

A forest fragment might seem like a poor habitat for birds. But in Costa Rica, scientists have found that one bird species is faring much better in fragments than in a nearby forest reserve, where snakes are killing nestlings at an alarming rate.

The researchers studied chestnut-backed antbirds, which live in the forest understory. This species is unusual because it tends to hang on in forest fragments, while other birds vanish.

The team looked for the antbirds in three types of landscapes in Costa Rica: an intact swath of old-growth forest, a protected peninsula at La Selva Biological Station, and two forest fragments. At each site, the scientists monitored nests with video cameras, collecting more than 22,000 hours of footage at 99 nests. The team then reviewed the videos to identify predators that had eaten the eggs or nestlings.

Surprisingly, the birds’ population density was the highest in the forest fragments. The team counted about 40 to 47 pairs of birds per 100 hectares in the fragments but only 17 and 9 pairs per 100 hectares in the intact forest and La Selva reserve, respectively.

The forest fragments also had among the lowest nest predation rates. Nests had a 64 and 72 percent chance of being raided in the two fragments, compared to 79 percent in the intact forest and 95 percent at the reserve. Predators included snakes, fire ants, an opossum, an ocelot, and a hawk. But the bird-eating snake (Pseustes poecilonotus) was by far the deadliest, accounting for about 80 percent of the nest attacks.

The results suggest that attacks on eggs and nestlings are not driving chestnut-backed antbirds to decline in forest fragments, the team says. “[F]ragments may instead provide refuge from nest predation, allowing population increase,” the authors write in Biological Conservation. However, it’s also possible that the species is just taking advantage of the disappearance of other, less hardy antbirds.

Bird-eating snakes might thrive at La Selva because they are protected from hunting and have few predators, the authors speculate. To find out more about the snakes’ activity, researchers could capture some of the animals and equip them with radio transmitters. Roberta Kwok | 26 February 2015

Correction: A previous version of this article incorrectly referred to antbirds as “ant-eating birds.”

Source: Increased abundance, but reduced nest predation in the chestnut-backed antbird in Costa Rican rainforest fragments: surprising impacts of a pervasive snake species. Biological Conservation doi: 10.1016/j.biocon.2015.01.015.

Image © Geoff Gallice | Flickr (Creative Commons Attribution 2.0 Generic license)