Shorebird feeding on stranded giant gastropod egg capsules of Adelomelon brasiliana (Volutidae) in coastal Argentina

We report predation by shorebirds on large egg capsules of the gastropod Ad elomelon brasiliana. This gastropod is a common inhabitant of sandy substra tes in the northern coastal area of Argentina and is the only South America n volutid known to have free, unattached egg capsules. These capsules are c ommonly found strewn on beaches, especially after storms. Two shorebirds we re observed feeding on embryos inside capsules, the American oystercatcher (Haematopus palliatus) and the South American stilt (Himantopus melanurus). During periods when egg capsules were stranded both birds switched to prey on embryos inside capsules stranded on the sandy beach. Field observations and a field experiment showed that birds prefer to prey on capsules that h ave embryos in advanced developmental stages. It is interesting to note tha t of all the shorebirds observed at the study site, only the local non-migr atory species preyed on capsules.