Community Organization in Hummingbirds: Relationships Between Morphology and Ecology

COMMUNITY ORGANIZATION IN HUMMINGBIRDS: RELATIONSHIPS BETWEEN MORPHOLOGY AND ECOLOGY JAMESH. BROWNAND MICHAEL A. BOWERS Departmentof Ecologyand Evolutionary Biology, University of Arizona, Tucson, Arizona 85721 USA AssTRACT.--Hummingbirdspecies inhabiting restrictedgeographic regions exhibit morphologicalpatterns that differ significantlyfrom thosepredicted by null modelsin which speciesare selectedat random from appropriatespecies pools. Temperate North American hummingbirds are convergent:more similar in bill length, body weight, and wing length than predictedby severalnull models.These temperate species also are more similar to each otherthan they are to moreclosely related (congeneric) species from subtropicaland tropical habitats.Hummingbirds of the Greater and LesserAntilles show a nonrandom distribution of speciesamong islands:all islandsinhabited by hummingbirdshave at least two species, and thesefall into two distinctly different size categories.Allometric scalingof bill length with respectto body massis distinctivein Antillean hummingbirds;bill length increases more rapidly with increasingbody weight in West Indian hummingbirdsthan in random samplesof hummingbirdsof the world or in other birds. These morphologicalpatterns appear to reflect two ecologicalprocesses: interspecific competition among hummingbirds and mutualisticcoevolution with flowers.Hummingbird speciesof similar morphologyuse similar floral resourcesbut rarely coexistin the samelocal areas.Species of divergent mor- phologyexploit differentfood resourcesand frequentlycoexist locally. Length of the bill, which influencesaccess to different kinds of flowers,is particularlyimportant in the organization of these simple hummingbird associations.Received 28 June1983, accepted 5 July 1984. EVOLUTIONARYecology has a long tradition lems can be overcome in some casesby reana- of inferring dynamicprocesses of ecologicalin- lyzing information already available for cer- teraction from static patterns in the morphol- tain groups of organisms. Although these ogies and geographic distributions of closely analysesare no substitute for careful experi- related species(e.g. Lack 1947,Brown and Wil- mental and observationalfield studies,they may son 1956, Hutchinson 1959, MacArthur 1972). clarify issuesand focus future empirical work. Such patterns have played a particularly im- Hummingbirds, the nonpasserineavian fam- portantrole in the developmentof currentideas ily Trochilidae,provide an excellentsystem for about the role of competition and other inter- addressing these issues. This presumably specificinteractions in the organization of nat- monophyleticgroup, containing more than 100 ural communities(e.g. MacArthur 1972, Cody genera and 300 species,is widely distributed in and Diamond 1975). Recently, however, these the New World, attaining its greatest diversity ideas have been criticized for two reasons. First, in tropical latitudes. Hummingbirds are spe- manyof the purportedpatterns have not been cialized for feeding on floral nectar and share rigorously tested against appropriate null a suite of characteristicsincluding long, slen- models that assumethey could result from ran- der bills, tiny feet, and wings adapted for hov- dom processesrather than deterministic bio- ering flight. Becausethey are easily observed logical mechanisms(e.g. Strong et al. 1979, and their food resources can be monitored ac- Connor and Simberloff 1979, Simberloff and curately,hummingbirds have been the subjects Boecklen 1981). Second, the relationship be- of many behavioral and ecologicalstudies. tween morphologyand ecologyoften is simply Several investigators have suggestedthat assumed or inferred from indirect evidence in- speciesin particular geographicareas exhibit stead of being documented rigorously by ob- morphological patterns that reflect evolution- servation and experiment (e.g. Lack 1947, ary adaptationsboth for competitive interac- Hutchinson 1959, Findley 1973, Cody 1974, tions with other hummingbirds and for mutu- Brown 1975, Karr and James1975). These prob- alistic interactionswith bird-pollinated flowers 251 The Auk 102:251-269. April 1985 252 BI•OWNAND BOWEI•S [Auk, Vol. 102 (e.g. Grant and Grant 1968, Kodric-Brown and clusive use of local areas by individuals and Brown 1978, and Brown and Kodric-Brown 1979 species,and its outcome depends largely on for temperate North America; Lack 1973, 1976, habitat(including flower density)and wing disc and Kodric-Brown et al. 1984 for Caribbean is- loading (the ratio of body weight to the area lands; Snow and Snow 1972, Colwell 1973, swept out by the wings in flight; Feinsinger Feinsinger 1976, and Stiles 1975 for tropical and Chaplin 1975, Kodric-Brown and Brown America; and Feinsinger and Chaplin 1975, 1978,Feinsinger et al. 1979). Feinsingerand Colwell 1978,and Feinsingeret The claim that temperatehummingbirds are al. 1979 for hummingbirds in general). Al- convergentin morphologyhas not been tested though these interactionshave been studied rigorouslyagainst a null hypothesis.If the claim carefully in certain instances,there has been is correct,then temperatehummingbirds should little attempt to test the purported patterns of be more similar to each other than a compara- morphologicalorganization against alternative ble number of speciesdrawn at random from models that assume no interspecific interac- an appropriatespecies pool. We testedthis hy- tions. We performed such testsfor the species pothesisby comparingmeasurements of body inhabiting temperate North America and the weight, bill (exposedculmen) length, and wing Antillean islands, and the results are discussed length for the temperate specieswith sets of here in terms of what is known about the pro- eightspecies randomly drawn from a largepool cessesof ecologicalinteraction. Our analysisis of speciesof hummingbirds of the world for basedon three quantitativemorphological traits which measurementswere available (see Ap- that are known to be important in humming- pendix). We conductedthree tests of the null bird ecology:culmen (bill) length, which influ- hypothesisthat the temperatehummingbirds ences the kinds of flowers from which the birds are no more similar in size than randomly as- can forage;body weight, which indicatestotal sembled associationsof species. energy requirements;and wing length, which First, we used computer simulation tech- together with body weight affectsthe aerody- niques to draw 8 speciesat random 1,000 dif- namicsof flight (Greenewalt 1975). ferent times from the pool of 123 speciesfor which all measurements were available. For TEMPERATE NORTH AMERICAN eachset of eight specieswe computedthe vari- ASSOCIATIONS ance for each of the three measurements, and Eight species,belonging to four genera,have we recorded the number of cases in which the breeding rangesthat extend well into temper- variancefor a null communityexceeded that of ate North America. Several other species,not the observed temperate assemblage.For all consideredfurther here, are primarily subtrop- three characters, variance in the random com- ical in distribution and reach their northern munities was greater than in the real commu- limits just across the Mexican-United States nity for all 1,000draws (Table 1). Clearly, tem- border. The collective breeding range of these perate hummingbirdsare much more similar temperate speciesextends from the deserts of in size and shape than a random subsetof the the southwestern United States and northern hummingbirds of the world (Fig. 1). Mexico to southern Alaska and from Florida to A random draw from the global speciespool southeasternCanada. All of these speciesare may not be an appropriatemethod for evalu- migratory, wintering in Mexico and Central ating certainnull hypotheses,however. For one America. thing, the temperatespecies probably are more Several investigatorshave noted that the closelyrelated to eachother than to certain ex- north temperatehummingbirds are extremely clusively tropical groups.Thus, the similarity similar in sizeand shape.Grant and Grant (1968; could be attributed to failure to diverge from a see also Brown and Kodric-Brown 1979) sug- recent common ancestor rather than to conver- gested that this similarity representsconver- gencein responseto similar ecologicalcondi- gence among speciesto utilize a common set tions. Biogeographicand taxonomicrelation- of widely distributedflower species,which also shipssuggest that temperateNorth American have converged in size, shape, and nectar re- hummingbirdsderived from tropical montane wards.Hummingbirds compete for thesefloral and subtropicalancestors that colonized from resourcesprimarily by aggressionand territo- Mexico. The morphology of temperate hum- riality. This interferenceusually results in ex- mingbirdsalso might be constrainedby the de- April1985] HummingbirdMorphology andEcology 253 TABLE1. Resultsof simulationstesting the null hypothesisthat the eight speciesof temperateNorth Amer- ican hummingbirdsare more similar than expectedfrom a random draw of eight speciessampled from the world speciespool. Simulationscompared the actualvariances of morphologicalcharacters of temperate speciesagainst variances of the entire world speciespool, the speciespool that inhabitscontinental North America north of the Isthmus of Tehuantepec(biogeographic constraints), and speciespool with wing lengthsand body weights within the range observedfor temperatespecies (morphological constraints). Number of random draws with variances greater than observedvariance World Variancein

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