EXPERIMENTAL-ANALYSIS OF DIET SPECIALIZATION IN THE SNAIL KITE - THE ROLE OF BEHAVIORAL CONSERVATISM

We examined factors maintaining extreme diet specialization in the sna il kite (Rostrhamus sociabilis), a medium-sized hawk which feed almost exclusively on Pomacea snails, by determining why during some months kites eat crabs (Dilocarcinus dentatus) in the llanos of Venezuela. We offered snails and crabs of different sizes to wild free-flying birds to develop estimates for a prey choice model. Handling times of Pomac ea doliodes snails averaged 90+/-39 s and were positively correlated w ith snail size. Handling times for crabs (xBAR=353+/-130 s) were signi ficantly longer and exhibited greater variation than for snails, and w ere not correlated with crab size. Edible crab tissues had greater dry weights and contained more energy (25.37 kJ/g) than tissues of snails (16.91 kJ/g). Total energy of crabs was much greater than that of sna ils, and total energy of both foods was highly related to body length. We constructed an allometric equation for profitability of snails and crabs. Snails were more profitable than all but the largest crabs, bu t estimates of variance in profitability were greater for crabs. Predi ctions from the model were tested by offering crabs that represented e qual, greater and much greater profitability than snails, to determine whether kites chose prey according to profitability. Only 15.6% of 28 9 food items chosen were crabs. Half of the 18 kites tested did not ea t crabs and only 3 birds switched from snails to more profitable crabs . Four fledglings showed no preference for snails. The role of neophob ia in food choice was investigated by offering unfamiliar snails (Poma cea urceus) to kites. Kites exhibited neophobic behaviors, and 5 of 12 birds chose not to capture P. urceus. Two-thirds of the 12 snails cho sen were rejected immediately, but the others were handled efficiently (xBAR=133+/-89 s). Although morphological adaptations allow kites to specialize on snails, the costs of specialization were overcome for ki tes when the profitability of alternative food increased sufficiently. Our results suggest a role for behavioral conservatism, in the form o f risk-averse foraging and neophobia, in maintaining severe diet speci alization in the snail kite.