j. Field Ornithol., 62(1):117-131 GEOGRAPHIC VARIATION IN THE GRAY KINGBIRD KARENHABERMAN, • D. I. MACKENZIE2 ANDJ. D. RISING3 Departmentof Zoology Universityof Toronto Toronto, Ontario M5S IA I, Canada Abstract.--We analyzedpatterns of geographicvariation in 11 measurementsof 369 Gray Kingbirds(Tyrannus dominicensis) from 10 localities.In bothsexes, kingbirds from migratory populationshad longerwings and larger bills than thosefrom non-migratorypopulations. In addition,there was significantsexual dimorphism in wing and tail length,as well as in somemeasures of bill size and wing shape.We interpretedthe sexualdifferences in wing and tail lengthand shapeto reflectsexual differences in courtshipdisplays and territorial defense.The differencesin bill size may reflect adaptationsfor niche partitioning,as the bill sizedimorphism tended to be greatestin localitieswhere Gray Kingbirdswere the only Tyrannuspresent. VARIACION GEOGRJ•FICA EN TYRANNUS DOMINICENSIS Sinopsis.--Analizamosel patron de variaciongeogrfifica de l 1 parfimetrostomados de 369 individuosde Tyrannusdominicensis pertenecientes a 10 localidadesdiferentes. Aves de ambos sexos,de poblacionesmigratorias, mostraron tener las alas y el picomils grande que indi- viduosde poblacionesno migratorias.Ademfis encontramos un dimorfismosexual signifi- cativoen el largo del ala y rabo, comotambi•n en algunasmedidas del tamafiodel pico y forma del ala. Interpretamosque el dimorfismoen el largo y forma de las alas y el rabo, esreflejo de diferencias sexuales en el patr6nde cortejoy la defenzaterritorial. Las diferencias en el tamafiodel picopoddan ser una adaptaci6npara la segregaci6nde nicho,ya que este dimorfismotendi6 a sermayor en localidadesen dondela especieera el finicorepresentativo de su g•nero. The Gray Kingbird (Tyrannusdominicensis) breeds from the southern Atlantic and Gulf coastsof the United States, south through the West Indies to the islandsoff South America (the Netherlands Antilles east to Trinidad and Tobago), and occasionallyin arid coastalregions of Ven- ezuelaand perhapsColombia. Individuals breeding in the northernparts of this range (United States,Bahamas, Jamaica and Cuba) migratesouth to northern SouthAmerica in winter, but Gray Kingbirdsare generally residentfrom Hispaniolaeastward (American Ornithologists' Union 1983, Bond 1985, Brodkorb 1950, Hilty and Brown 1986, Voous 1983). Brodkorb(1950) studiedgeographic variation in this species.He pre- sortedhis specimensinto five different subspecies,and measuredbirds taken throughoutthe year. The subspeciesdescribed by Brodkorbare not generallyrecognized today (Mayr and Short 1970, Voous 1983), and it is generally agreed that the geographicalvariation, particularly in bill shape,is not well understood(Voous 1983). Here we describepatterns of geographicvariation and sexualdimorphism in the species. Currentaddress: 312 Durant Way,Mill Valley,California 9d9dI USA. Deceased. Authorto whomcorrespondence should be sent. 117 118] K. Habermanet al. J.Field Ornithol. Winter 1991 Hypotheses Size variation.--Bergmann's Rule, the tendency for individuals of a speciesto be relativelylarge in the coldest,or coldestand dryestparts of the species'range, is the mostcommonly described pattern of geographic variationin sizein birds (James1970, Zink and Remsen1986). However, we cannotassess size variationin Gray Kingbirdsbecause we have no measure of body size. Weights were not indicated on enough of the specimenlabels for us to use them as a measureof body size; of the featureswe could measureon the museum specimensavailable, tarsus length might reflectbody size,but others(measures of wing, tail and bill size) probablyare little correlatedwith bodysize (Freemanand Jackson 1990, Rising and Somers1989). Ornithologistshave frequently used wing length as sucha measure,but that is precludedin this studybecause T. dominicensisis migratory in part of its range and sedentaryin others. Generally,within a speciesor amongclosely-related species, individuals from non-migratorypopulations have smaller wings, relative to bodysize, than those from migratory populations[e.g., Passerculussandwichensis (Rising, pers. obs.) and in Vireo (Barlow, pers. comm.)], and, indeed, Brodkorb (1950:334) notesthat Gray Kingbirds that migrate have rel- atively longer wings and shortertails than do thosethat do not. Sexualselection.--Commonly, size dimorphismin animals appearsto have evolvedas a consequenceof intrasexual aggressionor mate choice, that is, of sexual selection.In many birds, males defendterritories, nest sitesand matesfrom othermales, and in suchspecies males are commonly larger than females.Alternatively, in specieswhere maleshave acrobatic aerial displays,such as many shorebirdsand hummingbirds,so-called reversedimorphism (males smaller than females)is common(Jehl and Murray 1986), presumablybecause smaller males are better able to perform suchacrobatic displays. Little has beenwritten on the courtship and territorialityof T. dominicensis.Bent (1942:31)cites Audubon's (1840) accountof the courtship,which indicatesthat both sexesdo aerial displays duringcourtship, and Smith (1966) describesthe "Wing Flutter" display, typical of Tyrannus,for T. dominicensis.Although both sexesperform courtship displays,in kingbirds in general only the males engagein territorial defenseand the "Tumble Flight" display associatedwith it, andtypically male Tyrannusare moreaggressive toward conspecifics than females(Smith 1966). There is no reasonto suspectthat the behaviorof T. dominicensisis substantiallydifferent from that of better-studiedTy- rannus (Smith 1966), and male T. dominicensishave the modified outer primary wing feathersthat are characteristicof adult male kingbirds (Pyle et al. 1987). This probablyreflects a specialrole of thesefeathers during courtshipand territorial displaysof male Tyrannusin general. Assumingthat male T. dominicensisengage in territorial fights more frequentlythan females,we predictthat maleswill be larger, on average, than females,particularly with regard to wing length, both becauseit doubtlessreflects overall body size to someextent as well as flying ability. Alternatively,we couldargue, as did Jehl and Murray (1986) for shore- Vol.62, No. 1 GeographicVariation ofGray Kingbirds [ 119 birds, that acrobaticdisplays select for relatively small males, but this seemsless likely to us becausethe Tumble Flight displaysof Tyrannus do not appear to be as acrobaticas thoseof someshorebirds and hum- mingbirds. Nichepartitioning.--Van Valen (1965) postulatedthat the variability in structuresused for obtainingfood will be greatestin populationswhere the diversityof differenttypes of foodsavailable is greatest.This is the so-called"niche variation hypothesis."The diversityof food availableis influencedboth by the kinds of foodspresent and by the amount of competitionfor those foods.Where the amount of food is potentially limiting, intraspecificcompetition for it is of selectiveimportance. Where sympatricspecies use similar foods,interspecific competition may alsobe important.However, intraspecific competition is probablymore important than interspecificcompetition even in crowdedcommunities because the nicheoverlap of conspecificsmust be greaterthan that of non-conspecifics, and if both individualsof a pair feedon their territory, their overlapping requirementsmay be to their mutual detriment.The magnitudeof this effect dependson resourceavailability and the extent to which the re- quirementsof the male and femaleoverlap, and presumablyselection to reduceintersexual competition would act to exaggeratea differencethat evolvedas a consequenceof sexual selection(Jehl and Murray 1986, Price 1984). Island populationsof melanerpinewoodpeckers, and other populationsthat are allopatricwith otherwoodpecker species of similar size,show significantly more dimorphism in bill sizethan do populations that are sympatricwith congeners(Selander 1966). A similar situation pertainsin southwesternPicoides woodpeckers (Short 1971), and hasbeen describedin other populationsof birds, especiallythose found on islands (Lack 1971). The Gray Kingbird is found throughoutthe Caribbeanregion, and is the commonestWest Indian flycatcher. On many islands, it is the only Tyrannusfound, whereason othersit is sympatricwith one or two other species.The Tropical Kingbird (T. melancholicus)is found on Grenada, Trinidad and in the NetherlandsAntilles; the Fork-tailed Flycatcher(T. savana)is of irregular occurrencein the NetherlandsAntilles; the Giant Kingbird (T. cubensis)is foundon Cuba andthe Isle of Pines,and formerly on Great Inagua; the LoggerheadKingbird (T. caudifasciatus)is found in the Bahamas,Cuba, the Isle of Pines, the Cayman Islands,Jamaica, Hispaniola and Puerto Rico (but not the Virgin Islands;Raffaele 1989). The Giant Kingbird is decidedlylarger than the Gray Kingbird, is un- commonand tends to be found in woodlandsand pine forests,whereas the Gray Kingbird is generallyfound in more open habitats.It is, thus, probably not in significantcompetition with T. dominicensis.The Log- gerheadKingbird likewise is more of a woodlandspecies than the Gray Kingbird, althoughit canbe seenin opencountry where Gray Kingbirds are found (Bond 1985, Raffaele 1989). In the Netherlands Antilles the Tropical and Gray Kingbirds,and the Fork-tailed Flycatcherhave similar habits, but the Gray Kingbird is far more commonthan the other two 120] t:. Habermanet al. J.Field Ornithol. Winter 1991 species(Voous 1983). The Gray Kingbird is sympatricwith the Eastern Kingbird (T. tyrannus)in Florida.