The Dispersal of Snails by Birds

Abstract

I WAS present at a meeting of the Malacological Society last May when Dr. Boycott read a very interesting paper, in which he showed how the small snail Balea perversa occurred on trees, walls, and rocks, but not on the ground. The question arose how it got from tree to tree; and in the resulting discussion the fact was brought out that it had exceptionally tenacious slime. There appears to be no doubt that it is carried by birds, to which it may become attached at night while they are asleep. The Island of Porto Santo possesses a large number of endemic snails, and also a certain number of species, such as Euparypha pisana and Cochlicella acuta (the latter, new to the island, I found abundant locally north of the Villa Baleira), which have been introduced by man. But there is one species, Balea perversa, which was found by Wollaston only on the isolated summit of the highest mountain in the island, the Pico do Facho. This is far from human habitation, and it is nearly certain that the snail was not brought there by man. It is equally improbable that it is a member of the ancient fauna, remaining unchanged while all the other species have diverged in various directions. The strong probability is that it was carried by birds, which could reach the Madeiras in a short time, while the snails remained dormant attached to their legs. Mr. J. Y. Johnson many years ago cited no less than seventy species of birds which had been observed as visitors or stragglers to the Madeira Islands.