The risk to birds has come from Polish researchers. According to Science News, Justyna Chachulska and Katarzyna Turzańska were monitoring nests of the common whitethroat (Sylvia communis), a mid-size European warbler on the outskirts of the city of Wrocław, when they noticed something unusual.
The researchers spotted a slug crawling into a nest containing three young birds. This seemed unusual although it didn’t seem especially significant. Completing their observation, the researchers moved onto to other tasks. When the researchers returned to the following day they found that two of the birds were dead and one was missing. With the dead birds, there were signs of parts of the birds having been eaten. Analysis of this suggested the slug was the likely candidate.
As to how common this is, it appears to be rare. A review of literature by the researchers found only one case ever described in the U.S., and a handful documented in Poland, Germany and the Netherlands. The species of slugs involved were red slug (Arion rufus), the black slug (Arion ater) and the invasive slug (Arion vulgaris). These are fairly large slugs, the approximate size of a human thumb. The slugs are native to the Iberian Peninsula.
Slugs normally feed on vegetation, mushrooms and dead animals. Cases of slugs eating live animals are very unusual. The researchers are searching for further incidents. One of their concerns is whether these unusual attacks are likely to increase, and whether this correlates with a rise in the number of slugs across Europe. Here the slug is regarded as a pest and an unwelcome invasive species.
With the most recent observation, involving the common whitethroat, the bird builds its nest in low shrub or brambles. In a typical nest three to seven eggs are laid. The low level of the nest, relative to other birds, increases the vulnerability to predators.
The findings are published in the Journal of Avian Biology. The research paper is titled “Arion slugs as nest predators of small passerine species – a review.”
