Rapid long-distance colonization of Lake Gatun, Panama, by snail kites

Abstract

-The distribution of the Snail Kite (Rostrhamus sociabilis) is closely tied to that of apple snails (Pomacea spp.), its nearly exclusive food. Before the early 1990s, the species occurred in Panama primarily as a vagrant. Apple snails were introduced to Lake Gatun in central Panama in the late 1980s, and by 1994 Snail Kites had colonized the lake from population sources at least 350 km away and initiated breeding. Since 1994 the population has increased rapidly and the species can now be found throughout the lake. Received 7 Oct. 1998, accepted 6 Jan. 1999. The Snail Kite (Rostrhamus sociabilis) is a highly specialized raptor that ranges from southern Florida and Mexico, through Central America, to Bolivia, northern Argentina and Uruguay (Beissinger 1988). Its distribution is closely tied to that of apple snails (Pomacea spp.), which form its diet almost exclusively (Beissinger 1988). The kite uses its exceptionally thin upper mandible to extract snails from their shells (Snyder and Snyder 1969). Other species of snails, turtles, crabs and other items are taken on occasion (Beissinger 1990a, Snyder and Kale 1983, Sykes and Kale 1974). The species is nomadic, moving in response to changes in the availability of its favored prey because of fluctuating water levels (Sykes 1979, 1983; Beissinger and Takekawa 1983, Takekawa and Beissinger 1989). Snail Kites are rare in southern Central America, and there have been only seven previous reports from Panama (Ridgely and Gwynne 1989). The sole Panama specimen, an immature female, was collected near the Colombian border at Perm6, near Puerto Obaldia, San Blas Province, in 1929 (Wetmore 1965). There were four reports of single birds, all either females or immatures, at marshes near Panama City, in 1971, 1973, 1977, and 1979. There were two reports from Chiriqui province in western Panama; an adult male and a female or immature near Gualaca in 1965, and several pairs and a nest in a marsh near Remedios in 1973, the latter being the only previous report of breeding activity in Panama (Ridgely and Gwynne 1989). The closest significant populations of Snail Kites to Panama are in western Colombia, on the west side of the Gulf of Uraba (Hilty and Brown 1986), about 350 km from the Panama Canal area, and the Tempisque Basin in northem Costa Rica (Stiles and Skutch 1989), approximately 650 km away.