• A slug caught in action, attacking baby birds in a nest. (K. Leniowski , E. Węgrzyn & A. Wojton/Taylor & Francis)Source: K. Leniowski , E. Węgrzyn & A. Wojton/Taylor & Francis
Turns out slugs are not picky eaters.
Julianna Photopoulos

New Scientist
29 Aug 2016 - 11:15 AM  UPDATED 29 Aug 2016 - 11:15 AM

Some baby birds are meeting a slimy end. Voracious supersized slugs have been seen chomping on chicks in nests on or near the ground.

“The actual moment of slugs predating on nestlings isn’t easy to observe,” says Katarzyna Turzańska at the University of Wroclaw in Poland. “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.”

She and Justyna Chachulska, a colleague at the University of Zielona Góra in Poland, were studying common whitethroat birds near Wroclaw, in Poland, when they spotted a slug of the Arion genus in a nest with newly hatched chicks. The next day, the slug was gone, and the chicks were dead with severe injuries on their bodies that hinted at the slug as the culprit.

They were gobsmacked. “We couldn’t believe that the slug had killed the nestlings,” says Turzańska. “We talked to many experienced ornithologists, but none of them had observed slug predatory behaviour towards birds before.”

But it turns out that this was not a one-off event: Turzańska and Chachulska found that there had also been reports of this type of behaviour before. Most of them occurred in Europe and concerned species that nest close to ground, such as the whitethroat, wood and reed warbler, and chiffchaff.

A few papers described an actual attack, showing unequivocally that slugs do predate on nestlings. Others described the injuries as being different from those left by other predators, which, together with mucus trails and droppings, suggested a slug as the cause of death.

But for many, this behaviour comes as a surprise. Ben Rowson, a zoologist at the National Museum Wales in the UK, says he was not aware of all these cases of slugs eating birds. 

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Slime wave

Slugs are not picky eaters: they will munch on anything from tender leaves to decomposing plants, animal carcasses, faeces and rotting paper. Some species are known to feed on earthworms and other slugs.

Slugs are known to detect odours, so Turzańska and Chachulska suggest that they may be able to catch the scent of nestlings and seek them out. But there is no evidence that they do this yet.

“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,” says Turzańska. “The nestlings are unable to defend themselves and are eaten alive.”

Surprisingly, the birds’ parents don’t seem to defend them, perhaps because such predation does not happen often enough for them to have evolved a defence response. A blackcap was even seen incubating a slug feeding on dead chicks.

The culprits are the three large European slug species from the Arionidae family of “roundback slugs”: the red slug (Arion rufus), the black slug (Arion ater), and the invasive Spanish slug (Arion vulgaris).

It is often difficult to work out which species of slug is responsible, but the signs that a slug attack has happened – including huge skin wounds, holes in beaks or muscles, and missing eyes – are obvious.

The Spanish slug has spread to many European countries, posing threats to plants and crops. These mass invasions could also affect local bird populations.

“If these invasive slugs are attacking birds, it’s one more thing that makes them unwelcome,” says Rowson. “Although we face additional foreign slug species in the UK, several of which are spreading, I think the amount of nestlings attacked by slugs would be very small compared to those by their vertebrate predators, such as magpies, grey squirrels and hedgehogs.”

Journal reference: Journal of Avian Biology, DOI: 10.1111/jav.01189

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