An exceptional event of predation on desert snails by migratory thrushes in the Negev Desert, Israel.

Migratory thrushes (mostly Turdus philomelos) preyed heavily upon desert land snails, Sphincterochila zonata and Trochoidea seetzenii in Sede Boqer, Israel, in late winter, 1972. The event lasted approx 55 d and occurred only once in 8 yr. Yet due to its low potential for population increase, S. zonata did not recover from the loss of c.65% of its population within the 7 subsequent yr. T. seetzenii, on the other hand, recovered rapidly from its loss of 10% of the population. The 2 snail species differed in the type and density of appropriate anvils on which they could be cracked, and in their density, availability, energy content, and ease of finding and cracking. These differences, associated with the gradual reduction in prey density due to depletion, acquisition of experience by the birds in locating anvils and prey and in handling them, were responsible for variation in predation rates and diet composition.-from Authors