SHORT COMMUNICATIONS 265

SIBLEY, C. G. AND B. L. MONROE, JR. 1990. Distri- STEPHENS,M. 1984b. Intraspecific distraction displays bution and taxonomy of birds of the world. Yale of the polyandrous Northern Jacana, Jacana spi- Univ. Press, New Haven, Connecticut. 11osa. Ibis 126:70-72. SKUTCH, A. E 1976. Parent birds and their young. STRAHL, S. D. 1987. The social organization and be- Univ. of Texas Press, Austin. haviour of the Hoatzin Opisthocomus hoazin in STEPHENS,M. 1984a. Interspecific aggresive behavior central Venezuela. Ibis 130:483-502. of the polyandrous Northern Jacana (Jucana spi- WILKINSON, L. 1997. SYSTAT 7.0: the system for sta- nosa). Auk 101:508-518. tistics. SYSTAT Inc., Evanston, Illinois.

Wilson Bull., 11 l(2), 1999, pp. 265-268

Rapid Long-distance Colonization of Lake Gatun, Panama, by Snail Kites

George R. Angehr ’

ABSTRACT-The distribution of the Snail Kite America, and there have been only seven pre- (Rostrhamus sociabilis) is closely tied to that of apple vious reports from Panama (Ridgely and snails (Pomacea spp.), its nearly exclusive food. Be- Gwynne 1989). The sole Panama specimen, fore the early 199Os, the species occurred in Panama an immature female, was collected near the primarily as a vagrant. Apple snails were introduced to Lake Gatun in central Panama in the late 198Os, and Colombian border at PermC, near Puerto Ob- by 1994 Snail Kites had colonized the lake from pop- aldia, San Blas Province, in 1929 (Wetmore ulation sources at least 350 km away and initiated 1965). There were four reports of single birds, breeding. Since 1994 the population has increased rap- all either females or immatures, at marshes idly and the species can now be found throughout the near Panama City, in 197 1, 1973, 1977, and lake. Received 7 Oct. 1998, accepted 6 Jan. 1999. 1979. There were two reports from Chiriqui province in western Panama; an adult male and a female or immature near Gualaca in The Snail Kite (Rostrhamus sociubilis) is a 1965, and several pairs and a nest in a marsh highly specialized raptor that ranges from near Remedios in 1973, the latter being the southern Florida and Mexico, through Central only previous report of breeding activity in America, to Bolivia, northern Argentina and Panama (Ridgely and Gwynne 1989). The Uruguay (Beissinger 1988). Its distribution is closest significant populations of Snail Kites closely tied to that of apple snails (Pomacea to Panama are in western Colombia, on the spp.), which form its diet almost exclusively west side of the Gulf of UrabB (Hilty and (Beissinger 1988). The kite uses its exception- Brown 1986), about 350 km from the Panama ally thin upper mandible to extract snails from Canal area, and the Tempisque Basin in north- their shells (Snyder and Snyder 1969). Other em Costa Rica (Stiles and Skutch 1989), ap- species of snails, turtles, crabs and other items proximately 650 km away. are taken on occasion (Beissinger 1990a, Sny- der and Kale 1983, Sykes and Kale 1974). STUDY AREA The species is nomadic, moving in response Lake Gatun (420 km2) is an artificial lake created to changes in the availability of its favored by the damming of the Chagres River to form the cen- prey because of fluctuating water levels tral part of the Panama Canal in 1914. The lake level (Sykes 1979, 1983; Beissinger and Takekawa is controlled by the Panama Canal Commission, and 1983, Takekawa and Beissinger 1989). may vary several meters between wet and dry seasons as water is released when ships pass through the locks. Snail Kites are rare in southern Central The Canal area experiences a strong four-month long dry season from mid-December to mid-April. ’ ’ Smithsonian Tropical Research Institute, Unit The introduced aquatic plant Hydrilla verticillata is 0948, APO AA 34002-0948; a major problem in the lake. It apparently first became E-mail: [email protected] established in the late 1920s or early 193Os, and had 266 THE WILSON BULLETIN * Vol. 111, No. 2, June 1999 become a significant problem by the 1960s (Mature11 matures and adult females have brown plum- and Salazar 1994). Rooting in shallow water, it now age and cannot reliably be distinguished in the occurs in huge beds in many parts of the lake. field (S. Beissinger, pers. comm.). The apple snail Pomacea lattrei was introduced to At four locations we observed nests con- Panama from Guatemala in the early 1980s by Pana- ma’s National Directorate of Aquaculture. Snails were sisting of loose platforms of twigs at various established at a research station at Divisa, Herrera stages of construction. One small vine-cov- Province, approximately 130 km west of Lake Gatun, ered tree standing in water (Site 1) had seven for use in rural aquaculture programs. Between 1986 nests while the other sites had one nest each. and 1988, the owner of a sportfishing business in the No eggs or chicks were evident at that time, town of La Arenosa on the southwestern arm of Lake although we could not see the contents of high Gatun obtained 15 snails from the Divisa station and introduced them to a small artificial pond next to Lake nests. Santamarfa recently had seen birds Gatun. After they had reproduced, she distributed bringing twigs to add to the nests. some to neighbors and introduced others into the lake. The area was visited again on 20 May 1994 A few months later it was noticed that the snails were by D. and L. Engleman. They did not see any feeding on and reducing the abundant Hydrilla around active nests at Site 1, but at least 17 kites were the community’s public dock and in surrounding areas. soaring or perched in the area. They observed Residents of other lakeside communities began delib- erately introducing snails to their areas in an effort to five nests at another site (Site 2) about 300 m control Hydrilla (Mature11 and Salazar 1994) a major north of Site 1. These were located on a small impediment to small boat travel. The snails are also island composed of Annona glubru shrubs used for food by some local people. overgrown with vines. Three nests were com- Surveys by the Panama Canal Commission found plete, one with two eggs and another with at that the snails were distributed throughout the south- least one egg and perhaps more. A third nest eastern arm of the lake by 1991, were in the north- western part by 1992, and had reached the town of appeared to have eggs but they could not be Gamboa at the east end of the lake by 1993. In some counted. The remaining two nests were under areas Hydrilla cover was reduced by as much as 94% construction and were visited by birds in in three years. Deliberate introduction by humans ev- brown plumage carrying twigs. At least 20 idently allowed the snail to disperse quickly around kites were present at this site, yielding a total the lake. The snails also spread by floating in currents of at least 37 in the cove (D. and L. Engleman, and by egg masses fixed to floating vegetation, logs, pers. comm.). and boats (Mature11 and Salazar 1994). A native species of apple snail, Pomacea cumingi, I visited Guindilla Cove again on 15 July occurs in the Canal area. It is a bottom-dwelling spe- 1994. At that time seven nests were present at cies that typically occurs in low densities. It mostly Site 2. Two nests had three well-feathered inhabits small streams, but is sometimes found in larg- nestlings, while two more fledglings were er rivers. The combination of habitat, bottom dwelling perched together on a branch near a third nest. habit, and low density evidently makes this species Three other birds which took flight from the unsuitable as prey for Snail Kites (E G. Thompson, island had very short tails and appeared to pers. comm.). have recently fledged. On 27 August 1994 two nests at Site 1 each had a single large downy RESULTS young. No active nests were present at Site 2, M. Santamarfa (pers. comm.), a game war- but I saw two birds that apparently had re- den at the Barro Colorado Nature Monument, cently fledged. On 14 October no activity was a reserve managed by the Smithsonian Trop- seen at Site 1, but three apparently recently ical Research Institute, first observed unusual fledged birds were present at Site 2. On this hawklike birds eating snails in Gigante Bay, date I also surveyed several other areas in Gi- south of Barr-o Colorado Island, in May or gante Bay that I had not previously investi- June 1994. On 3 February 1995 Santamaria gated. One nest with two small nestlings was and I visited an area where he had recently found near an island at the mouth of Guindilla seen birds building nests in Guindilla Cove, a Cove, and a second nest with two fledglings narrow inlet about 1500 m long on the south on the verge of flying was found on a large shore of Gigante Bay. We saw at least 14 Snail Annona glabra island about 3 km west of Kites in the cove at that time, including at Guindilla. Approximately 15 adults and three least two adult males in black plumage. The apparently recently fledged young were also remaining birds were in brown plumage. Im- seen at this site. SHORT COMMUNICATIONS 267

DISCUSSION though there are few data, clutch size else- where in the kite’s range seems typically to be Apple snails probably reached the area of two to three, although four-egg clutches often Gigante Bay in 1991 (Mature11 and Salazar occur in Argentina (Beissinger 1990b) and 1994). I am certain no kites were present in clutch size may range up to six (Beissinger the area before 1992, because I made three 1988). Despite a presumably superabundant surveys of the shoreline of parts of the bay by food supply at Gigante in 1995, there was no canoe in 199 1 and 1992, and surveyed Guin- evidence that kites increased their clutch size dilla Cove itself on 23 August 1992. Guindilla in response. Cove probably was colonized by kites in ei- The Snail Kite colony in Guindilla cove in- ther 1993 or 1994. creased from no birds in 1992 to at least 37 In 1995, in the central part of the lake near in 1995, an extraordinarily high rate of in- the Panama Canal channel, Snail Kites were crease if only a single founding pair had been restricted to Gigante Bay, based on informa- involved. Because Kites are highly sociable, tion from Smithsonian game wardens and re- initial colonization could have been by a small searchers who worked on the lake. Kites were flock of birds rather than a single pair. It is rarely if ever seen at the Smithsonian research possible that Guindilla Cove was not the ini- station on the north side of Barro Colorado tial site of colonization of Lake Gatun. Snails Island 4 km away. were present in the southwestern arm of the Since 1995, kites have been seen more of- lake by the late 1980s. Kites could have col- ten in other parts of Lake Gatun and the Canal onized that area first, and then spread to Guin- area in general, ranging from the northern end dilla. The southwestern arm of the lake is of the lake near the Gatun Locks to Miraflores large and remote from population centers, so Locks near the Pacific entrance to the Panama a colony there could easily have gone unde- Canal. Although no comprehensive surveys of tected. kite distribution and numbers have been made, Nevertheless, Snail Kite populations in- Snail Kites appear to have spread throughout crease rapidly under favorable conditions the lake and occur wherever suitable habitat (Snyder et al. 1989, Beissinger 1995), such as is present. the essentially unlimited food supply Lake In 1995 nest construction evidently began Gatun would have offered the first colonists. in January, during the early dry season. A few The age at first reproduction in Snail Kites is small nestlings were still present in mid-Oc- very low, with some females nesting at 10 tober, during the late rainy season. These latter months. In favorable years in Florida, the birds would probably have fledged in Novem- breeding season may last up to 10 months, and ber, so that in 1995 the breeding season was some kites may re-nest and raise second at least 10 months long. In Florida, the main broods (Beissinger 1986, Snyder et al. 1989). nesting season is January-August, peaking in Parents of either sex may desert a nest leaving February-June, although in years of high wa- the other member of the pair to continue to ter breeding may begin in December and ex- raise the young alone (Beissinger and Snyder tend to September (Beissinger 1986, 1988; 1987). The remaining parent almost always is Snyder et al. 1989). Nesting seasonality is able to rear the young to independence by it- poorly known elsewhere in the tropics, but self, while the deserting parent has the oppor- may be tied to the rainy season. Nesting takes tunity to re-nest with another partner. place during the wettest period in Surinam, The rapid colonization by Snail Kites of Argentina and Venezuela (Beissinger 1988). Lake Gatun and subsequent population expan- At Gigante, clutch or brood size ranged sion provides an interesting example of the from one to three. In Florida, clutch size pres- dispersal capabilities of this highly opportu- ently ranges from one to three with the latter nistic species. The most likely source of col- much more frequent. However, clutch size ap- onizing birds would have been the Colombian pears to have declined historically in Florida, population, approximately 350 km from Lake with four-egg clutches previously having been Gatun. This population is much closer than common (Beissinger 1986). The decline may the one in Costa Rica, and there are several be attributable to habitat deterioration. Al- large areas of freshwater habitat, including 268 THE WILSON BULLETIN l Vol. Ill, No. 2, June 1999

Lake Bayano and several large rivers, in the BEISSINGER,S. R. AND J. E. TAKEKAWA. 1983. Habitat intervening area. Dispersers discovered the use and dispersal by Snail Kites in Florida during drought conditions. Fla. Field Nat. 11:89-106. newly suitable habitat of Lake Gatun within HAVERSCHMIDT, E 1970. Notes on the Snail Kite in only a few years after the introduction of their Surinam. Auk 87:580-584. preferred food, and rapidly spread throughout HILTY, S. L. AND W. L. BROWN. 1986. A guide to the the available area. birds of Colombia. Princeton Univ. Press, Prince- ton, New Jersey. ACKNOWLEDGMENTS MATLJRELL,J. C. AND A. J. SALAZAR. 1994. Aspectos de la introduccidn y diseminacibn de1 caracol gi- I would particularly like to thank M. Santamaria, gante Pomacea sp. en el Lago Gatdn y sus efectos who first told me of the kites, D. and L. Engleman, sobre la abundlncia de Hydrilla verticillata. Pan- who provided additional information, and D. George, ama Canal Commission, Bureau of Engineering who organized two of our visits to the colony. K. and Construction, Balboa, Panama. Aparicio, S. Follett, K. Kaufmann, and J. Nason also RIDGELY, R. AND J. GWYNNE. 1989. A guide to the assisted with field observations. I thank E G. Thomp- birds of Panama. Princeton Univ. Press, Princeton, son for identifying snail specimens and providing ad- New Jersey. ditional information on Pomacea biology. S. R. Beis- SNYDER, N. E AND H. A. SNYDER. 1969. A compara- singer and I? W. Sykes Jr. provided helpful reviews that tive study of mollusc predation by Limpkins, Ev- improved the manuscript. I would also like to thank erglade Kites, and Boat-tailed Grackles. Living ANAM, Panama’s environmental authority, for grant- Bird 8:177-233. ing the permits under which this research was con- SNYDER, N. E AND H. W. KALE, II. 1983. Mollusk ducted. predation by Snail Kites in Colombia. Auk 100: 93-97. LITERATURE CITED SNYDER, N. F., S. R. BEISSINGER,AND R. E CHANDLER. 1989. Reproduction and demography of the Flor- BEISSINGER,S. R. 1986. Demography, environmental ida Everglade (Snail) Kite. Condor 91:300-316. uncertainty, and the evolution of mate desertion STILES, E G. AND A. E SKUTCH. 1989. A guide to the in the Snail Kite. Ecology 67:1445-1459. birds of Costa Rica. Cornell Univ. Press, Ithaca, BEISSINGER,S. R. 1988. Snail Kite. Pp. 148-165 in New York. Handbook of North American birds. Vol. 4 (R. S. SYKES, I? W., JR. 1979. Status of the Everglade Kite Palmer, Ed.). Yale Univ. Press, New Haven, Con- in Florida 1968-1978. Wilson Bull. 91:495-511. necticut. SYKES, P. W., JR. 1983. Recent population trends of BEISSINGER,S. R. 1990a. Alternative foods of a diet the Everglade Kite in Florida and its relationship specialist, the Snail Kite. Auk 107:327-333. to water levels. J. Field Omithol. 54:237-246. BEISSINGER,S. R. 1990b. Experimental brood manip- SYKES, I? W., JR. AND H. W. KALE, II. 1974. Ever- ulations and the monoparental threshold in Snail glades Kites feed on non-snail prey. Auk 91:818- Kites. Am. Nat. 136:20-38. 820. BEISSINGER,S. R. 1995. Modeling extinction in peri- TAKEKAWA, J. E. AND S. R. BEISSINGER. 1989. Cyclic odic environments: everglades water levels and drought, dispersal, and the conservation of the Snail Kite population viability. Ecol. Applications Snail Kite in Florida: lessons in critical habitat. 5:618-631. Conserv. Biol. 3:302-3 11. BEISSINGER,S. R. AND N. E R. SNYDER. 1987. Mate WETMORE, A. 1965. The birds of the Republic of Pan- desertion in the Snail Kite. Anim. Behav. 35:477- ama. Vol. I. Tinamidae to Rynchopidae. Smith- 487. sonian Institution Press, Washington, D.C.