IRIDESCENT POLYCHROMATISM IN A FEMALE : IS IT RELATED TO FEEDING STRATEGIES?

ROBERT BLEIWEISS l Museumof ComparativeZoology, Harvard University, Cambridge, Massachusetts 02138 USA

ABSTP,ACT.--Conspicuous coloration may evolve through aggressive competition either for mates,in sexualselection, or for any limiting resource,in socialcompetition. Most species of (Trochilidae) have polygynous (promiscuous) mating systems, bright male plumage,and conspicuoussexual dichromatism. These patterns have led to the view that their iridescentplumage has evolved by sexualselection. I reportan iridescentpolychro- matismin femalesof the Andeanhummingbird Heliangelus exortis exortis that suggeststhe evolutionof iridescentplumage via aggressivefeeding behavior. Female H. e. exortisvary within and amongpopulations in the developmentof the largeiridescent gorget character- isticof males.I documenta regularnorth-south pattern of geographicvariation in the poly- chromatism.In samplesfrom northernColombia, female gorgets vary from a noniridescent (femalelike)to a fully iridescent(malelike) condition, whereas females from southernCo- lombiahave only the discretenoniridescent and fully iridescentmorphs. Samples of females from Ecuadorare comprisedpredominantly of the noniridescentmorph. The colorforms are not due to seasonalplumage differences or ontogeneticchanges. A significantnegative correlationbetween bill lengthand degree of iridescencein the largestgeographic sample suggeststhat femaleswith iridescentgorgets are moreaggressive territorial foragers than lessiridescent females. Received 16 July1984, accepted 8 April 1985.

NUMEROUSfactors have been proposedto ex- socialcompetition (Crook 1972;West-Eberhard plain the evolution of conspicuouscoloration 1979, 1984) recognizes that analogous interac- in .These include predatorwarning and tions may extendto competitionover limiting avoidance (Curio 1976, Baker and Parker 1979, resourcesother than mates, and promote the Endler 1979), thermoregulation (Watt 1968, elaborationof conspicuoussignals used in non- Clark 1974, Walsberget al. 1978), and behav- sexual contexts. ioral communicationas influenced by the biot- The effects of sexual selection and of nonsex- ic (Nobel and Vogt 1935,Tinbergen 1951,Mur- ual social competition are difficult to distin- ray 1981) or physical (Burtt and Gatz 1982) guishin practicebecause both phenomena lead environment. Evaluationof the relative impor- to the predictionthat increaseddominance and tance of these potential influences presentsa aggressivebehaviors can lead to the elabora- major challenge to evolutionists. tion of conspicuoussignals such as bright col- Within the category of behavioral commu- oration. Moreover, the effects of sexual selec- nication, the effects of nonsexual social com- tion are difficult to control in sexually petition are particularly difficult to distinguish reproducinganimals. Specifically, variation in from those of sexual selection. Darwin (1871) male coloration (e.g. Rohwer and Niles 1979, proposedthat conspicuoussexually dimorphic Dominey 1980,Ewald and Rohwer 1980,Gross featuresevolve via sexually selectedcompeti- and Charnov 1980, Rohwer et al. 1980, Procter- tion for mates, through the advantage these Grey and Holmes 1981, Edwards1982, Flood features confer either in mate choice (epigamic 1984) often fails to distinguish these hypothe- selection) or in aggressiveinteractions among ses becausemale dominance and territoriality members of one sex that determine access to invariably are tied to breeding activities(Wil- the other (intrasexual selection). The theory of son 1975, O'Donald 1977). This ubiquitous cor- relation makes it difficult to determine wheth- er nonsexualsocial competition is a sufficient • Present address: Department of Ornithology, explanationfor the evolutionof conspicuous American Museum of Natural History, New York, coloration in males. New York 10024 USA. The occurrence of intraspecific variation in

701 The Auk 102: 701-713. October 1985 702 ROBERTBLEIWEISS [Auk,Vol. 102 female colorationprovides a natural control of recordedonly for the tropicalWhite-necked Ja- this difficulty that has gone unappreciated.In cobin (FlorisugameIIivora; Zimmer 1950b, Mon- many species,especially those in which males roe 1968, Elgar 1978).Although H. e. exortisis mate promiscuously,females are less aggres- consideredto be sexually dichromatic,with a sive sexually,thereby providing somecontrol highly iridescentgulat patch or "gorget" in for the influence of sexual selection. Variation malesand a white gorgetin females(e.g. Mey- in female plumage is particularly relevant for er de Schauensee1964), Chapman (1917) noted determining the evolutionary significanceof that some females of H. e. exortis from the West- iridescentplumage in the largely promiscuous ern Cordilleraof Colombiahad fully iridescent hummingbirds (Trochilidae), which exhibit gorgets like those of males. I show here that someanomalous female plumagesthat are dif- femaleH. e. exortisvary dramaticallyboth with- ficult to explain by sexualselection. in and among populationsin the malelike iri- Male hummingbirdsemploy their iridescent descenceon their gorgets,and I arguethat the plumage in a variety of contexts,including polychromaticvariation supports the origin of nuptial displays,aggressive sexual displays, and iridescentcoloration in hummingbirdsthrough aggressivedisplays associated with nectar-cen- selectionfor aggressivesignals associated with tered feeding territoriality (Stiles 1973, 1982). foraging behavior. Because promiscuous breeding systems are thought to be associatedwith high levels of MATERIALS AND METHODS sexualselection (Payne 1984), the bright male In Hellangelusexortis, the violet-chinned and rose- plumageand sexualdichromatism characteris- throatednominate form, H. e. exortis,occurs through- tic of most hummingbirds are consistentwith out the three Andean Cordilleras of Colombia and sexual selection theory (Selander 1972). How- through the EcuadorJanAndes, primarily on the ever, sexualselection does not explain the evo- eastern slope. Two orange-throated forms, H. e. mi- lution of "bright monomorphic" specieschar- craster and H. e. cutervensis,occur in the Andes of acterizedby a brilliant female plumage similar southwestern Ecuador and northern Peru, respec- to that of the male (e.g. in the genera Panterpe, tively (Peters 1945, Zimmer 1951), but are rare in collections and are not treated here. I examined 275 EuIampis,AmaziIia). Bright monomorphicspecies do not show a consistent reduction in the in- individuals from the entire range of H. e. exortisin this study. Sample size and geographicdistribution tensityof sexualselection as inferred from their of specimensare given in Table 1. A more complete breeding systems(Wolf and Stiles 1970, Wolf descriptionof the geographicrelationships among 1975a).Rather, both sexesin bright monomor- localities can be found in Bleiweiss (1985). phic speciesexhibit nonbreeding nectar-cen- Determinationof ageand sex.--Only two specimens, tered feeding territoriality (Wolf 1969, 1975a, both immatures, had gonad data noted on their la- b; Ingles 1976).Feeding territoriality is usually bels. Therefore, alternative methods had to be used lacking in the dull-colored femalesof mostsex- to determineage and sex.To distinguishimmatures ually dichromaticspecies (Wolf 1969, 1975a,b; from adults,I relied on the presenceof corrugations Linhart 1973). Therefore, the correlation be- on the culmensof immaturebirds (Ortiz-Crespo1972, Stiles and Wolf 1974). Specimenswere examined un- tween bright female plumageand feeding ter- der a Bauschand Lomb zoom dissectingmicroscope ritoriality strongly suggeststhat conspicuous (7-30 x) to look for the presenceof corrugations.A iridescence may evolve solely for its impor- specimen was considered adult if the bill was per- tance as a nonsexualaggressive signal (Pitelka fectly smooth outside the nasal depression.The in- 1942, Wolf and Stiles 1970, Stiles 1973). verse relationship between the number of bill cor- In this paper I addressa previously unana- rugationsand age of immature permits one to lyzed color pattern that involves the variable study plumage ontogeny (Ortiz-Crespo 1972, Ewald expressionof iridescentmalelike featuresin fe- and Rohwer 1980).To determinethe relative age of males of an Andean hummingbird, the Tour- immatures,bill corrugationswere counted with the maline (HeIiangelusexortis exortis). aid of a Wild microscopewith cameralucida attach- ment. Wing and tail measurementsare considerably Subtle examplesof this "polychromatic"vari- larger in males of many except the smallest hum- ation in hummingbirds have been noted in the mingbird species(Zimmer 1950a, 1952; Stiles 1983; temperate species Calypte anna (Williamson Payne 1984). I have verified this relationship for He- 1956) and in several tropical genera, including liangeluswith large seriesof H. amethysticollisfor which HeIiangelus(Berlioz 1944, 1949), but distinctive gonadal data were available (Bleiweissin prep.). To femalevariants in iridescentmale plumageare determine the sex of H. e. exortisspecimens, I em- October 1985] FemaleVariation in Iridescence 703

TABLE1. Distribution and samplesize for Heliangelusexortis exortis specimens examined.

Cordillera b

Locality Pooled' West Central East Adults ttlres 1. El Pefi6n A West 1 2. La Aguadita A West 1 3. E1 Robie A West 6 1 4. Fusugasug•-Silvania A West 4 5. Hacidenda Zuliaba East 7 1 6. Santa Elena East 2 7. P•ramo de Sons6n West 6 2 8. E1 Zancudo B West 17 1 9. Santa Isabel B West 1 1 10. Laguneta West 43 15 11. Toche; Tolima West 7 6 12. Purac• West 1 13. Coconuco West 1 14. Paletara West 2 15. Almaguer West 6 16. Hacienda La Ilusi6n West 8 17. La Florida C East 1 18. Coast Range C East 3 19. Cerro Munchique C East 25 1 20. Munchique-E1 Tambo C East 2 1 21. Tijeras-Moscopin East 7 4 22. Guanderal D East 3 23. Cerro Pax D East 8 5 24. Papallacta E East 8 25. Cuyujfia E East 14 4 26. Baeza E East 3 1 27. Volcln Sumaco Isolated 10 1 peak 28. San Antonio F East 1 1 29. Ambato F East 1 30. Barios F East 6 1 31. Volcln Tungurahua F East 4 32. Planchas F East 2 33. Coraz6n West 1 34. Pallatanga West 6 3 35. Macas F East 4 Total 219 56

Letters A-F designate pooled localities used for analysisof geographic variation (see text). East and west designationsunder each Cordillera refer to the slope of the range. ployed the two measurementsthat best separatedthe The violet chin and rose throat feathers are always sexesin H. amethysticollis:length of unflattenedouter glittering in both sexesof H. e. exortis.Nonglittering primary from point of emergencefrom skin, and chin and throat feathersvary geographicallyfrom length of outer tail feather (rectrix 5) from point of white (eastern and southern Colombia through Ec- emergencefrom skin. The extrapolation from H. ame- uador) to white with a shining green to dusky ter- thysticollisto H. e. exortisis justifiedbecause they are minal dot (northern and central Colombia) [see Blei- congeners. There is no reason to assume that what weiss (1985) for a discussionof this variation]. The holds for H. amethysticollisand many other hum- female polychromatism involves variation in the mingbirds does not also hold for H. exortis. number of glittering gorget feathers.I quantified the Morphologicalvariation.--I use the term "gorget" to polychromatic variation either by directly counting denote the entire gular region, and "chin" and the number of glittering iridescentfeathers or, when "throat" to denoteits uppermostand remainingpor- very numerous, by estimating their numbers by tions, respectively.The color variation describedin- counting the tracingsof all glittering gorget feathers volvesiridescent structural colors that vary in inten- made with the camera lucida. sity: "glittering" indicates intense, mirrorlike To quantify the pattern of geographicvariation in iridescence,while "shining" indicatesless intense ir- glittering gorget iridescence,I divided the variation idescence with a metallic luster (Greenewalt 1960). in glittering feather counts into four gorget color 704 ROBERTBLE•WE•SS [Auk,Vol. 102

tionship betweenpolychromatic variation and feed- ing behavior:(1) length of exposedculmen (EC), (2) length of culmenfrom flangeof the nasaloperculum to tip (CF), (3) length of outer primary (FP), (4) chord of unflattened wing from the bend to the tip (WB), > (5) length of inner (IT) tail feather (rectrix 1), and (6) length of outer (OT) tail feather (rectrix 5) (seeBald- win et al. 1931). I used the two measuresof wing lengthas indices of wing discloading (assuming equal weights among females)because weights were not available.Specimens were measuredunder the Bausch and Lomb zoom dissectingmicroscope with needle- point dividers, that were then remeasuredwith dial calipers. I omitted individual measurementswhen Fig. 1. Distribution of glittering gorget feather featherswere molting or very worn. countsfor all adult females.Qualitative gorget color Statisticalanalysis.--Because of small sample sizes, classesII-IV were designatedon the basisof the two specimensfrom severallocalities were pooledfor sta- largestdistributional gaps in feathercounts (between tistical analysisof geographicvariation (pooled lo- 18 and 20 and between29 and 32, as indicatedby calitiesare designatedby lettersA-F in Table 1). Close dashed lines). geographicproximity and absenceof obviousphys- ical barriers between localities were the criteria used classes(I-IV) based on the number of glittering for pooling (Bleiweiss1985). Geographic variation in feathers.Class I denotesabsence of glittering irides- gorget color classeswas analyzedby R x C contin- cence.The frequencydistribution of glittering feath- gency tables using Chi-squarestatistics with at least er countsamong all adult females(n = 94) revealsan one expected observation per cell (Shedecor and uneven distribution, with gaps at 18-20, 29-31, and Cochran 1969:235, 241). I calculatedSpearman rank correlation coefficients between mensural characters 46-49 (Fig. 1). The first two gaps help divide the sample into three qualitative categories(little, mod- and gorget coloration for each geographic sample erate, and full iridescence)and permit the assign- separately becausethere is significant geographic ment of individuals to different count classes(class variation in most of the measurements (Bleiweiss II = 1-17, class III = 21-28, class IV = 32+). These 1985). classesare consistentwith a subjectiveimpression of differencesin appearance.Classes II and III have both RESULTS glitteringand nonglitteringfeathers. Class I hasonly nonglitteringfeathers, and classIV usuallyonly glit- Sexual variation in adult Colombian and Ecua- tering ones. doriansamples.--Adult Colombian H. e. exortis Mensural charactersmay be correlatedwith differ- show continuous variation in glittering gorget ences in feeding behavior between the sexes and iridescence. Adult H. e. exortis from Ecuador ex- among different hummingbird species(Feinsinger hibit only two gorget conditions:glittering vi- and Colwell 1978). I used such characters as an in- olet chin and rose throat, or pure white chin directtest of the relationshipbetween polychromatic and throat. Because variation in Ecuador is sim- variationand feeding behaviorin the absenceof be- havioral observations.Territorial speciesin general, plified, I used these samplesas the standard for and malesin particular, often have shorter bills, and determining sex in the variable Colombian shorter wings relative to body mass(wing disc load- samples.A bivariate scatterplot of wing length ing), than nonterritorial speciesand females,which (FP) vs. tail length (OT) for all adult specimens are lessaggressive (Stiles 1973, Feinsinger and Chap- from Ecuador(localities E, F, 27, 34) separated lin 1975, Feinsinger and Colwell 1978, Snow and adults into two groups (Fig. 2A). Individuals Snow 1980). These differencesin morphology are with wing and tail measurementsgreater than thought to increasethe energeticefficiency of terri- 45.5 mm have highly iridescent gorgets,which torial and nonterritorial foraging strategies(Wolf et are violet on the chin and rose on the throat. al. 1972,Feinsinger and Chaplin 1975,Feinsinger and Colwell 1978, Ewald and Williams 1982). Similar Smaller individuals have pure white, noniri- descent chins and throats in 24 of 25 individ- morphologicaldifferences therefore may evolve to- gether with evolutionary divergencein foraging be- uals. This clear morphological division in the havior among individuals. EcuadorJansample reflectssexual differencesin I used six charactersto examine the possiblerela- measurements.Thus, all males have highly ir- October1985] FemaleVariation in Iridescence 705

39-

35 37 39 4l 43 45 47 49 5'l 5'3 35 37 39 4l 43 45 47 49 5l LENGTHOf OUTERTAIL FEATHER (ram) LENGTHOF OUTER TAIL FEATHER (ram)

41'

LENGTHOF OUTER TAIL FEATHER {ram) LENGTHOF OUTER TAIL FEATHER {ram)

Fig. 2. Scatterplots of lengthsof outer tail feather(OT) vs. first (= outer)primary (FP) for Ecuadorian localitiesE, F, 27, 34 (A), and the threelargest Colombian samples: B = loc B (El Zancudoregion), C = Laguneta,D = loc C (Munchiqueregion). Solid symbols are males;open symbols are females.The number of glitteringgorget feathers is indicatedabove each circle. Triangles are females with no glitteringgorget feathers.Specimens for whichonly oneof the measurementscould be takenare alsoplotted; they fall on the appropriateaxis. Arrows indicate questionable females (see text). Scatter plots of additionallocalities can be found in Bleiweiss(1983). idescentgorgets and all femalesbut one have body coloration.Plots of Colombianspecimens pure white gorgets. for wing length (FP) and tail length (OT) fall, In addition to the gorget color differences, like EcuadorJanones, into two groupsthat are the sexesalso differ in body colorationin Ec- roughly separatedby a size of 45.5 mm for FP uador.Although both sexes are predominantly and OT (Fig. 2B-D). Largerindividuals (males) shining green, malesare much darker than fe- have the samegorget color as males from Ec- males,with a bronzecast on the backand up- uador. The female portion of each sample, per tail coverts. Male crowns look blackish however,is highly variable, with from 0 to 55 when seenfrom the front and have a highly glittering iridescentgorget feathers.For indi- contrastingglittering green forecrown. Female vidualssexed by bodyplumage, 2 of 75 females crownsare green,with lesscontrasting glitter- (indicatedby arrowsin Fig. 2C, D) and 7 of 104 ing forecrowns. Female belly feathers are maleshad one of the two measurementslarger broadly edged with tan, and their outer tail or smaller than expectedbased on their plum- feathershave light tips, featuresthat are absent age.Of these,the sexof 4 of the malesand the in males. classI femalefrom Laguneta(Fig. 2C) are prob- Colombianspecimens were first sexedby ably correctbecause my evaluationagrees with 706 ROBERTBLEIWEISS [Auk,Vol. 102

81' 78' 75' 72' T

r2 ø

WESTERN CENTRAL

i0 ß CLASS

8 o

s ß

œo

• OVER3000 METERS '"'"'•oN=,Oß"IOOO METER CONTOUR4 o / ; • , I00 KILOMETERS

78 ø 75 ø 72 ø

Fig. 3. Geographicvariation in glittering gorget iridescencein adult female H. e. exortis.Gorget color classesare basedon Fig. 1 and are as describedin text. Separatehistograms for each locality are on the left side.The principal geographicpattern is summarizedby the histogramson the right side.The Bogot•sample (top center)has all four gorget color classesand is used as a key for the other histograms.Localities were plottedwith the aid of ornithologicalgazetteers (Paynter and Traylor 1977,1981). Numbers under histograms refer to samplesincluded (seeTable 1). There was no significantdifferentiation among localitiesin the central sectionsof the Central and Easterncordilleras (A, B, and loc 10; 3 x 4 contingencyX 2 = 0.809,v = 6, NS). the sex noted on labelsby the collectors,who having broad white or light gray concealed sexedbirds by examining their gonads(Chap- bases,and in having violet chin feathersonly man 1917: 304-305). The other three proble- when the glittering gorgetiridescence is exten- maticalmales are all fully iridescentand do not sive.Although the number of glittering gorget affect the general conclusions. feathers in males also varies, this variation is Femaleswith the highestcounts of glittering not related to qualitative changesin gorget col- gorget feathershave gorgetsthat are malelike or and pattern. Within the accuracyof the sex- in color, but this color is more limited in extent ing criterion,adult malesfrom all geographic (maximum count 56 vs. a range of 43-95 for regionshave the gorget color and pattern de- males). Iridescent gorgets in females differ in scribedfor malesfrom the Ecuadoriansample. detail from thoseof typicalmales in frequently The variation in number of glittering gorget October1985] FemaleVariation in Iridescence 707 feathersfor combinedadult femalesamples was significantly greater than that for males [F-test of In (number of glittering feathers+ 1) (Sokal and Rohlf 1981: 421), F = 14.88, v• = 93, v2= 113; P < 0.001 (one-tailed test)]. Geographicvariation in femalecoloration.--The pattern of geographic variation in all four classesof glittering iridescenceamong adult fe- malesis summarizedin Fig. 3. There is striking geographicdifferentiation in the best-sampled region from the Central Cordillera of Colombia NUMBER OF BILL CORRUGAT)ONS NUMBER Of BILL CORRUGATIONS southward into Ecuador.Samples are highly variable (classesI-IV) in the north, but exhibit Fig. 4. Ontogeneticvariation in glittering gorget decreasedvariability southward,with only the iridescenceamong immature male (left) and female (right) H. e. exortisfrom combinedgeographic sam- extreme noniridescent and fully iridescent pies. morphspresent in southernColombia and with decreasingfrequency of the iridescentmorph (classIV) in Ecuador.Differences among sam- bill corrugations disappear. Males with <20 pieswithin this region were highly significant corrugationsalready have gorgets similar to (3 x 4 contingency table, X2 = 48.265, v = 6, those in smooth-billed males (immature range P < 0.001).The occurrenceof only the two ex- 36-76, adult range 43-95). Immature females treme gorget classesI and IV in the southern with comparablenumbers of bill corrugations section of the Central Cordillera of Colombia, (<20) exhibit the full range of variation ob- though based upon small individual collec- servedamong smooth-billedfemales. Taken as tions,is repeatedfor the severalcontiguous lo- a whole, this information doesnot rule out age- calities. Other small collections from elsewhere relatedor other facultativechanges in adult fe- in the range do not show this pattern. male gorgetcoloration, but it doesindicate that Two other relatively well-sampled regions any gorget coloration may exist at the incep- alsoexhibit suggestivepatterns. The sampleof tion of maturity. female H. e. exortiswith accuratelocality infor- I found no evidence of significant seasonal mation from the Bogot• region was limited or annual changesin gorget color class fre- (n = 7), but the high variability of femalesfrom quencies.Within localities,the range of poly- this region (classesI-IV) is further supported chromaticvariants in subsamplescollected over by their high variability among the numerous a brief time interval is similar to that for the Bogot• trade skins. Trade skins were not in- entire sample and for other large subsamples cluded in the analysisbecause they lack spe- collected during different months and years cific locality information. These collections (Table 2). For example,most of the large, vari- generally were made in the Bogot• region or, able Laguneta sample (locality 10) comprises lessfrequently, in the central part of the Cen- two collections obtained in different months tral Cordillera (Chapman 1917, Berlioz and and years,one from Augustto September1911, Jouanin 1944). Bogot• trade skins also exhibit the other from March to April 1942.There was all four gorget classes.Unlike all other large no significantdifference (by Chi-square)in the samplesof females,the samplesfrom the Cerro frequencyof the different gorget color classes Munchique region (loc C) lack individuals in betweenthese two samples.Similarly, the pre- the noniridescentclass (I). dominantly white-gorgeted samplesfrom Ec- Age and temporalvariation in gorget irides- uador have been collected in various months cence.--Theontogeny of plumagecoloration in and years (see Table 2), indicating that the H. e. exortisindicates that the adult plumage, prevalence of class I is not due to temporal and the completerange of variability in gorget samplingbias. This particular pattern was sta- colorationamong smooth-billed females,is at- ble over at least 41 years (see Table 2). These tained before the bill corrugationsdisappear dataalso indicate that the basicpattern of geo- (Fig. 4). Adult males have a single gorget color graphicvariation is not the result of temporal that iramaturesacquire gradually before their sampling bias (Table 2). 708 ROBERTBLEIWEISS [Auk, Vol. 102

TABLE2. Temporalsampling of adult polychromaticvariants with respectto month and year.a

Total classes Locality n Month Year for locality Total classesfor sample A. Bogotfiregion 7 24 Mar-6 Apr 1913 I-IV I-IV (2, i, 2, 2) 8. E1Zancudo 7 2-31 Aug 1918 I-IV I-IV (2, 2, 1, 2)• 10. Laguneta 9 4 Aug-6 Sept 1911 I-IV I-IV (5, 1, 2, 1)c 9 17 Mar-15 Apr 1942 I-IV I-IV (1, 1, 5, 2) 15. Almaguer 2 11-16 Mar 1912 I, IV I, IV (1, 0, 0, 1) C. Munchiqueregion 4 15-22Apr 1957 II-IV II-IV (0, 2, 1, 1) 21. Tijeras-Moscopfin 2 23 Feb-3Mar 1952 I, IV I, IV (1, 0, 0, 1) 23. Cerro Pax 4 20-23 Dec 1950 I, IV I, IV (2, 0, 0, 2) 24. Papallacta 2 Feb 1899 I I (2, 0, 0, 0) 25. Cuyujfia 2 Aug 1932 I, IV I, IV (1, 0, 0, 1) 5 19 June 1928 I, IV I (5, 0, 0, 0) 26. Baeza 3 2-10 Nov 1922 I I (3, 0, 0, 0) 27. Volcin Sumaco 2 12-14 June 1924 I I (2, 0, 0, 0) 31. Volcan Tungurahua 2 June 1939 I I (2, 0, 0, 0) 34. Pallatanga 2 19-20 Oct 1931 I I (2, 0, 0, 0) ' Localityletters and numbersrefer to designationsin Table 1. Numbersin parenthesesare the numberof individuals in each of the four gorget color classes(I-IV). • Locality8 vs. locality 10, Aug; Yatescorrected for continuityx 2 = 0.29, v = 3, NS at 0.05 level. cLocality 10, Aug vs. locality 10, Mar-Apr; Yatescorrected for continuity x 2 = 2.57, v = 3, NS at 0.05 level.

Trophicmorphology.--In the largestpolychro- unlikely explanation for the origin of poly- matic female sample, from Laguneta (locality chromatic variation in female hummingbirds. 10, n = 21; all other polychromatic localities, By contrast,aggressive competition for nectar n < 10),bills vary by morethan 2 mm in length mayoccur in both sexesin hummingbirds,and (EC 15.3-17.6),and both measuresof bill length it helps explain the evolution of the brilliant were significantly negatively correlatedwith female plumageof bright monomorphicspecies numbersof glittering feathers(Table 3). There (Wolf 1969, Wolf and Stiles 1970). These con- were no significantcorrelations between gor- siderationssuggest that competition for nectar get color and the other four measurementsat is a more plausible basisfor the origin of po- Lagunetaor in any characterfrom other large lychromaticvariation in femaleH. e. exortisthan samples(n > 4). Male and female classIV in- is sexual selection.Social competition is de- dividuals from Lagunetawere also similar in fined in terms of intraspecific competition, bill length (for example,for EC, Mann-Whit- whereas aggressivecompetition for nectar in ney U-test;U, = 52, t, = 0.77, v = 23; two-tailed hummingbirdsmay include interspecificinter- test, P > 0.5), while bills of classI females were actions(Moynihan 1979). I therefore refer to significantlylonger than bills of males(U, = the hypothesisfor the evolution of polychro- 94, t, = 2.69, v = 24; one-tailed test, P < 0.01). matic variation via nonsexual competition for Thus,in the Lagunetasample, individuals with nectaras the "feeding strategyhypothesis" in iridescentgorgets in either sex have compara- order to include the possibleeffects of inter- ble bill lengthsthat are shorterthan in females specificaggression. with nonglitteringgorgets (class I). The principal prediction of the feeding strat- egy hypothesisis that polychromaticvariation DISCUSSION in the gorgetof femaleH. e. exortisreflects vari- ation in foragingbehavior. Iridescent females In promiscuous mating systems,which are of bright monomorphichummingbirds active- characteristic of most hummingbird species ly defend their territoriesand are codominant (Stiles and Wolf 1979), the potential for sexual with males (Wolf 1969, Stiles and Wolf 1970, selectionas measuredby variance in reproduc- Wolf and Stiles 1970). On this basis, I further tive successis high in malesbut low in females predict that increasing gorget iridescence (Payne 1984). This makes sexual selectionan should be positively correlatedwith increasing October1985] FemaleVariation in Iridescence 709

T^I•LE3. Spearmanrank correlationcoefficients for female gorget iridescenceand six mensuralcharacters for the Lagunetasample (locality 10).a Sample sizesare given in parentheses.

EC CF FP WB IT OT -0.59** (21) -0.51' (21) 0.32 (20) 0.37 (17) 0.29 (20) 0.21 (19) ' EC = length of exposedculmen, CF = length of culmen from flange of nasal operculumto tip, FP = length of outer primary, WB = chord of unflattenedwing from bend to tip, IT = length of inner tail feather, OT = length of outer tail feather. * P < 0.02, ** P < 0.01 (two-tailed tests).

dominanceand degree of feeding territoriality variation. The polychromatic variation de- among female H. e. exortis.There is as yet no scribed in this paper differs from ontogeneti- direct proof of these predictionsfrom field ob- cally developedcolor variation in at leastthree servations. However, aggressiveterritorial ex- important respects:(1) occurrencewithin broad ploitation of nectar is characteristicof male H. age categoriesof immatures and adults, (2) e. exortis(Moynihan 1979, Hilty and Brown in temporalstability in the relative frequenciesof press).In other territorial hummingbird species the gorget color classesat somelocalities such where males have a glittering gorget, the gor- as Laguneta,and (3) female sex-biasedpattern get is used in aggressivedisplays associated (polychromaticvariation limited to females).I with feeding territoriality (Ewald and Carpen- explore below the possibleimplications of these ter 1978). Feeding territoriality in male H. e. characteristicsfor the feeding strategyhypoth- exortistherefore provides a necessarycondition esisto provide a specificframework for future for my inference that female H. e. exortiswith research. iridescentgorgets are territorial nectar feeders. Age independence.--Ageindependence of The negativecorrelation between bill length polychromaticvariants contrasts with the strong and the degreeof gorget iridescencein females ontogeneticcomponent of many casesof vari- from Lagunetasupports the predictedassocia- able male plumage ascribedto sexual selection tion of an iridescent gorget with aggressive (Gilliard 1969, Wilson 1975). Dull, unorna- feeding territoriality (in light of the possible mented males are often subordinate immatures general relationship between decreasingbill that do not achieve full ornamentation or re- length and increasedterritoriality discussedin productive potential until they move up the the Materials and Methods). However, this cor- dominance hierarchy. Such males may even relation is not consistentamong all localities. delay attemptsto reproduceuntil they are old- Perhaps,variation in bill length is not neces- er, but may gain in lifetime reproductive fit- sary for the evolution of individual differences nessby achieving greater reproductivesuccess in the degree of feeding territoriality. at an older age. Thus, sexually selectedplum- The only direct observationsof behavioral age variation in males may arise out of its re- correlatesof intrapopulationalvariation in gor- flection of age rather than from the selective get coloration involve ontogenetic variation in value of different levels of ornamentation per male Calypteanna. Adult male C. annahave glit- se (Price 1984). The appearanceof polychro- tering red crowns and gorgets that develop matic variation early in the ontogeny of H. e. gradually with age. As is consistent with the exortis,and its persistencein adults, suggests predictionsfor polychromaticvariation in H. e. that the different phenotypesare viable at any exortis,adult male C. annaare territorial during age. Competition for nectar may produce age- the nonbreeding seasonwhile the less irides- independent polychromatic variation in fe- cent juveniles hold poor-quality territories or males; unlike competition for mates among are not territorial (Ewald and Rohwer 1980). It males,females have no apparent fitnesspayoff is difficult, however, to assessthe evolutionary for delaying accessto nectar. significanceof polychromatismsusing ontoge- Temporalstability.--Temporal stability of netic variation becauseindividuals differ in age polychromatic variants suggeststhat they can (experience)and sexual maturity. These two coexistwithout evolutionary exclusion.Coexis- factors can determine the outcome of aggres- tence of polychromatic variants that differ in sive interactions even in the absence of color foraging behavior is possibleif bright plumage 710 ROBERTBLEIWEISS [Auk, Vol. 102 confers different costs and benefits from dull appear to be lessdiscriminating in mate choice plumage in different socialcontexts associated (Sibley 1957). Thus, male choiceacting on fe- with foraging (Ewald and Robwet 1980, Rob- males would be lesslikely to eliminate female wet 1982). For example, bright plumage, al- color morphs that evolved through nonsexual though possibly advantageousin aggressive selection. encounterswith conspecificsor other species, Both proximate mechanisms and broader may be costly for subordinate birds if it in- ecologicalcauses probably will be important creasesthe rate and intensity of attacks by for analyzing the evolution of hummingbird dominants. Conversely, dull plumage in sub- polychromatisms.Hummingbirds as a whole ordinatesmay increasetheir successat intrud- meet the requirementsfor the evolution of in- ing onto nectar territories and reduce the se- trapopulationalvariation in conspicuouscol- verity of aggressive responsesby territorial oration as outlined by Robwet (1982):presence birds (Ewald and Robwet 1980). Further mor- of aggressivecompetition for a resource(nec- phologicalevolution for increasedefficiency in tar), variation in dominance, and frequent ag- different foraging strategiesby dominantsand gressiveinteractions among individuals. The subordinates should enhance stable coexis- simplestmechanism for the origin of variation tence. This possibility is suggestedby the cor- in phenotypic traits associatedwith variation relation between bills and coloration in Lagu- in dominancebehavior is a physiologicallink neta, where the frequency characteristicsof the between coloration and behavior (Keeler et al. gorget color classesdid not changesignificant- 1970, Watt et al. 1984). The final form of the ly over at least 30 years. Temporal and geo- variation may then depend upon how alter- graphic shifts in polychromaticvariation may native individual strategiesof resourceexploi- be causedby extrinsic shifts in nectar resource tation are limited by resource availability. availability through changes in the flora or Presenceand absenceof territoriality in hum- changes in avian nectar competitors such as mingbirdsdepends upon nectarresource den- other hummingbirds or the nectarivorousDi- sity, with territoriality around denser clumps glossa(Coeribidae). of nectar. Therefore, female polychromatisms Femalesex-biased pattern.--Intersexual differ- may evolve when femaleswithin a population encesin variability may be due to sexualselec- forage over a broad range of nectar densities, tion. The sex bias in the polychromatismrep- while sexually dichromatic and bright mono- resentsa simplecase of reducedmale variability chromatic speciesmay evolve when respective documentedin many taxa (Pearseand Murray femalesspecialize at low or high densities. 1982,Stamps and Gon 1983).Stabilizing sexual selection by female choice and intrasexual ACKNOWLEDGMENTS aggressionhave been proposedas alternative explanationsof this pattern (Pearseand Mur- I thank my thesisadvisor, E. E. Williams, for many ray 1982, Robwet 1982, Stampsand Gon 1983). helpful discussionson general evolutionary topics over the years and W. Bock,W. Bosseft,P. Ewald, P. Both processesmay operate in conjunction to Feinsinger, S. Rohwer, J. Shopland, F. Vuilleumier, reduce variation in aggressivesignals in male and L. L. Wolf for helpful criticism on earlier drafts hummingbirds.Female hummingbirds of some of this paper. I gratefully acknowledgethe helpful speciesseem to choosemales on the basis of discussionand commentsof K. Fristrup, D. Geary, A. occupancyand quality of a nectarterritory (e.g. Haynes,and G. Mayer. I thank the following curators Calypteanna, Panterpe insignis; Stiles 1973).This for kindly providing specimensunder their care:L. indirect female choiceof male aggressioncan Baptista(California Academy of Sciences),F. Gill lead to strong selectionfor aggressivenessand (Academy of Natural Sciences,Philadelphia), J. C. aggressivesignals among all malesif all males Hafner (Moore collection, Occidental College), J. compete for territories during the breeding Jennings(Western Foundation of VertebrateZoolo- gy), R. A. Paynter (Museum of Comparative Zoolo- seasonwithout adopting alternative nonterri- gy), J. V. Reinsen (Louisiana State University Mu- torial mating strategies(Robwet 1982).Indeed, seumof Zoology),R. Schreiber(Los Angeles County these conditions appear to exist in Calypteanna Museum), L. L. Short (American Museum of Natural (Stiles 1973), where most males are iridescent, History), S. Wood (CarnegieMuseum of Natural His- but females are polychromatic (Williamson tory), and R. L. Zusi (U.S. National Museum). This 1956).Conversely, males of promiscuousspecies work was supportedin part by grantsfrom the De- October1985] FemaleVariation in Iridescence 711 partment of Biology, Harvard University, and by a patterns.Pp. 319-364in Evolutionarybiology (M. ChapmanMemorial Fund grant from the American K. Hecht, W. C. Steere, and B. Wallaceß Eds.). Museum of Natural History. New York, Plenum Press. EWALD,P. W., & F. L. CARPENTER.1978. Territorial

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From "General Notes" (1885 Auk 4: 382-383): strike me at first that it might be this bird; moreover, its actionwas soodd that I hardly knew what to make "On the FeedingHabits of Phalaenoptilusnut- of it. At any rate one soonnoiselessly lit, like a great, talli.--Just without the picket fence that enclosesin gray moth, directly in front of me in the road, but a part the parking of my presentresidence at Fort Win- few feet distant.It wasextremely difficult to seehim, gate,New Mexico,then [sic]runs a wide board-walk. and it was more by good luck than good shooting Beyondthis is a broad, well-kept gravel road, stand- that the little pinch of shot from my cane-gun ing between the former and an open level plot of knocked him over, though the weapon rarely fails ground of about an acre's extent. For a number of me in daytime. I immediately ran up to my study eveningspast my neighborshave tried to induce me with my prize, where I discoveredI had killed a fine to come out and see a strange-actingbird that dis- specimenof Nuttall's Poor-will. As the skeleton of ported itself in this roadway,between twilight and this bird had long been amongmy desiderata,the dark. I paid little heed to this, as from its description skin and its beautiful plumagewas soon stripped off, I believedit to be the half-grown young of the Chor- whereuponI was much surprisedto find in its mouth dedilesof this region, which is very abundantin the somefour or five quite sizablemoths, and the upper neighborhood.Last night, however,the bird having portionof the oesophagusfilled with a wad of a doz- been described to me as a small Owl with a white en or fifteenmore. Fully half of thesewere yet alive, throat, by one of its observers,I took my cane-gun and two or three managedto fly away when freed and made a searchfor it up and down the road-way. from the bodiesof their more disabledcompanions. I had not far to go, when, as well as I could seeby This, then, is what the bird was up to; instead of the light of a very young moon, I noticed a small, flying about as a Nighthawk does, taking his insect dark-brownish looking bird apparently amusing prey in a conspicuousmanner upon the wing, he himself by making short jumps of two feet or more capturesit in the way I have describedabove. up in the air, then restingon the road to repeatthe "To-night the moon is twenty-four hours older, performancein a momentor so. Another was going and the eveningproportionately brighter, but a care- through similar capers on the broad walk. They ful search for over half an hour failed to discover a seemedto be perfectlyoblivious to my presence,and, single specimenof the bird on the same ground. I indeed, some children further along were trying to am not aware that any of the other Caprimulgidae catch them with their hands. As I had never heard have similar habits.--DR. R. W. SHUFELDT,Fort Win- the note of the Poor-willsin the vicinity, it did not gate,New Mexico."