Death by slug: It's not a good way to go, but that's exactly what's happening to baby birds in Poland. Researchers there have witnessed a surprising pattern of large slugs of the Arion genus crawling into the nests of birds and eating the hatchlings alive, reports New Scientist.

A trail of slime is the only evidence remaining from such attacks. The behavior is so unusual that the birds' parents don't even try to defend them, possibly because they don't see the slugs as a threat until it's too late. One parent bird was even witnessed incubating a slug feeding on its dead chicks.

“The actual moment of slugs predating on nestlings isn’t easy to observe,” explained Katarzyna Turzańska from the University of Wroclaw in Poland, one of the researchers in the study. "You are more likely to come across the traces of the ‘tragedy’: dead or alive nestlings with heavy injuries, covered in slime – and often slugs’ droppings found nearby."

The researchers' work appeared in the Journal of Avian Biology.

Turzańska and colleague Justyna Chachulska have been researching this behavior since they observed it for the first time while studying whitethroat birds near Wroclaw, in Poland. Although predation by slugs has been recorded before across Europe, almost all of these reports pertain to bird species that nest close to the ground. That giant slugs would seek out treetop-nesting birds as food is a surprise, however. It appears unprecedented.

Slugs in the Arion genus can grow quite large, and they'll eat almost anything. While most slugs spend their time dining on leaves and rotting plants, they have been known to devour earthworms and other smaller slugs. Decomposing animals are certainly on the menu as well. And, apparently, now they've expanded their appetite to include live chicks.

While much of a slug's diet involves items they just stumble upon, these gastropods have a sensitive sense of smell that can guide them to a meal. It's certainly possible that they are seeking out chicks because they have developed a taste for them.

“When a slug finds itself inside a nest — probably accidentally, or maybe by actively searching for this type of food — it just starts foraging on the living nestlings using its radula, or tongue covered in tiny teeth,” explained Turzańska. “The nestlings are unable to defend themselves and are eaten alive.”

The deaths are particularly tragic because it ought to be easy for bird parents to flick an invading slug away. No doubt, if slugs continue to develop a taste for hatchlings, birds will evolve defense mechanisms in time. For now, though, Polish slugs appear to be feasting on easy meals.