ALTERNATIVE FOODS OF A DIET SPECIALIST, THE SNAIL KITE

STEVEN R. BEISSINGER Yale University,School of Forestryand EnvironmentalStudies, New Haven, Connecticut06511 USA, • and Departmentof ZoologicalResearch, National ZoologicalPark, SmithsonianInstitution, Washington,D.C. 20008 USA

ABSTRACT.--AlthoughSnail Kites (Rostrhamus sociabilis) in Florida and Venezuela fed mostly on Pomaceasnails, I documentedthree alternativefoods. In Florida, kites fed on five species of smallturtles, but especiallyon Sternotherusodoratus and Kinosternonbauri. During the height ofa drought,one kite in Floridafed on a smallaquatic snail (Viviparus georgianus) for5 weeks.' In Venezuela,freshwater crabs (Dilocarcinus dentatus) made up on average10% of the Snail Kite's diet, but more than 25% during Septemberand October. Kites consumedalternative foodswith techniquesthat resemblesnail-eating behavior, such as enteringa turtle'sbody cavityby piercingthe only leg shapedlike a snailoperculum. Handling times for turtles(76 rain) and crabs(5.4 rain) were much longer than for Pomaceasnails (1.5 rain). Viviparussnails requiredapproximately one-third less handling time but containedone-fifth less body mass than Pomaceasnails. Handling time (30 s) for V. georgianusdid not differ betweenthe Snail Kite and the Boat-tailedGrackle (Quiscalus major), a diet generalist. Thesefindings are related to the factorsthat reinforcediet specialization and the ecological conditionsthat promote diet diversificationin specialists.Like Pomaceasnails, alternative foodshave shells or carapacesand moverelatively slowly. Alternative foods are probably lessprofitable than Pomaceasnails, except for large crabs.Although crabs were regularly eatenby kitesin Venezuela,turtles and Viviparussnails were eatenin Floridaonly during times of food scarcity.Received 9 June1989, accepted 6 November1989.

DEPENDENCEon a single prey type is unusual Marisa snails for short periods (Mader 1981, among vertebrates,but may be more common Snyder and Kale 1983). in invertebrates (Bristow 1988). A notable ex- The evolutionary pathway to specialization ception, the Snail Kite (Rostrhamussociabilis), is often difficult to identify from ecological feedsalmost exclusively on one genusof fresh- studies(Futuyma and Moreno 1988). However, water snails (Pomacea).In Florida the kite eats someunderstanding of the factorsthat promote only Pomaceapaludosa (Howell 1932,Snyder and and maintain diet specialization may emerge Snyder 1969, Sykes 1987), while in Central and by examining the causesand consequencesfor SouthAmerica several Pomacea species are eaten specialistswhen they select alternative foods. (Hayerschmidt 1962, 1970; Beissinger 1983; For instance,two morphological specializations Snyder and Kale 1983; Bourne 1985). Alterna- allow Snail Kites to efficiently catch and eat tive foods are taken on rare occasions. These snails (Snyder and Snyder 1969). These are a include turtles (Sykes and Kale 1974, Woodin long, slender, hooked bill to sever the snail's and Woodin 1981, Takekawa and Beissinger columellar muscleand detach the snail's body 1989)and a smallmammal (Sykes and Kale 1974). from its shell, and long toesfor seizing the snail Other less carefully documentedobservations by the shell. These morphological specializa- of nonsnail foods include a "small yellowish tions could be expected to carry a cost of re- serpent"(Slud 1980),a ring-neckedsnake (Dia- ducedefficiency for feeding on alternativefoods dophispunctatus), and a deadAmerican Coot (Fu- (MacArthur 1972, Benkman 1988). lica americana)(Sykes 1987). In Venezuela and Here I documentalternative foods regularly Colombia, kites fed on freshwater crabs and eaten by Snail Kites in Florida and Venezuela. I report behavior associatedwith diet general- ization by a specialistand the ecologicalcon- Addressto which reprint requestsshould be sent. ditions that favored diet diversification.

327 The Auk 107:327-333. April 1990 328 STEVENR. BEISSINGER [Auk,Vol. 107

METHODS Turtles were eaten most frequently when

I observed the food habits of Snail Kites in Florida water levelswere low during drought(n = 34 during studiesof their reproductivebehavior (1979- turtles;71%) or in yearsfollowing drought(n 1983). Studies were conducted on Lake Okeechobee = 10 turtles; 21%). Kites also ate turtles in winter (Glades, Hendry, and Okeechobee counties), State months(n = 4 turtles;8%) during or just after Water Conservation Areas 2 and 3 (Dade and Broward the passageof cold fronts. The seasonaldistri- counties), and Lakes Tohopekaliga and Kissimmee bution of turtle predation was as follows: 2 in (OsceolaCounty). Beissinger(1986) and Beissinger January,4 in February,9 in March, 28 in May, and Snyder (1987) describedthe study areasand cli- and 5 in July. matologicalconditions. Fieldwork was conducted from During the height of a drought in Florida in Januaryto August each year, except 1980 (July to 1981, one banded 2-year-old male Snail Kite August)and 1983(March to June).Additional obser- (#230) fed almostexclusively on a very small vations were made during February and April 1986. In Venezuela, studies were conducted in the llanos aquaticsnail (Viviparusgeorgianus) at Lake Okee- chobee. The kite was observed 300-500 m north at the ranch "Fundo Pequario Masaguaral" (8ø34'N, 67ø35'W), 45 km south of Calabozo in Guarico, and of the Clewistonlocks on sixdifferent days dur- on other ranchesalong the main highway 30 km to ing a 5-week period (11 June-13 July). Lake the north and south. ! observed food habits of kites levels were so low (ca. 3.0 m above mean sea from July to December1985, and from July to No- level) that V. georgianussnails, normally found vember 1986and 1987,usually by watchingfood de- on the bottomof lakesand streams(Snyder and livered to nestlingsby parents.No observationswere Snyder1969), lay exposedon a shallowmudflat. madeduring the dry season(January-May) when most During 5 hoursof observations,male #230 rap- kites leavethe llanos.Eisenberg (1979) and Beissinger idly ate 46 Viviparussnails and captured12 oth- et al. (1988) describedthe study areas and climato- logical conditions. ersthat he discarded, possibly because they were I observed behavior with 10 x binoculars and 15- empty shells.Although no Pomaceasnails were 60 x spottingscopes. When kites were feeding,their capturedduring these observationperiods, he behavior was described and timed to the nearest sec- caughtone Pomaceasnail during the five weeks ond. Handlingtime is definedas the ti.ne requiredby that he foragedalong the mudflat. Remainsof a kite to extractand consumecompletely a food item 170 Viviparusgeorgianus, 4 PomaceapaIudosa, and after perching with it. Whenever possible,remains 1 unopened freshwater musselwere under male of nonsnail foods were recovered, identified, and #230's extractionperch on 3 July. measured.During this process,I often collectedother During thissame period, another kite foraged food items beneath perches where kites had eaten snails or nonsnail foods. near the mudflaton four occasions.Although Data were analyzed by SYSTAT microcomputer this otherkite frequentlyhad difficultyfinding programs.As the datawere normallydistributed and Pomaceasnails, it never fed on Viviparusgeor- varianceswere equal, I used Student'st-tests to com- gianuseven though it watched male #230 feed pare handling timesbetween Pomacea snails and var- on Viviparussnails. ious alternative foods. Means (g) are reported with On one other occasion (during the 1981 standard deviations (SD). drought), kites fed on Viviparusgeorgianus on Lake Okeechobee.Twelve empty snail shells RESULTS were found under an extractionperch used ex- clusively by kites. Selectionand occurrenceof alternativefoods.-- In the llanos of Venezuela, Snail Kites aug- Small turtles were the alternative food most fre- mented their diet with freshwater crabs (Dilo- quently eaten by kites in Florida (Table 1). I carcinusdentatus). Of 3,866 food items captured watched kites eat 9 turtles and found 39 turtles by kites, 82.9% were positively identified. dead under perchesthat kites used regularly Freshwatercrabs made up 10.3%of the diet and for extractingsnails. Musk turtles (SternotherusPomacea doIiodes the remainder; but, the con- odoratus)accounted for 58.3% and mud turtles sumptionof crabsby Snail Kites varied season- (Kinosternonbauri) 31.3% of the turtles eaten. Soft- ally (Fig. 1). Crabswere rarely eatenfrom July shelledturtles (Trionyx ferox), and peninsulaand through early August, the first half of the wet Florida cooters(Chrysemys neIsoni and C. fiori- season.The frequency of crabsin the diet in- dana)made up the remainder.Most turtles eaten creased in September and October, and crabs by kiteswere smallerthan full-sizedadults, es- constituted more than one fourth of the food peciallythe larger species(Table 1). items in some months. Crab-eating declined in April 1990] Whena SpecialistDiversifies 329

40 ß 81 0 0 1985 / •- -• 1986 / • 3o / •- ß- • 1987 / / m 25

,, 20 III /•8o o 228 / / • "' 727 "/ / 'Z•...• '• 267 ,•- -:." -O ,,, 56,,,' ..K 307 / _. •'• 0 45;381,L--•'•'•,,_.:' ' ' ' ' 91 : : -:' 7 8 9 10 11 12

MONTH Fig. 1. Seasonalvariation in the diet of Snail Kites in the llanos of Venezuela during three years of study. Only two foods were identified: Pomaceadollodes snails and freshwater crabs (Dilocarcinusdentatus). The percentageof crabsin the diet and the total number of food items identified are given for each month that >20 food items were identified.

November and Decemberin 1985,the only year kite pulled small piecesof meat from inside the with sufficient sample sizes to characterizekite turtle'sbody cavity with its bill and swallowed diets during these months. them. Methodsof catchingand consumingalternative SnailKites rarely completelyconsumed a tur- foods.--Snail Kites consumedturtles with tech- tle. Often organsin the anterior part of the body niques that resembled snail-eating behavior cavity (heart and lungs) were not eaten and (Snyder and Snyder 1969,Beissinger 1988). Af- intact eggsremained in 4 turtles. In only one ter arriving at a perch, the turtle was placed on third of the freshlydiscovered turtle bodieswere its back with its head near the branch and its the internal parts completely eaten; in nearly posterior away (Fig. 2). Then the kite would half of the turtles, more than two thirds of the grip the looseskin around a leg and pull, in an internal parts were eaten; and in one fifth of attempt to break the skin and enter the turtle's the turtles,only half was eaten (n = 15). Usually body cavity. From its position looking down on the head and front legs (40%), or these anterior the turtle, the kite could choosewhich append- external body parts and the left-rear leg (33%) age among the four legs and neck it should were not eaten (n = 15); but in 20% of the cases attack. In all instances (direct observations: n = all limbsand the headwere gone,and onceonly 3; and recovered carcasses:n = 3), kites attacked the front legs remained intact. the turtle's right-rear leg. From a ventral view, Viviparus snails were captured and eaten the turtle'sright-rear leg is the only appendage somewhatdifferently than Pomaceasnails. First, that is oriented in a manner which resembles as V. georgianuswas quite abundant and espe- the sickle shape of a Pomaceasnail operculum cially visible in shallow water, male #230 cap- and shell when positioned in a kite's talons for tured them quickly using short flights (14.6 _+ extraction (Fig. 2). 10.8 s, n = 27). In the Everglades,capturing Often a kite had trouble gripping both the Pomaceasnails commonly required flights of 90- turtle and the perch, and on two occasionskites 180 s (pers.obs.). Second, male #230 frequently dropped an uneaten turtle prematurely. One extractedViviparus snails while standingon the kite required5-8 min to tearaway enough flesh exposedmudflat instead of flying to a perch, from the turtle'sright-rear leg to enterthe body the usual substrate for extraction of Pomacea cavity. Once the right-rear leg was pierced, the snails.Third, to eat Viviparus,male #230 simply 330 S•vœ• R. BœI$$IIqGœR [Auk,Vol. 107

T^BI•œ1. Speciescomposition and size of turtles eaten by Snail Kites in Florida. Adult size ranges(Conant 1975) are in parentheses.

Carapacelength (mm) Species n œ+ SD Range Sternotherus odoratus 28 62.8 + 12.5 30.0-76.8 (80-115) Kinosternon bauri 15 82.5 + 5.1 69.8-89.8 (75-100) Chrysemysfioridana 3a 65.3 +_ 1.8 64.0-66.5 (320-330) C. nelsoni 1 57.2 -- (200-305) Trionyxferox 1 45.0 -- (150-498) Totals 48 68.8 + 14.1 30.0-89.8 (75-498)

Only two carcasseswere measured.

pulled on the snail's operculum until both the the abdomen, near the head, and inside the car- operculum and body separatedfrom the shell. apace.Kites usually ate most of the soft parts Viviparussnails cannot closethemselves tightly in the crab'sbody cavity,but the legsand claws in their shells and only moderate force is re- were never eaten and often were not removed. quired to pull the snail'sbody from its shell. In When feeding crabs to nestlings, kites often contrast,before a Pomaceasnail's body can be removed the carapaceand most of the append- removed from its shell, kites must first remove agesbefore carrying the crab to the nest. Kites the operculum and then cut the columellar frequently made two or more trips to the nest muscle (Snyder and Snyder 1969). with pieces from the same crab. The feeding Crab extraction techniques resembled those process,which usually required only one trip usedfor turtles.First, the crabwas pinned to a to the nest lasting 1-2 min for snails,frequently perch (usually by holding one foot on the car- required 6-8 min for crabs. apaceand the other on a claw). Crabswere usu- Handlingtimes of alternativefoods.--Exact han- ally positioneddorsal side down with the head dling times for turtles often could not be mea- toward the perch. Then the carapacewas lifted suredbecause the kites were already in the pro- from the ventral side and pulled off. Very small cessof eating when I found them. In three cases, bits of meat and organs were eaten from under however, I encounteredthe kite shortly after it

Fig. 2. The orientationof a turtle and Pomaceasnail as viewed by a kite when positionedon a perch for extraction.Note how only the fleshy area around the turtle's right-rear leg, indicated by the arrow, has the samesickle shape of the apertureopening of the Pomaceasnail. Kites always began extracting turtle meatby piercing the right-rear leg to enter the body cavity. April1990] Whena SpecialistDiversifies 331

T^BLœ2. Comparisonof the time required to extractand eat (handling time) Pomaceasnails and alternative foods by Snail Kites in Florida (FL) and Venezuela (VZ).

Diet/food item Area n Handling time (s) Preferred Pomaceapaludosa FL 197 95.7 + 37.3 P. doliodes• VZ 248 93.0 + 48.0

Alternative Small turtles b FL 3 4,579.8 + 864.0 Viviparussnails FL 46 31.3 + 15.5 Dilocarcinus crabs VZ 14 329.1 + 119.0

Data from T. Donnay and S. Beissinger(unpubl.). Minimum handling times for Sternotherusodoratus and Kinosternonbauri. (See text for explanation.) began to eat. Handling times for turtles aver- all have shells or carapaces,which permit kites aged > 60 rain (range:63-85 rain) comparedwith to utilize extractionprocedures similar to those just 1.5 rain for Pomaceapaludosa (Table 2). usedfor eatingPomacea snails. For example,the Extractingand eatingcrabs was also relatively techniqueof extractingturtle meat closelyre- time consuming(Table 2). Handling times for sembled the methods that kites use to remove crabswere significantlylonger than for Pomacea a snail from its shell (Fig. 2). Second,alternative doliodes(t = 7.4, df = 260, P < 0.01). Although foods,like Pomaceasnails, move relatively slow- kitesusually were ableto begin eatingcrabmeat ly in the water and cannot struggle vigorously within 30 s, additional time was required to to escapeonce caught. Becausekites descend remove and eat the small bits of meat from the slowly to the water surfacerather than dive on body. prey during capture(Snyder and Snyder 1969), Viviparussnails required approximatelyone- prey that move rapidly would probably be dif- third lesshandling time (t = 11.5, df = 241, P ficult for a kite to capture.Also, kites might find < 0.001) by kites than Pomaceasnails (Table 2). it difficultto grip a strugglingfish, frog, or snake However, the averagewet weight of extracted with their long, thin toes and weak talons. Pomaceasnail bodies (2.9 + 0.6 g, n = 15) on The specializationsthat enable kites to extract Lake Okeechobeewas more than 5 timesgreater and eat Pomaceasnails so efficiently appear to (t = 12.1, df = 24, P < 0.001) than Viviparus make it difficult for kites to extract and eat some extractedwet weights (0.5 + 0.2 g, n = 11). alternative foods (Table 2). Handling times for Boat-tailedGrackles (Quiscalus major) also fed small turtles were >60 rain comparedwith just on Viviparussnails on the mudflatwith kite #230. 1.5 rain for Pomaceasnails. Although a small The gracklescaptured snails by walking, hop- turtle probably provides more energy than a ping, and sometimesmaking short flights. Kite snail, it might not provide more energy than 3 #230 usually capturedsnails more quickly than or 4 snails.The inability of Snail Kites to reach the grackles(14.6 + 10.8 s, n = 27 vs. 22.9 + all of the meat in the anterior portion of a turtle 20.4 s, n = 19), but the differencewas not quite body cavity, comparedwith other turtle-eating statisticallysignificant (t = 1.8, df = 44, P = raptors (e.g. Auffenberg 1981, Woodall 1982), 0.08).Also, there was no differencein the length suggeststhat the kite's hookedbill may not be of time required for the Snail Kite (Table 2) or able to penetratedeeply enoughinto the turtle's the grackles(33.0 + 15.4 s, n = 42) to extract body cavity. Kites presumably selected rela- and eat Viviparussnails (t = 0.5, df = 86, P = tively small turtles (Table 1) possiblybecause 0.61). of the difficulty in manipulating and eating larger ones. Although kites had no difficulty eating crabs, DISCUSSION handling times were 3-4 times longer than for Pomaceasnails. The time-consumingportion of Snail Kites fed mostlyon Pomaceasnails, but extraction was the removal of the small bits of small turtles and another species of snail in meat from the crab's exoskeleton. The kite's Florida, and freshwater crabs in Venezuela, stronglydecurved bill did not appearto hinder served as alternative foods. Alternative foods this process. resemblePomacea snails in severalways. First, Kites are preadaptedto extractand eat Vivip- 332 STEVENR. BEISSINGER [Auk, Vol. 107 arussnails. But apparently little morphological Crabs might also be included in the diet if specialization is required to feed on these mol- they contained some nutrient which snails lusks because the Boat-tailed Grackle, a diet lacked. Because kites in Florida have no nutri- generalist(Snyder and Snyder 1969), extracted tional or growth problemson strictly a Pomacea and ate Viviparusas efficiently as did kite #230. diet (Beissinger 1987, Beissingerand Snyder This kite required approximatelyone-third less 1987), nutrient needs seem an unlikely expla- time to extractand eat Viviparussnails compared nation for crab-eating.If snail abundancefluc- to Pomaceasnails. However, the profitability tuated seasonally,this might also account for (energy consumed/handling time) of Pomacea the switch to include crabs in the diet. Low snail snails should be ca. 1.4 times greater than Vi- abundancein Venezuela is suggestedby lower viparusbecause they are approximatelyone-fifth food delivery rates to nestlings and smaller the size of a Pomacea(assuming the energy and brood sizescompared with Florida (Beissinger water content of snail tissueto be equivalent). 1990). Feeding on such a small snail may have been Natural selection should favor increased ef- an act of desperation for kite #230. Another ficiencyin food handling. If efficiencyis gained kite nearby continued to search for Pomacea, through morphological specialization, diet even though they were apparently difficult to breadth may contract(MacArthur 1972, Futuy- find, rather than eat Viviparussnails. ma and Moreno 1988). I suggestthat even se- Alternative foods in Florida were eaten only vere diet specialistswith strong trophic adap- during times of apparent food scarcitycaused tationscan be somewhatflexible. Specialistscan by regional drought or the passageof a winter feed on alternative foods if the foods structur- cold front. Pomaceasnails estivate in response ally resemblepreferred food, but efficiencyin to drying conditions,which causeskites to dis- handling alternative foods may be low. Al- persefrom the Evergladesthroughout the Flor- though diet diversificationby specialistsoccurs ida peninsulato searchfor food (Beissingerand most often during times of food scarcity,spe- Takekawa 1983,Takekawa and Beissinger1989). cialization may not preclude regularly eating Cold fronts causewater temperaturesto fall rap- some alternative foods that may be profitable idly, andsnails become inactive. Both situations and can be processed with extraction tech- result in snails becoming temporarily unavail- niques already mastered. able to kites. Diet diversification under such circumstancesis in accordancewith optimality ACKNOWLEDGMENTS modelsof diet choice(Stephens and Krebs 1986). Thisstudy was supported by grantsfrom the Smith- These models predict that food items should be sonian Institution, the U.S. Fish and Wildlife Service, included in the diet, not on the basis of their and a National Science Foundation Postdoctoral Fel- abundance,but dependent upon their profit- lowship in Environmental Biology. Sefior Tomas ability and the abundance of more profitable Blohm graciouslyallowed me to live and work on his food items already being eaten. ranch.Russell Thorstrom, Manuel Felipe Rodriguez, Freshwater crabs were the only alternative David Jickling,and Kitsten Silvius assistedwith field foodregularly included in the diet of kites.Crabs observations.Tim Donnay kindly allowed me to use madeup morethan 25%of the diet during some unpublished data. Peter Meylan identified Florida months in Venezuela (no freshwater crabs are turtles and placed them in the Florida StateMuseum collection(catalog numbers 54089-54120, 54174-54183). found in Florida), but the percentageof crabs JohnAnderton drew Fig.2. CraigBenkman, Tim Don- eatenby kitesvaried seasonally(Fig. 1). Because nay, RussellGreenberg, Mickey Hetmeyer,Noel Sny- handling times for crabsare 3-4 times longer der, and Steve Zack made helpful commentson the than Pomaceasnails, crabs would have to yield manuscript. significantlymore energyto be includedin the diet on the basis of profitability alone. Large LITERATURE CITED crabsbecome big enough that they can be more AUFFENI•ERC,W.F. 1981. Behaviourof L1ssemyspunc- profitablethan snails(S. Beissingerand T. Don- tata (Reptilia, Testudinata, Trionychidae) in a nay unpubl. data). Crab size increasesgreatly drying lake in Rajasthan,India. J. BombayNat. throughout the wet seasonin the llanos (Ta- Hist. Soc. 78: 487-493. phorn and Lilyestrom1984) and this could ac- BEISSINGER,S. R. 1983. Hunting behavior, prey se- count for the increasein crab-eatingby kites lection, and energeticsof Snail Kites in Guyana: during these months (Fig. 1). consumerchoice by a specialist.Auk 100:84-92. April 1990] Whena SpecialistDiversifies 333

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