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