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The predation of slugs on passerine bird nestlings is the topic of the article which will soon be published in Journal of Avian Biology (it has already caught the attention of, among others, New Scientist). We talk about the research on atypical slug behaviour with Katarzyna Turańska (pictured), doctoral student at the Natural History Museum of the University of Wrocław, who wrote the article in co-operation with Justyna Chachulska from the University of Zielona Góra.

How did you get interested in this unusual topic?

Katarzyna Turańska: During field studies for my doctoral dissertation, in spring 2014, we found in common whitethroat’s nest a large slug along newly hatched chicks. The next day the nestlings were already dead, with serious tissue damage, and the slug had left mucus and droppings. At first we didn’t believe that the slug could have killed the chicks. However, after long search, we collected more than ten publications on the topic. We decided to sum them up and describe the phenomenon in a review article which would help the observers such as ourselves understand what could have happened in the nest.

How slugs „attack” baby birds? Such scenes remind me only of cheap horrors.

Slugs don’t attack in some spectacular way. They just glide into a nest with helpless chicks and start preying on them. With the anatomical structure they use for feeding, radula, they cause damage to soft tissue or even bones or beaks.

Is it difficult to document such behaviour?

The available photographic documentation is very scarce. The best photos showing a slug preying on blackcap’s nestlings can be found in the article of Dr Leniowski and his co-authors from 2013. It’s difficult to spot how a slug preys on baby birds, because it rarely happens, probably in a limited group of bird species which nest close to the ground.

What are the effects of these attacks?

A slug preying on chicks can lead to their death, if it makes enough damage to weaken them – when they no longer have the strength to beg for food, parents stop feeding them. Some nestlings, however, are strong enough to leave the nest and survive in spite of their injuries.

Why bird parents don’t protect their chicks?

It is quite strange. In theory the birds should at least try to remove the slug, just as they do with other „garbage” in their nest, leaves, for example. The blackcaps from the experiment of Rzeszów ornitologists did not even try to throw the slug out of the nest. We don’t know why. It’s possible that slug is so rare a predator that birds have not yet developed evolutionary tools to address this threat.

Which birds are at biggest risk?

Species nesting on the ground or close to it, where slugs are found most often. The cases reported so far involved, among others, yellowhammer, marsh and wood warbler, common whitethroat, reed bunting.

What was the aim of your work?

Above all we wanted to reach a broad range of biologists, our goal was to familiarise them with the topic. We wanted to create a source where an observer such as ourselves could find the answer to the question about what a slug might be doing in a bird’s nest. We hope that our work may inspire someone to study this further. Two issues are particularly interesting here: is breeding success of birds living in places with mass occurrence of slugs really significantly decreased by this predator and can slugs memorise the smell of nestlings and let it be their guide when looking for food.

Do similar, peculiar for a layman, aggressive behaviours of animals usually considered peaceful often occur in nature?

If this layman reads some into the literature concerning bird breeding biology, he can be really  surprised with the fact that uninteresting slugs can kill vertebrates or that fluffy squirrels or beautiful, colourful woodpeckers take chicks out of their nests.

Interview by Michał Raińczuk

Photo by Wojciech Pietras

The article above may be copied and distributed under CC BY 2.0. licence.