An Exceptional Event of Predation on Desert Snails by Migratory Thrushes in the Negev Desert, Israel

Abstract

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 °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 °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 predation rates as a function of time and of prey densities produced dome—shaped curves. Mostly S. zonata were taken first, but later preferences switched to T. seetzenii. This was associated with a switch in the use of anvils, from stones, which were rare, to the shells of S. zonata, which are thicker than those of T. seetzenii, and were far commoner than stones. Thus the two species differed in the type and density of appropriate anvils on which they could be crackled, 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, the acquisition of experience by the birds in locating anvils and prey and in handling them, were responsible for the variation in predation rates and diet composition observed throughout the course of the predation event.