Alternative Foods of a Diet Specialist, the Snail Kite

-Although Snail Kites (Rostrhamus sociabilis) in Florida and Venezuela fed mostly on Pomacea snails, I documented three alternative foods. In Florida, kites fed on five species of small turtles, but especially on Sternotherus odoratus and Kinosternon bauri. During the height of a drought, one kite in Florida fed on a small aquatic snail (Viviparus georgianus) for 5 weeks.In Venezuela, freshwater crabs (Dilocarcinus dentatus) made up on average 10% of the Snail Kite's diet, but more than 25% during September and October. Kites consumed alternative foods with techniques that resemble snail-eating behavior, such as entering a turtle's body cavity by piercing the only leg shaped like a snail operculum. Handling times for turtles (76 min) and crabs (5.4 min) were much longer than for Pomacea snails (1.5 min). Viviparus snails required approximately one-third less handling time but contained one-fifth less body mass than Pomacea snails. Handling time (30 s) for V. georgianus did not differ between the Snail Kite and the Boat-tailed Grackle (Quiscalus major), a diet generalist. These findings are related to the factors that reinforce diet specialization and the ecological conditions that promote diet diversification in specialists. Like Pomacea snails, alternative foods have shells or carapaces and move relatively slowly. Alternative foods are probably less profitable than Pomacea snails, except for large crabs. Although crabs were regularly eaten by kites in Venezuela, turtles and Viviparus snails were eaten in Florida only during times of food scarcity. Received 9 June 1989, accepted 6 November 1989. DEPENDENCE on a single prey type is unusual among vertebrates, but may be more common in invertebrates (Bristow 1988). A notable exception, the Snail Kite (Rostrhamus sociabilis), feeds almost exclusively on one genus of freshwater snails (Pomacea). In Florida the kite eats only Pomacea paludosa (Howell 1932, Snyder and Snyder 1969, Sykes 1987), while in Central and South America several Pomacea species are eaten (Haverschmidt 1962, 1970; Beissinger 1983; Snyder and Kale 1983; Bourne 1985). Alternative foods are taken on rare occasions. These include turtles (Sykes and Kale 1974, Woodin and Woodin 1981, Takekawa and Beissinger 1989) and a small mammal (Sykes and Kale 1974). Other less carefully documented observations of nonsnail foods include a "small yellowish serpent" (Slud 1980), a ring-necked snake (Diadophis punctatus), and a dead American Coot (Fulica americana) (Sykes 1987). In Venezuela and Colombia, kites fed on freshwater crabs and 1 Address to which reprint requests should be sent. Marisa snails for short periods (Mader 1981, Snyder and Kale 1983). The evolutionary pathway to specialization is often difficult to identify from ecological studies (Futuyma and Moreno 1988). However, some understanding of the factors that promote and maintain diet specialization may emerge by examining the causes and consequences for specialists when they select alternative foods. For instance, two morphological specializations allow Snail Kites to efficiently catch and eat snails (Snyder and Snyder 1969). These are a long, slender, hooked bill to sever the snail's columellar muscle and detach the snail's body from its shell, and long toes for seizing the snail by the shell. These morphological specializations could be expected to carry a cost of reduced efficiency for feeding on alternative foods (MacArthur 1972, Benkman 1988). Here I document alternative foods regularly eaten by Snail Kites in Florida and Venezuela. I report behavior associated with diet generalization by a specialist and the ecological conditions that favored diet diversification. 327 The Auk 107: 327-333. April 1990 328 STEVEN R. BEISSINGER [Auk, Vol. 107