2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Polish ornithologists Katarzyna Turzanska and Justyna Czachulska from Wroclaw and Zelenogorsk Universities studied the gray warbler (Sylvia communis), a songbird smaller than a sparrow, and found a Spanish slug (Arion vulgaris) in one of the nests where chicks had recently hatched. The next day, the slug was no longer in the nest, and all the chicks were dead. Scientists discovered that they had severe injuries, which pointed to the slug as the culprit in the death of the chicks. The stunned researchers decided to study deeper the relationship between birds and gastropods and as a result published a review in the Journal of Avian Biology.
Among gastropods, there are predatory species, in particular European snails from the genus Poiretia, but they feed on invertebrates - mainly other mollusks and earthworms. However, since the 1920s, there have been periodic reports of slugs, mostly from the genus Arion, attacking chicks of small birds from the order of passerines. Most often, scientists caught red (Arion rufus) and black slugs (Arion ater), as well as a Spanish slug (Arion vulgaris) attacking bird nests.
Slugs attack chicks and inflict severe injuries on them, which usually lead to death. After "communication" with mollusks, the chicks found bleeding wounds, partially eaten beaks and eyes, extensive lesions of the skin of the wings, back and head. Scientists have observed damage to both soft tissue and bones. The chicks were covered with mucus, and snail droppings often remained around the nest. Researchers have recorded not only cases of slugs attacking chicks, but also eating bird eggs.
“When a slug enters the nest - perhaps by accident or in search of this kind of food - it starts devouring the chicks with a radula, or tongue covered with tiny teeth,” explains Katarzyna Tuжanska. "The chicks are unable to defend themselves, they are eaten alive."
When slugs attack the nest, adult birds behave very strangely: they do not protect the chicks in any way and may even leave their brood. Moreover, the researchers describe a case when the black-headed warbler began to incubate a nest in which the slug fed on its chicks. Perhaps due to the fact that slugs attack the nests quite rarely, the birds have not developed a protective behavior and they do not see enemies in the snails.
Slugs crawl into nests on the ground and bushes. The highest height at which the researchers observed a mollusk in a bird's nest is 81 centimeters. Snails are most active at temperatures from 4–5 to 22 degrees Celsius and at high humidity, respectively, the greatest number of attacks on nests was recorded in cool and humid weather. Interestingly, some slugs can sense odors, learn, and change their eating habits. However, so far there has been no research whether slugs can recognize the presence of chicks at a distance and purposefully crawl to the nest.
How often slugs hunt bird nests, the authors of the review find it difficult to say: the sample of nests on which the studies were carried out was small. However, Polish ornithologists note that one of the most common representatives of the genus Arion, the Spanish slug, whose homeland was the Iberian Peninsula, has penetrated most of Europe and the United States over the past 60 years. These mollusks 8–12 centimeters long can lay up to 400 eggs annually and withstand quite extreme conditions (for example, in Switzerland they were found at an altitude of 1,700 meters above sea level). With such vitality and aggressiveness, slugs can seriously affect the population of small birds.