Display Behavior of Male Calliope Hummingbirds During the Breeding Season’

The Condor91x272-279 0 TheCooper Ornithological Society 1989

DISPLAY BEHAVIOR OF MALE CALLIOPE HUMMINGBIRDS DURING THE BREEDING SEASON’

STAFFAN TAMM,~ DOUG P. ARMSTRONG~ AND ZENA J. TOOZE~ Departmentsof Zoology and ResourceEcology/Resource Management Science, Universityof British Columbia, Vancouver,BC V6T 2A9, Canada

Abstract. We studied the breeding behavior of male Calliope Hummingbirds (Stellula calliope)in south-central British Columbia where they defended breeding territories from late April through late June each summer. We describedive displays,hover displays,“buzzing,” chasing, and vocalizing, and include a description of a complete mating sequence. Territorial males always performed dive displays in responseto intrusions by female conspecifics.In contrast, they usuallychased male conspecifics,although chases were occasionally preceded by dive displays. We conclude that dive displays play an important role in the courtship of Calliope Hummingbirds, although this does not preclude a possiblerole in aggressiveinteractions. We detected no seasonaltrend in overall display activity, but at least in 1 year, more dive displayswere given to males than to females early in the breeding season,and the reverse was true later in the season. There was a strong die1 pattern in display behavior in that both dive and hover displaysdecreased gradually over the day. We discussdifferences between the courtship behavior of Calliope Hummingbirds and Anna’s Hummingbirds (Calypte anna). Key words: CalliopeHummingbird: Stellula calliope; courtship;mating; display behavior; die1variation; interspecific@erences.

tailed observations of male-female interactions, INTRODUCTION it is difficult to determine which of these are Most evidence suggeststhat contact between sexes important in courtship, agonistic interactions, or in North American hummingbirds is limited to both. courtship and copulation (Johnsgard 1983), al- Conspicuous dive displays have been de- though male North American hummingbirds scribed for most North American species(Bent have occasionally been reported to feed nestlings 1940, Stiles 1982, Johnsgard 1983), and have or nesting females (Bailey 1927, Welter 1935, traditionally been assumed to be courtship dis- Cottam 194 1 as cited in Pitelka 1942) or to re- plays (e.g., Grinnell and Storer 1924, Bent 1940 main perched near nests (Wheelock 19 16 as cited and referencestherein, Skutch 1977). However, in Moore 1947, Clyde 1972). Except for a few Woods (1927, 1940), Pitelka (1942), and Stiles cases of females nesting in males’ territories (Stiles (1982) noted that male hummingbirds dive dis- 1973), there is a fairly clear habitat separation play towards both male and female intruders, between sexes during the breeding season; fe- and suggestedthat the display is primarily ago- males nest in partially wooded areas and males nistic in nature. Stiles (1973, 1982) also de- defend territories in more open areas nearby (Pi- scribed a number of close-rangedisplays that he telka 1942, 195 1; Legg and Pitelka 1956; Wil- interpreted as being more important in court- liamson 1956; Stiles 1973). On their territories, ship. The most extensive and detailed observa- males exhibit many conspicuous displays and tions are those in Stiles (1973, 1982). More de- vocalizations which have often been interpreted scriptions of the social context of displays are as courtship behavior. However, without de- required if we are to determine whether courtship behavior maintains reproductive isolation LReceived 8 February 1988. Final acceptance 30 among North American hummingbirds, and January 1989. 2 Present address:National Environmental Protec- whether display behavior by males is a basis for tion Agency, Box 1302, S-17 1 25 Solna, Sweden. choice of mates by females. 3Present address:School of Biological Sciences,Zo- In this paper we report behavioral observa- ologyBuilding (AO8), University of Sydney,NSW 2006, tions of Calliope Hummingbirds (Stellulu cal- Australia. 4 Present address:Department of Zoology, Univer- liope) during three breeding seasons. Calliope sity of Alberta, Edmonton, AL T6G 2E9, Canada. Hummingbirds breed in the mountains of west-

~721 DISPLAY BEHAVIOR BY CALLIOPE HUMMINGBIRDS 273 ern North America, from northern Mexico to We observed for 67.5 hr on five territories in southern Canada (Bent 1940, Johnsgard 1983). 1983,67.5 hr on five territories in 1984, and 88.5 Except for general descriptions of their natural hr on six territories in 1985. On each observation history (Bent 1940, Johnsgard 1983) and a few day, one or two of us observed males’ behavior analyses of the energetics of their territoriality during three 30-min sessionsstarting at 06:30, (Armstrong 1987) and their nesting(Calder 197 l), 1 l:OO, and 15:30. We recorded and timed all little is known about their reproductive behav- behaviors used in interactions with territory in- ior. We used our data to determine what behav- truders. We also recorded and timed apparent iors males use in interactions with other hum- displays performed in the absence of obvious mingbirds on their territories, and to suggestsome intruders. We examined die1 and seasonal vari- conclusions regarding the “functions” of those ation in display data collected by all observers. behaviors. However, because intruders were often difficult to detect and identify, we compared responses STUDY AREA AND METHODS to different types of intruders using only data Our study site is in a narrow valley in the Twin collectedby ST in 1984 and DPA in 1985, when Lakes area of the Ashnola Provincial Forest about each of us had had a previous year’s observa- 25 km SW of Penticton, British Columbia, Can- tional experience. ada (119”47’W, 49”18’N). The elevation of the Each year, there were virtually no flowers con- bottom of the valley is about 800 m. The male taining nectar in the valley or on the slopeswhen Calliope Hummingbirds we studied defended the males arrived. Squaw currant (Ribes cereum) territories in a strip of meadow about 100 m wide bloomed in early May, about the time we saw on the valley floor (seemap in Armstrong 1987). the first females, and was the main source of The females we observed nested in Douglas-fir nectar during most of May. During this time, (Pseudotsugamenziesii) forest on the north side males may have obtained much of their nectar of this meadow. A few Calliope Hummingbird from sources on their territories (Armstrong males held territories in the nesting area, and 1987). Although plants of other speciesbloomed both Calliope Hummingbird and Rufous Hum- throughout June, far less nectar than the birds mingbird (Selasphorus rujks) males defended required was produced on the territories (Arm- territories 100-200 m above the meadow on the strong 1987), and we observed almost no for- north-facing slope of the valley. aging on the territories in June. During this time Male Calliope and Rufous hummingbirds ar- both males and females visited undefended rived and established territories in this area in patches of flowers on the slopes south of and late April, and stayeduntil late June or early July. above the meadow and in the woods north of Since females were less conspicuous, it was dif- the meadow. In addition, sapsuckerswere pres- ficult to determine exactly when they arrived, but ent and we saw one Calliope Hummingbird fe- our first sightings were in early May each year. male feed at a drilling (seeSutherland et al. 1982, On 3 May 1985, we placed feederswhere females Miller and Nero 1983, Kattan and Murcia 1985) had fed from feedersof the same type in previous but the drillings we found were not used regularly years. No females appeared at these feeders,but by hummingbirds. several females appearedwhen we placed feeders at the same locations on 7 May. At least some BEHAVIOR OF TERRITORIAL MALES females stayedwell into July. Egglaying occurred Up to 12 males defended territories in the mead- from late May through early June 1983 (Tamm ow in May, and at least some of the territories 1985). The latest clutch we observed was laid on were contiguous. However, by June each year, or about 3 July 1984, but was later abandoned, no more than six of the territories remained, and We marked birds individually, either by spray- these were all well separated, with on average ing them with thinned enamel paint when they over 100 m between territory centers. Three of visited feedersor perches(Ewald and Carpenter the five banded males that we observed defend- 1978, Ewald and Rohwer 1980) or by catching ing territories in 1984 defended the same terri- them in a feeder-baited trap and applying paint tories at the beginning of the 1985 season. Ter- by hand. The latter birds were banded. In 1984, ritory owners were occasionally challenged by we banded 11 birds (eight males, three females); one or more intruding males over a period of in 1985, we banded 13 (11 males, two females). several days. In some cases, challengers dive- 214 S. TAMM, D. P. ARMSTRONG AND Z. J. TOOZE displayed to owners. Some individuals that we circle dance of Calliope Hummingbirds. We often marked in the meadow in early May were not saw buzzing bouts that appeared to be directed observed again until they took over one of the towards birds of other species,or nonliving ob- territories in late June. jects such as twigs or leaves. In 1984, a male Territory owners were visible 57% of the ob- performed a seriesof dive displays and buzzings servation time in 1983, 55% in 1984, and 65% to two fledgling conspecifics(Armstrong 1988). in 1985. An unknown portion of the time out of Buzzing is similar to the shuttle display of Anna’s sight was spent off the territories. At least some Hummingbird males (Stiles 197 1, 1973, 1982). of this time was spent feeding on flowers in other Chasing. Males chased conspecifics and Ru- areas (Armstrong 1987). fous Hummingbirds of both sexes, passerine During 10 hr of observation of two incubating birds, and less often bumblebees. Sometimes females, and over 30 hr of observation of three more than two hummingbirds were involved in nests at the nestling stage,we never saw a male a chase.Although we have no firm evidence for approach a nest. We therefore have no evidence this, we felt that chasesoften were less vigorous that males assistedin the rearing of young. when the chased bird was a female hummingbird, suggestingthat the male was “following” DESCRIPTION OF INTERACTIVE BEHAVIORS rather than “chasing” her. Dive displays.Males first ascended to 20-30 m, Vocalizing. Males typically chattered during then dived, producing a loud whistle at the bot- chases.They sometimes chatteredwhen perched, tom of the dive. After a dive they either ascended often in responseto chatteringby neighbors.Some again until they reached approximately the same males often chattered when leaving their terri- altitude and made another dive, or changed di- tories. This behavior was similar to the “low rection early in the ascending phase. Most dive intensity threat” describedby Stiles (197 1, 1973, displays were directed towards another bird 1982), Ewald and Carpenter (1978) and Ewald perched on the territory, but displays were oc- and Orians (1983) in Anna’s Hummingbirds. casionally given when we detectedno other bird. Hover displays. Males often started hovering RESPONSES TO DIFFERENT bouts several meters above the ground (some- TYPES OF INTRUDERS times 10 m or more) and slowly descended in Territory owners usually displayed towards fe- discontinuous stages, alternating between sud- male Calliope Hummingbirds on their territo- den vertical drops of a few dm to 0.5 m, and ries. In contrast, they almost always chasedmale stationary hovering or slow descent. While hov- intruders (both conspecificsand Rufous Hum- ering, males often turned from side to side. Hov- mingbirds), although dive displays sometimes ering males usually faced the display object (usu- precededthe chases(Table 1). Males often dived ally a female, see below) but did not necessarily at passerinebirds (flycatchers,Empidonax spp.; hover directly above it. Similar behavior has been Nashville Warblers, Vermivora rujicapilla; and observed in Ruby-throated Hummingbirds, Ar- Chipping Sparrows, Spizella passerina) that chilochuscolubris (Tyler 1940) and Broad-tailed perchedconspicuously on their territories (Table Hummingbirds (Selasphorusplatycercus) (Bent 1). Responsesto males were significantly differ- 1940). Males sometimes hovered after returning ent than responsesto both females and passer- from chases.We usually detectedno display ob- ines in 1984 (Chi-square test, P < 0.001 for both ject in such cases. comparisons), but responsesto females were not Buzzing. When a female perched on a male’s significantly different than those to passerines(P territory, the male often hovered in front of and > 0.1). All three comparisons were statistically slightly above her, and produced a loud buzz. significant in 1985 (P c 0.001). Sometimes the female left her perch and both Dive displays directed towards conspecificfe- birds spun around in circles (“circle dance”), fac- males included about three times as many dives ing each other with the male slightly higher than per encounter as displays directed towards either the female. Wyman (1920) reported a male and unidentified or Rufous Hummingbirds, passer- a female holding each other’s bills during a circle ines, or unknown/no objects (Fig. 1). We have dance. We believe that many of the “mid-air insufficient data to estimate mean number of matings” described for several hummingbird dives in encounters with males in 1984. Hover species(Bent 1940) were displays similar to the displays towards females were also of much lon- DISPLAY BEHAVIOR BY CALLIOPE HUMMINGBIRDS 275

TABLE 1. Differencesin responsesof territorialmales to intrusionsby male conspecifics,female conspecifics, and passerinesin 2 years.Intrusions that evokedno responseby territoryowners were excluded because we couldnot be certainthat suchintruders were detected by the resident.Because many encountersinvolved more than one type of recordedbehavior, the sum of the percentagesfor the threebehaviors exceeds 100% for all displayobjects.

% responses involving: No. of intruders Display object responded to Dive displays Hover displays Chases 1984 Male 54 2 9 98

PasserineFemale 2215 10086 2713 :: 1985 Male 151 9 11 92 Female 30 100 61 9 Passerine 65 85 9 15 ger duration than displays towards all other ob- long buzzing bout, the female perched with her jects (Fig. 2). body almost horizontal, and wings partially open. After buzzing in front of her, possibly in direct MATING SEQUENCE contact, the male mounted for 2 sec. Immedi- The following description illustrates use of div- ately after the mating the female flew to another ing, hovering, and buzzing displays in a complete bush. The male made 13 more dives, two hover courtship sequence that culminated in copula- bouts (27 set), and one buzzing bout (1 set) be- tion. A marked male encountered an unmarked fore the female left the territory 3.5 min after the female near the center of his territory at 16:23 mating. The female chirped (“chip note,” Stiles on 3 June 1984. The female moved from perch 197 1) throughout the interaction. to perch, all on vegetation near the ground but Earlier the same day, an unmarked female dis- clearly visible, while the male dive-displayed. placed this male from his perch and landed on The mating took place about 0.5 m above the it herself. We noticed similar behavior by un- ground in a bush about 3 m from the perch that marked females on another territory twice the had been the male’s main perch that day. Mount- same day. In all three casesthe males responded ing occurred 7 min after the female appeared on with series of dive displays, and at least one of the territory, and was precededby a seriesof 19 these interactions involved a circle dance. dives interspersedwith four hover-display bouts (total duration 256 set), and six buzzing bouts (total duration 5 1 set) the longest of which (26 set) immediately precededmounting. During this

22 80 -

s n 1984 J‘ L 0 1985

n 1984 2 40- q 1985

Female HB P ? Male

Female HB P ? M2.k Display Object Display Object FIGURE 2. Mean durationof hoverdisplays per en- FIGURE 1. Mean numberof divesper encounterin counterwith femaleconspecifics (“Female”), uniden- displaysto femaleconspecifics (“Female”), unidenti- tified or Rufous Hummingbirds (“HB”), passerines fiedor RufousHummingbirds (“ HB”), passerines(“I” ‘), (“P”), no or undetectedobjects ( “?“), and male con- no or undetectedobjects ( “?“), and male conspecifics specifics(“Male”). 95% confidenceintervals are indi- (“Male”). 95% confidenceintervals are indicated. cated. 276 S. TAMM, D. P. ARMSTRONG AND Z. J. TOOZE

in the other years, presumably becausewe started observations later in those years (Fig. 3). Both dive and hover displays decreasedmark- edly during the day. Median number of dives per hour was 9, 4, and 1, for morning, midday, and evening sessions, respectively (Kruskal-Wallis test, df = 2, H = 22.91, P +z 0.001). Median FIGURE 3. Mean frequenciesof dive displaysbe- duration of hover displays was 27, 14, and 2 set, tween06:30 and 09:45 by CalliopeHummingbird males duringthree breeding seasons. The linesconnect mean for the three sessions(H = 14.32, P -c 0.001). displayrates on five territoriesfor 1983and 1984,and six for 1985.Open squaresindicate that eachterritory DISCUSSION was observedfor 30 min by one observer, and filled The courtship behavior of Calliope Humming- squaresindicate that each territory was observed for 30 min by one observer, and then for another 30 min birds, while similar to that of Anna’s Hum- by a different observer. Seasonaland yearly variation mingbirds as described by Stiles (1973, 1982), is not statistically significant. appearsto differ in at least two ways. First, copulation in Anna’s Hummingbirds is precededby a chase from the male’s territory, usually to the SEASONALAND DIEL PATTERNS OF vicinity of the female’s nest. In contrast, the Cal- DISPLAY BEHAVIOR liope Hummingbird mating we observed took We detected no difference in number of displays place near the center of a male’s territory and per hour between years (Kruskal-Wallis test, df was not immediately preceded by a chase, al- = 2; dive displays: H = 3.29, P > 0.1; hover though the male followed the female when she displays:H = 1X3, P > 0.1; two-tailed testshere moved from perch to perch (usually by adjusting and below), nor did we detect any seasonaltrend his position while hovering above her). It should in overall display activity (Fig. 3; Spearman’s be pointed out that we observedonly one mating, rank correlation between number of dive dis- and found no nests prior to egglaying, and most plays per sessionand date, rs = -0.099, df = 60, of our intensive observation was on territories. P > 0.4; between duration of hover displays and Therefore we probably would have missed any date, r. = -0.007, df = 60, P > 0.5). However, matings that took place away from the males’ there was a seasonalpattern in 1985 in that more territories. dive displays were directed towards conspecific Second, Stiles mentioned no female initiatives males than towards conspecific females early in other than visiting males’ territories to feed. Stiles the season, whereas the opposite was true later used expressionslike “the male forcesher down” in the season(Fig. 4). No suchtrend was apparent (fig. 6 in Stiles 1982). In contrast, the Calliope

n Female Unidentified hummingbird Passerine

May 4 June 14 FIGURE 4. Frequenciesof dive displaysby categoryof displayobject in 1985 averagedover all times of day. DISPLAY BEHAVIOR BY CALLIOPE HUMMINGBIRDS 277

Hummingbird female we observed did not ap- telka 1942 and references therein; Stiles 1982; pear to be trying to escapethe male. She chirped ST, pers. observ.), and juvenile Anna’s Hum- continuously and moved between fairly visible mingbird males perform rudimentary dive dis- perches in the core area of the male’s territory. plays within 2 weeks of fledging (Stiles 1973, Furthermore, we saw at least two different fe- 1982). However, only mature males perform males actively approach, and displace perched dives in the highly ritualized manner character- males on three separateoccasions the same day. istic of each North American hummingbird Also, females often visited territories that con- species.Furthermore, although Woods (1940) re- tained no profitable flowers (Armstrong 1987). ported that dive displays by Anna’s Humming- Pitelka (1942) and Stiles (1973, 1982) sug- birds occur during “at least the greater part of gestedthat mate selection in Anna’s Humming- the year,” and Wagner (1954) reported display birds may be mediated through male territory flights by Black-chinned Hummingbird (Archilo- quality: females visit territories of males pri- cusalexandri) males on wintering grounds, Stiles marily to feed, and males who have successfully (1973) reported that dive displays were rare or competed with other males for high quality ter- absent outside the breeding season in several ritories encounter more females than do males species,but became progressivelymore frequent on poorer territories. Female choice would act as the breeding seasonapproached. According to through acceptance or rejection of particular Pitelka (1942) the peak of diving coincides with males. In contrast, it is our impression that fe- the peak of gonad development, and may be male Calliope Hummingbirds may visit males closely correlated with testosteronelevels. for the purpose of mating, as do female hermit Stiles (1982) recorded almost as many dive hummingbirds (e.g., Snow 1974, Stiles and Wolf displays towards conspecific males (18) as to- 1979). The breeding system of the Calliope wards females (21) by Anna’s Hummingbird Hummingbirds at this site is perhaps best de- males, but more chases of males (63) than of scribed by the term “exploded lek.” Calliope females (35). The Calliope Hummingbird males Hummingbird females might still use energetic we studied always dive-displayed towards all fe- quality of territories as an index of male quality males they encountered on their territories, and in other areas, but at our study site there was rarely chasedthem, but typically chasedintrud- little nectar available on any territory when mating males, and rarely dived at them (Table 1; ings occurred (Tamm 1985, Armstrong 1987) Tamm 1985). Furthermore, the only mating we so under this hypothesis many females would observed was accompanied by the most intense have to choosemates one or more weeks before seriesof displayswe recorded(32 dives, six hover they laid their eggs. displays,and sevenbuzzing displaysin 10.5 min), It is not surprising to find behavioral differ- and displays directed to females usually consist- encesbetween these two species,given that Cal- ed of more dives than displays to other objects liope Hummingbirds migrate whereasmany An- (Fig. 1). We therefore conclude that dive displays na’s Hummingbirds remain in the same general play an important role in the courtship of at least area all year. Furthermore, while Calliope Hum- this species,although this does not preclude a mingbird males are smaller than the females, the possible role in aggressiveinteractions (Tinber- reverse is true for Anna’s Hummingbirds. This gen 1952, Wingfield 1984 and referencesthere- suggeststhat the mating systemsare different. in). Alternatively, it is possiblethat there are two The dives of North American hummingbird types of dive display that differ in ways too subtle males have traditionally been regarded as court- for us to detect. ship displays(e.g., Grinnell and Storer 1924, Bent It is conceivable that females could compare 1940 and referencestherein, Skutch 1977) but the relative responsesof males whose territories this interpretation has been questioned on the they visit, but we have no evidence for any in- grounds that dives are directed to conspecificsof fluence of display rates on female choice. How- both sexes,and to members of other humming- ever, given the risk of interspecific matings (e.g., bird and nonhummingbird species(Woods 1927, Banks and Johnson 196 1, Wells et al. 1978, Wells 1940). In contrast,Pitelka (1942) and Stiles (1973, and Baptista 1979) it would be surprising if fe- 1982) considered the dives primarily, but not males did not use the conspicuous and highly exclusively, aggressively motivated. Female species-specificdive displays in addition to other hummingbirds sometimes dive at intruders (Pi- means of identifying conspecific males. In par- 218 S. TAMM, D. P. ARMSTRONG AND Z. J. TOOZE titular, the soundsproduced by displaying males ACKNOWLEDGMENTS of different speciesare both loud and easily dis- We thank Lee Gass for suggestingthe study, Karen tinguishable by humans (Bent 1940, Woods Price for assisting in the field in 1983. and Si and 1940). Doreen Sieben for their hospitality throughout the We do not know why male hummingbirds dis- project. The following provided helpful comments on the manuscript:L. F. Baptista, C. L. Gass,M. Roberts, play to passerines.Stiles (1982) pointed out that F. G. Stiles, and an anonymous reviewer. Financial male-male and male-female encounters are very supportwas provided by grants from the Royal Swed- similar in many hummingbird species,and sug- ish Academy of Sciencesand the University of Stock- gestedthat females signal their sex by perching, holm to ST, an NSERC (Natural Sciencesand Engi- whereas males leave. Pitelka (1942) and Stiles neering Research Council of Canada) postgraduate scholarshipand a University of British Columbia grad- (1973, 1982) noticed that males were more likely uate fellowship to DPA, and NSERC grant 67-9876 to to display towards perching than towards flying C. L. Gass. intruders. It is therefore possible that passerines often elicit courtshipbehavior becausethey perch on males’ territories. A Calliope Hummingbird LITERATURE CITED male at our site displayed to and appeared to ARMSTRONG,D. P. 1987. Economicsofbreeding ter- attempt copulation with juveniles (Armstrong ritoriality in male Calliope Hummingbirds. Auk 1988), and similar behavior has been observed 104:242-253. in other hummingbird species(Snow 1974, Stiles ARMSTRONG,D. P. 1988. Persistent attempts by a male Calliope Hummingbird, Stellulucalliope, to and Wolf 1979, Stiles 1982). However, when we copulate with newly fledged conspecifics. Can. attracted both male and female intruders to feed- Field-Nat. 102:259-260. ers on males’ territories, owners dive-displayed BAILEY,A. M. 1927. Notes on the birds in south- to females and chased males although birds of easternAlaska. Auk 44~35l-367. BANKS,R. C., AND N. K. JOHNSON.196 1. A review neither sex perched. of North American hybrid hummingbirds. Con- There are several possibleexplanations for the dor 63:3-28. decrease in display activity over the day. We BENT,A. C. 1940. Life histories of North American initially hypothesized that this decreasewas re- cuckoos,goatsuckers, hummingbirds, and their al- lated to die1 variations in nectar availability, be- lies. U.S. Natl. Mus. Bull. 176. CALDER,W. A. 1971. Temperature relationshipsand cause males dramatically increase their display nestingof the Calliope Hummingbird. Condor 73: rates when they have accessto artificial feeders 314-321. (Tamm 1985). Such a decreasein nectar avail- CLYDE,D. P. 1972. Anna’s Hummingbird in adult ability did occur on 19 and 21 May 1985, when male plumage feeds nestling. Condor 74: 102. EWALD,P. W., AND F. L. CARPENTER.1978. Terri- the average afternoon standing crop of Ribes torial responsesto energy manipulations in the flowers on territories dropped to 8% of the early Anna Hummingbird. Oecologia (Berl.) 56:359- morning level (Armstrong 1987). However, this 364. pattern was not observed in Ribes flowers sam- EWALD,P. W., AND G. H. O~ANS. 1983. Effects of pled on 10 and 13 May, nor in Castillejaminiata resourcedepression on use of inexpensive and es- calatedaggressive behavior: experimental testsus- flowers sampled off territories in June when that ing Anna Hummingbirds. Behav. Ecol. Sociobiol. speciesappeared to be the birds’ primary source 12:95-101. of nectar (Armstrong 1987). Given that our ob- EWALD,P. W., ANDS. ROHWER. 1980. Age, coloration servations suggestthat the die1 variation in dis- and dominance in non-breeding hummingbirds: a test of the asymmetry hypothesis. Behav. Ecol. play activity persiststhroughout both Ribesand Sociobiol. 71273-279. Castilleja-basedphases of the breeding season, GRINNELL,J., AND T. I. STORER. 1924. Animal life we conclude that nectar availability is unlikely in the Yosemite. Univ. of California Press,Berke- to be the primary reason for this variation. Other ley. explanations include the possibilities that there JOHNSGARD,P. A. 1983. The hummingbirds ofNorth America. Smithsonian Institution Press. Wash- are more or more active competitors in the ington, DC. morning, that it is important for territory owners KATTAN,G., AND C. MURCIA. 1985. Hummingbird to establish their presenceearly in the morning, associationwith Acorn Woodpecker sap trees in and that females are more likely to visit or to Colombia. Condor 87:542-543. LEGG,K., ANDF. A. PITELKA. 1956. Ecologicalover- observe territories in the morning than later in lap of Anna and Allen hummingbirds nesting at the day. We have no data that would allow us Santa Cruz, California. Condor 58:393-405. to distinguish between alternative explanations. MILLER, R. S., AND R. W. NERO. 1983. Humming- DISPLAY BEHAVIOR BY CALLIOPE HUMMINGBIRDS 279

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