"Some species of fresh-water shells have a very wide range, and allied species, which, on my theory, are descended from a common parent and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much, as their ova are not likely to be transported by birds, and they are immediately killed by sea water, as are the adults. I could not even understand how some naturalised species have rapidly spread throughout the same country. But two facts, which I have observed—and no doubt many others remain to be observed—throw some light on this subject. When a duck suddenly emerges from a pond covered with duck-weed, I have twice seen these little plants adhering to its back; and it has happened to me, in removing a little duck-weed from one aquarium to another, that I have quite unintentionally stocked the one with fresh-water shells from the other. But another agency is perhaps more effectual: I suspended a duck's feet, which might represent those of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and just hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point."
Charles Darwin. 1859. On the Origin of Species by Means of Natural Selection
A recent post at Research at a snail's pace on land snail dispersal has prompted me to write this. I have already written briefly about the speculations that very small snails may be distributed by the wind. There is, however, quite a bit of evidence to support Darwin's prediction that birds may disperse mollusks.
The oldest published record on this subject that I have in my collection is an anonymous 1936 note from the Nautilus1 that reported that "a tiny fresh-water mollusk [sic], Succinea" had been found in the back feathers of a Vesper Sparrow, Pooecetes gramineus.
Rees2 cited several records of aquatic gastropods and bivalves recovered from the feet of birds. He also gave records of the land snails Vitrina pellucida in Europe and Succinea species in the U.S. that had been found in the plumage of migrant birds. Interestingly, he also gave records of both the land snail Pomatias elegans and several species of freshwater limpets found on the legs of bees and aquatic beetles, respectively.
Dundee et al.,3 reported finding Succinea unicolor among the feathers of the Woodcock (Scolopax minor), the Common Snipe (Gallinago gallinago) and the Whippoorwill (Caprimulgus vociferous) in Louisiana.
Vagvolgyi4 also discussed the dispersal of land snails by birds and gave citations to several relevant papers some of which I haven't read.
Baur & Bengtsson5 briefly discussed land snail dispersal by birds.
Wesselingh et al.,6 proposed 2 ways birds can transport live mollusks: attached to their feathers or feet or in their digestive tracts. They gave examples of live mollusks recovered from birds' feces. They also implicated dispersal by birds in the distribution of the freshwater snails Tryonia and Planorbarius.
Oxyloma retusa preparing for takeoff.
1. Anonymous. 1936. Succinea carried by a bird. Nautilus 50:31. [Note that Succinea is actually a land snail genus.]
2. Rees, W.J. 1965. The aerial dispersal of mollusca. Proc. Malac. Soc. London 36:269-282.
3. Dundee et al. 1967. Snails on migratory birds. Nautilus 80:89-91.
4. Vagvolgyi, J. 1975. Body size, aerial dispersal & origin of the Pacific land snail fauna. Systematic Zoology 24:465-488.
5. Baur & Bengtsson. 1987. Colonizing ability in land snails on Baltic uplift archipelagos. Journal of Biogeography 14:329-341.
6. Wesselingh, F.P., G.C. Cadée & W. Renema. 1999. Flying high: on the airborne dispersal of aquatic organisms as illustrated by the distribution histories of the gastropod genera Tryonia and Planorbarius. Geologie en Mijnbouw 78:165-174.