154 SHORT COMMUNICATIONS

burrows sampled in this study had been previously used Burrowing Owls in southeasternIdaho. M.Sc. thesis, by Burrowing Owls. Univ. of Idaho, Moscow. Burrows used for nesting were in soils with a greater HANSEN,R. M., AND I. K. GOLD. 1977. Blacktail prairie sand content than non-nest burrows. Although this dif- dogs, desert cottontails, and cattle trophic relations ferencewas statisticallyinsignificant (P i 0.14), it may be on shortgrassrange. J. Range Manage. 30:2 1O-2 14. biologically significant. The significant difference in bur- HOPE,K. 1969. Methods of multivariate analysis.Gor- row entrancediameter between nest and non-nest burrows don and Breach, Inc. New York. indicatesthat BurrowingOwls modify prairie dog burrows MERRIAM,C. 1902. The prairie dog of the Great Plains, used as nest sites. Presumably, sandy soil would facilitate p. 257-270. In Yearbook of the United States De- enlarging burrow passageways.Coulombe (197 1) stated partment of Agriculture. Government Printing Office, that in California burrow diameters averaged20 cm, and Washington, DC. suggestedthat owls may modify burrows that have been RICH, T. 1984. Monitoring Burrowing Owl populations: abandonedby rodents. In addition, sandy soils drain rap- imolications of burrow re-use. Wildl. Sot. Bull. 12: idly, which would reduce nest flooding during frequent 178-180. spring and summer rainstorms. THOMSEN,L. 1971. Behavior and ecologyof Burrowing Owls on the Oakland Municipal Airnort. Condor 73: We thank C. Sorg and M. Rumble for assistancein _ _ collectingdata. R. T. Reynolds, D. Finch, and S. Anderson 177-192. provided helpful comments on earlier drafts of this manu- URESK,D. W., J. G. MACCRACKEN,AND A. J. BJUGSTAD. 1982. Prairie dog density and cattle grazingrelation- script. ships, p. 199-201. In R. M. Timm and R. J. Johnson [eds.], ProceedingsFifth Great Plains Wildlife Dam- LITERATURE CITED age Control Workshop. Institute of Agriculture and Natural Resources,Univ. of Nebraska, Lincoln. BONHAM,C. D., AND A. LERWICK. 1976. Vegetation WEDGEWOOD,J. A. 1976. Burrowing Owls in southcen- changesinduced by prairie dogs on shortgrassrange. tral Saskatchewan.Blue Jay 34:26-44. J. Range Manage. 29:221-225. BRADY,N. C. 1974. The nature and properties of soils. Department of Range Science,Colorado State University, 8th ed. Macmillan Publ. Co., New York. Fort Collins, Colorado80523; United States Department COULOMBE,N. H. 1971. Behavior and population ecol- OfAgriculture,Forest Service,Rocky Mountain Forestand ogy of the Burrowing Owl, Speotytocunicularia, in Range Experiment Station, Rapid City, South Dakota the Imperial Valley of California. Condor 73:162- 57701; and Departmentof Range Science,Colorado State 176. - University,Fort Collins, Colorado80523. Presentaddress DAUBENMIRE,R. 1959. A canopy coverage method of offirst author: SR Box 51370, Fairbanks, Alaska 99701. vegetation analysis.Northwest Sci. 33:43-64. Received 19 March 1984. Final acceptance14 September GLEASON,R. S. 1978. Aspects of the breeding biology of 1984.

TheCondor87:154-157 terfowl rarely involves combat and generallyis shown only Q The Cooper Ornithological Society 1985 towardconspecifics(McKinney 1965, Seymour 1974, Sey- mour and Titman 1978, Stewart and Titman 1980), al- TERRITORIALITY AND though exceptionsare known (McKinney et al. 1978, Sa- vard 1982). INTERSPECIFIC AGGRESSION Recently, Nuechterlein and Storer (1985) described IN STEAMER-DUCKS interspecific aggressionof Plying Steamer-Ducks on sev- eral freshwater lakes in the Argentine Andes. These au- thors frequently observed combat between male T. pa- BRADLEY C. LIVEZEY tachonicusand regularly noted “mass spooks” of grebes, AND coots, and duckswhen territorial pairs of T. patachonicus PHILIP S. HUMPHREY called or approached.They also witnessedtwo severe at- tacks (one fatal) by males on Red Shovelers(Anas plata- lea), and found carcassesof five more shovelers and one Steamer-ducks(Tachyeres spp.) are large diving ducks of Yellow-billed Pintail (A. georgica)which were determined southern Argentina and Chile, and the Falkland Islands to have been victims of steamer-ducks.Nuechterlein and (Murphy 1936). Four species are recognized currently Storer arguedthat the massive skeletaland muscularmor- (Humphrey and Thompson 1981): Flying Steamer-Duck phology of steamer-ducksmakes the “costs,” or risks, of (T. patachonicus),Magellanic FlightlessSteamer-Duck ( T. interspecific aggressionnegligible, and that this renders pteneres),Falkland Flightless Steamer-Duck (T. brachyp- “profitable” the attackson the possibly food-competitive terus),and White-headed FliahtlessSteamer-Duck (T. leu- shovelers.They also suggestedthat suchkillings may serve coce&alus). Flying Steamer-Ducks breed in both fresh- as displaysof fightingability in males, and hencebe main- water and marine habitats; the three flightlessspecies are tained partly by sexual selection. strictly marine in distribution (Humphrey and Livezey, in As part of an ongoingstudy of the morphology, system- press). All members of the genus feed primarily on large atics, and ecologyof steamer-ducks,we have observedall molluscs and crustaceansobtained from the bottom by four speciesof Tachyeresat a number of ecologicallydi- diving and shallow-water foraging, or found exposeddur- verselocalities: Ushuaia, Tierra de1Fuego, Argentina (De- ing low tide (Murphy 1936; Weller 1972; Livezey and cember 1980-January 1981); Puerto Deseado,Santa Cruz, Humphrey, unpubl.). Argentina (January-February 1981); Puerto Melo, Chu- Steamer-ducksare renowned for their pugnacity; nu- but, Argentina (February 1981, December 1981, January merous observershave describedtheir intense, sometimes 1982);Andean lakesof SantaCruz and Chubut (December fatal territorial combat (Vallentin 1924;Reynolds in Lowe 198l-January 1982); Puerto Montt and nearby lakes, Re- 1934; Murphy 1936; Pettingill 1965; Cawkell and Ham- gion X, Chile (December 1982-January 1983); and Port ilton 1961; Weller 1972, 1976). Territoriality in other wa- Stanleyand Lively Island, east Falkland Islands (January- SHORT COMMUNICATIONS 155

February 1984). While we agreein part with the inferences the structurallysimple, open, almost linear nature of their of Nuechterlein and Storer (1985), our studies prompt littoral habitat and the associatedsessile, predictable, and us to presenta different perspectiveon territoriality in the defendable food resources.In the three flightless species genus,and to offer somealternate interpretations of attacks and marine populations of T. patachonicus,these habitat on noncongeners,including protection ofbroods, adaptive characteristicsoccur in combination with a relatively be- “play,” and nonadaptive “inertial” aggression. nign maritime climate which permits year-round residen- Most of the aggressionshown by steamer-ducksis dis- cy. Together, these conditions may have favored the evo- tinctly territorial (sensudefense of an area; Noble 1939), lution of birds capableof year-round sequesteringof food, although we observed some aggressionamong birds in i.e., with intenseterritoriality and morphologicalweapons. non-breeding flocks or feeding on highly tidal waterfronts We hypothesize that this evolutionary trend was selec- where no static territory was obvious. Territoriality in tively maintainedthrough intrageneric competition for food steamer-duckshas been interpreted as defenseof nest site, and that, as at present, steamer-duckshad few or no non- food supplies, brood, and/or loafing site, and probably congeneric food competitors. These selection pressures servesmost or all of thesefunctions (Vallentin 1924; Wel- probably affected marine populations of steamer-ducks ler 1972, 1976). Both sexesregularly are involved in ter- most intensely, primarily becauseof the year-round de- ritorial disputes,although males are the primary combat- fendability of food supplies.This generaladaptive regime, ants(Cawkell and Hamilton 1961, Pettingill 1965, Schmidt however, probably also applies to freshwater T. pata- 1969). Methods of attack are described by Nuechterlein chonicus,because the feeding and nestingecology, general and Storer (1985). Defended areas generally are contig- features of breeding habitat, sympatric water birds, and uous,regularly spacedsegments of shorelineand adjacent probable marine wintering areas of lake-nesting T. pata- water, and marine territories of at least 7’. bruchypterus chonicusare similar to those of marine steamer-ducks. are defended all year, and perhapsfor life (Vallentin 1924; The selective advantage of intrageneric territorial be- Cawkell and Hamilton 1961; Pettingill 1965;Weller 1972, havior and associatedmorphology in steamer-ducks is 1976). Territorial dispersionof nestingpairs evidently af- enhanced through the defense of preferred nesting and fects some regulation of local population densities of brood-rearingsites, and perhaps,to a lesserextent, through steamer-ducks,in that large flocks of non-breeding birds, the protection of nesting females from attack and/or rape which are excluded from territories and loiter at com- by other males. We know of no observation of rape in munal loafing sites, have been observed for T. brachyp- Tachyeres,however, and we suspectthat males have rel- term, T. leucocephalus,and marine T. patachonicus(Mur- atively little need to defend their mates because of the phy 1936;Pettingill 1965;Weller 1972, 1976;pers. observ.) fightingabilities of the females, the low mobility of males, Steamer-duck aggressionis most intense among con- probably long-term pair bonds, and the localized ac- geners. During late summer, we observed numerous in- tivity of birds during nesting. Territoriality and large size traspecificterritorial disputesin both sexesof T. bruchyp- probably co-evolved with other “K-selected” traits of term, several of which culminated in prolonged physical steamer-ducks:increased longevity, late sexual maturity, combat. Also, 12 of 18 territorial encountersof continental and the importance of competitive interactions (relative steamer-ducksfor which we have field notes were intra- to environmental factors) in reproductive success. generic: seven between T. pteneresand T. patachonicus, In all speciesof Tuchyeres,it is evident that failure to four within T. pteneres,and one within T. leucocephalus. acquireand hold a territory precludesbreeding. We believe Invariably, in territorial encountersinvolving two species that this territorial “arms race” (Dawkins and Krebs 1979) of Tuchyeres,the individual of the larger flightlessspecies is the cause, in part, of weight-related flightlessnessin was dominant. steamer-ducks,which includes25% of the males of marine We found mended fracturesin 22 of 170 complete post- T. patachonicusat two localities in Argentina (Humphrey cranial skeletonsof steamer-ducks(13 of 89 males, 9 of and Livezey 1982). In addition, such selection probably 8 1 females; 13% overall); the count of fractures excludes affects males more than females, and undoubtedly con- one bird with a mended humerus that obviously was bro- tributes to the relatively great sexual dimorphism in the ken by a .22-caliber bullet. Tiemeier (194 1) found that 33 genus (Livezey and Humphrey 1984). We suspectthat of 256 (13%) assortedanatid skeletonscontained healed from this several-faceted, mostly intrageneric regime of fractures,but these included six that obviously had been selection emerged a heavy-bodied, hyperaggressive,pre- causedby gunshot. Based on our discussionswith local dominantly sedentary, and largely flightlessgenus of wa- residents, steamer-ducksare not considered good eating terfowl, which is preadapted for aggressiontoward other and are hunted seldom if at all. Hence, most of the frac- species.African Black Ducks (Anussparsu), which inhabit tures we found must have resulted from fights, falls, or linear, well-defined territories on rivers that permit year- oredators. Elements of the wine. and pectoral girdle com- round residency, show a similar combination of life-his- prised 2 1 of the 25 “naturally” broken bones; and six of tory characteristics:long-term pair bonds, year-round ter- these were in the carpometacarpus,which supports the ritoriality in both sexes,absence of rape, frequent physical wing knobs that are used in fighting. This finding strongly combat, and aggressiontoward several other species(Ball suggeststhat the majority of fracturessuffered by steamer- et al. 1978, McKinney et al. 1978). ducks resulted from combat. Although we believe that Red Shovelersare not serious Unlike most anatids (McKinney 1965), steamer-ducks competitors for food with steamer-duckson fresh or salt frequently attack other (non-congeneric)species, often fa- water, we agree with Nuechterlein and Storer (1985) tally (Table 1). Speciesfrom seventaxonomic orders have that the risks or costsincurred by steamer-ducksin inter- been attacked. involving birds in the wild and in aviaries; specific aggressionare negligible, hence, making such at- captive victims may have been less able to escape than tacks competitively neutral or marginally favorable. The those in natural habitat. In addition, a captive female T. likelihood of interspecificaggression probably is especially bruchypteruskilled a wild rat (Rattus sp.; Sea World per- great becausethe simple structureof the habitat permits sonnel,pers. comm.), and a captive pair of T. brachypterus good visibility and allows for some ecologicaloverlap (Or- with youngattacked a 1.5-m catfish(Silurus glanis), which ians and Willson 1964). Marginal competition for food, later died (Schmidt 1969). however, is not a sufficient explanation for attacks by Competition and the defendability of food suppliesare steamer-duckson other species,even other anatids, be- minimal prerequisites for the evolution of food-related causea variety of specieshave been attackedon salt water, territoriality (Brown 1964, Nudds and Ankney 1982). We where dietary overlap is insignificant. suggestthat the largesize, physicalstrength, and aggressive Many instancesof interspecific aggressionby steamer- tendencies of steamer-ducksevolved in part becauseof duckscan be most readily interpreted as defenseof duck- 156 SHORT COMMUNICATIONS

TABLE 1. Records of attacksby steamer-duckson non-congeners,including birds that were pursuedbut not engaged physically, birds that were killed, and the circumstancesof the attacks,if known. Speciesof Tuchyeresare abbreviated: pat = patachonicus,bra = brachypterus,leu = leucocephalus.

Attacking steamer-duck(s) “&;$) With Spaes attacked(no., age,sex) Species Sex Paired brood captive

Podicepsmajor (1, ad) No pat” Yes No No Pers. observ. Diomedea sp. (1) Yes bra - - Yes Todd 1979 Phalacrocoraxolivaceus (1) No path Yes Yes No Pers. observ. Phalacrocoraxsp. Yes bra No No Yes Hubbs-Sea World personnel Nycticorax nycticorax Yes bra No No Yes Hubbs-Sea World personnel Chloephaguhybridu (1) No bra - - No Pettingill 1965 Cygnussp. Yes bra - - Yes Todd 1979 Anas sibilatrix (2,4 8~) No pat= - No No Pers. observ. No pap - No No Pers. observ. iiS,a “Z c Yes bra No No Yes D. Crompton and J. Kear, pers. comm. A. strepera(1, ad)d Yes bra No No Yes D. Crompton and J. Kear, pers. comm. A. platyrhynchos(2, ad, ~9) Yes bra No No Yes D. Crompton and J. Kear, pers. comm. A. georgica(1, adp Yes - No Nuechterlein and Storer 1985 A. platalea (1, ad)f Yes pat” Yes No No Nuechterlein and Storer 1985 (1, ad, aY No Dats Yes No No Nuechterlein and Storer 1985 (5, ad)‘” Yes pap - - No Nuechterlein and Storer 1985 (1, adY Yes bra No No Yes D. Crompton and J. Kear, pers. comm. Lophonettaspecularioides No path Yes No No Pers. observ. i:; No leu Yes Yes No Pers. observ. Netta erythrophthalma Yes bra Yes No Yes D. Crompton and J. Kear, (5, downy) pers. comm. Netta sp. (several, ad) Yes bra Yes No Yes Delacour 1954, Schmidt 1969 Fulica americana (several) Yes bra No No Yes’ Hubbs-Sea World personnel Larus dominicanus(several) No bra Yes Yes No Pettingill 1965, Weller 1972 aOn freshwater. bOn salt water. EPinioned hirds. d Full-wingedbirds. s Diagnosedpost-mortem. ‘In full wing molt. 1 Four birds in wing molt. h IO-month-oldbird. ’ ’ Duck wasfree at time of killings. lings againstpotential avian predators, made effective by would be especially important for young birds and may morphologicalrefinements for combat. Attacks by steam- be consideredan example of “functionalist” animal play er-ducks on albatrosses,herons, and gulls (Table 1) can (Fagen 1974). The killing of five downy African Pochards be explained most simply as anti-predator actions (Hum- (Net& erythrophthalma)by a ten-month-old male T. bru- phrey and Livezey, in press). This explanation also may chypterus(Table 1) conforms well to this interpretation. apply to attacks on grebes and coots; related species in Nuechterlein and Storer (1985) rejected the hypoth- both groupsare known to attack ducks and broods (Gul- esis that interspecifickilling by Flying Steamer-Ducksre- lion 1953, Kirby 1976). sultsfrom mistaken identification of the attackedbirds as We agree with Nuechterlein and Storer (1985) that conspecifics,a view proposedfor birds generally by Mur- ritualization of interspecific attacks is likely in steamer- ray (1971, 1981). We suggestthat the aggressiveadapta- ducks. The marginal or neutral competitive importance tions of steamer-ducksand the associatedlow cost of in- of the aggression,the behavior of females during attacks terspecific attack make it likely that most sizable birds by their mates on other species,and the intensity of attack encountered on defended waters are attacked as if they suggestthat theseacts are sexuallyselected to some extent. were predatorsof ducklings,or competitors for food, nest Interspecific attacks by steamer-ducksmay also serve sites,or mates. For steamer-ducks,there probably is little as adaptive learning experiencesor practice for combat. selective pressurefor discrimination of targets for inter- The incessantfighting among young birds that we observed specificaggression. in wandering, non-breeding flocks of T. brachypterus In conclusion,we view interspecificaggression in steam- probably represents,in part, informative bouts in which er-ducksas a suite of secondaryadaptations for protection dominance hierarchies are established,fighting skills are of young, defense of food resourcesfrom marginal com- improved, and pair bonds are formed. Other species,par- petitors, sexually selectedritualized behavior for assess- ticularly other anatids, also may be safe targets for im- ment of males by females, and practice for intrageneric provement of the steamer-ducks’ complex combat skills combat. Some attacks may represent nonadaptive, “in- that are essentialfor reproductive success.Such practice ertial” aggression.Consequently, we suggestthat much of SHORT COMMUNICATIONS 157 the aggressionof steamer-duckstoward other birds is not morphism in continental steamer-ducks.Condor 86: directly related to competition for food, in contrast to the 368-377. findings of several authors from studies of interspecific LOWE,P. R. 1934. On the evidence for the existenceof aggressionin other avian species (e.g., Simmons 1951, two speciesof steamerduck (Tuchyeres),and primary Kruuk 1967. Codv 1968. Dow 1977. Savard 1982). We and secondary flightlessnessin birds. Ibis 76:467- emphasize, however, that the presumed low proximate 495. costsof this varied interspecificaggression were made pos- MCKINNEY, F. 1965. Spacing and chasing in breeding sible only by the extensiveand costly competition-related ducks. Wildfowl 16:92-106. adaptations of steamer-ducks for intrageneric combat, MCKINNEY,F., W. R. SIEGFRIED,I. J. BALL,AND P. G. H. which in turn are related to predictable, defendable food FROST. 1978. Behavioral specializationsfor river life supplies.These adaptationsfor aggressionare presumably in the African Black Duck (Anus sparsa Eyton). Z. costly throughout life, in terms of energy and materials. Tierpsychol. 48:349-400. In addition, they probably have had important influence MURPHY,R. C. 1936. Oceanic birds of South America. on other aspects of the life history and morphology of Vol. 2. American Museum of Natural History, New steamer-ducks,including sexual dimorphism, flightless- York. nessand associatedlow mobility, and budgetingof energy MURRAY,B. G., JR. 1971. The ecologicalconsequences reservesfor territorial behavior, nesting, and brood-rear- of interspecific territorial behavior in birds. Ecology ing. 52:414-423. MURRAY,B. G., JR. 1981. The origin of adaptive inter- Our studieswere supportedby National ScienceFoun- specificterritorialism. Biol. Rev. Camb. Philos. Sot. dation grantsDEB-80-12403 and DEB-8 1-17942. We are 56:l-22. grateful to G. L. Nuechterlein and R. W. Storer for the NOBLE,G. K. 1939. The role of dominance in the social opportunity to review early drafts of their paper. We thank life of birds. Auk 56:263-273. J. Kear and the late D. Crompton of the Wildfowl Trust, NUDDS, T. D., AND C. D. ANKNEY. 1982. Ecological Martin Mere, and C. Laughlin of Sea World, San Diego correlatesof territory and home range size in North for sharing their observationsof steamer-ducks.We also American dabbling ducks. Wildfowl 33:58-62. are grateful to F. McKinney for his helpful comments on NUECHTERLEIN,G. L., AND R. W. STORER. 1985. Ag- the manuscript. gressivebehavior and interspecific killing by Flying Steamer-Ducksin Argentina. Condor 87:87-9 1. LITERATURE CITED ORIANS, G. H., AND M. F. WILLSON. 1964. Interspecific territories of birds. Ecology 45:736-745. BALL, I. J., P. G. H. FROST,W. R. SIEGFRIED,AND F. PETTINGILL,0. S., JR. 1965. Kelp Geese and Flightless MCKINNEY. 1978. Territories and local movements Steamer Ducks in the Falkland Islands. Living Bird of African Black Ducks. Wildfowl 19:6l-79. 4:65-78. BROWN,J. L. 1964. The evolution of diversity in avian SAVARD,J.-P. L. 1982. Intra- and inter-specific com- territorial systems.Wilson Bull. 76: 160-169. petition between Barrow’s Goldeneye (Bucephulais- CAWKELL,E. M., ANDJ. E. HAMILTON. 1961. The birds lundicu)and Bufflehead(Buceuhulu ulbeola). Can. J. of the Falkland Islands. Ibis 103:l-27. Zool. 6&343:-3446. . - CODY, M. L. 1968. Interspecific territorialitv among SCHMIDT,C. R. 1969. Preliminary notes on breedingthe hummingbird species.Condor 70~270-27l._ - Falkland Islands Flightless Steamer Duck. Int. Zoo DAWKINS.R.. AND J. R. KREBS. 1979. Arms races be- Yearb. 9:125-127. tween and within species.Proc. R. Sot. Lond. B Biol. SEYMOUR,N. R. 1974. Territorial behaviour of wild Sci. 205:489-5 11. shovelersat Delta, Manitoba. Wildfowl 25:49-55. DELACOUR.J. 1954. The waterfowl of the world. Vol. 1 SEYMOUR,N. R., AND R. D. TITMAN. 1978. Changesin Country Life, London. activity patterns, agonistic behaviour, and territori- Dow, D. D. 1977. Indiscriminate interspecificaggression ality of Black Ducks (Anus rubripes)during the breed- leading to almost sole occupancyof spaceby a single ing seasonin a Nova Scotiatidal marsh. Can. J. Zool. speciesof bird. Emu 77: 115-l 2 1. 56:1773-1785. FAGEN,R. 1974. Selective and evolutionary aspectsof SIMMONS,K. E. L. 1951. Interspecificterritorialism. Ibis animal play. Am. Nat. 108:850-858. 93:407-413. GULLION,G. W. 1953. Territorial behavior of the Amer- STEWART,G. R., AND R. D. TITMAN. 1980. Territorial ican Coot. Condor 55:169-l 86. behaviour by prairie pothole Blue-winged Teal. Can. HUMPHREY,P. S., AND B. C. LIVEZEY. 1982. Fliahtless- J. Zool. 58:639-649. ness in’ Flying Steamer-Ducks.Auk 99:368-372. TIEMEIER,0. W. 1941. Repaired bone injuries in birds. HUMPHREY,P. S., AND B. C. LIVEZEY. In press. Nest, Auk 58:350-359. eggs,and downy youngofthe White-headed Flightless TODD,F. S. 1979. Waterfowl: ducks, geeseand swansof Steamer-Duck. In P. A. Buckley, M. S. Foster, E. S. the world. Harcourt-Brace Jovanovich, New York. Morton, R. S. Ridgely, and F. G. Buckley [eds.],Neo- VALLENTIN,R. 1924. Zoology, p. 285-335. In V. F. Boy- tropical ornithology. Omithol. Monogr. No. 36. son, The Falkland Islands. Clarendon Press, Oxford. HUMPHREY,P. S., AND M. C. THOMPSON.198 1. A new WELLER,M. W. 1972. Ecological studies of Falkland speciesof steamer-duck(Tuchyeres) from Argentina. Islands’ waterfowl. Wildfowl 23:25-44. Univ. Kans. Mus. Nat. Hist. Occas. Pap. 95:1-12. WELLER,M. W. 1976. Ecologyand behaviour of steamer KIRBY, R. E. 1976. Breedingchronology and interspecific ducks. Wildfowl 27:45-53. relations of Pied-billed Grebes in northern Minne- sota. Wilson Bull. 88:4931495. Museum of Natural History and Department of System- K~UUK, H. 1967. Competition for food betweenvultures atics and Ecology, Universityof Kansas, Lawrence, Kun- in east Africa. Ardea 55:171-203. sas 66045. Received 9 April 1984. Final acceptance 13 LIVEZEY,B. C., AND P. S. HUMPHREY. 1984. Sexual di- October 1984.