The Condor93279-285 0 The cooper olnithologicalsociety 199 1 THE EFFECT OF FLORAL ABUNDANCE ON FEEDER CENSUSES OF HUMMINGBIRD POPULATIONS DAVID W. INOUYE~ Departmentsof Zoology and Botany, Universityof Maryland, CollegePark, MD 20742 WILLIAM A. CALDER~ Department of Ecology and Evolutionary Biology, Universityof Arizona, Tucson,AZ 85721 NICKOLAS M. WASER~ Department of Biology, Universityof Cahfornia, Riverside,CA 92521 Abstract. Numbersof Broad-tailedHummingbirds (Sehzsphorous platycercus) captured eachsummer from 1979-1989 at the Rocky Mountain BiologicalLaboratory were quite variable, rangingfrom 115 (1981) to 348 (1989) with new birds usuallyoutnumbering returning(previously banded) birds. Capturenumbers were negatively correlated with the abundanceof four speciesof flowersthey visited,Erythronium grandiflorum, Delphinium nelsonii, Zpomopsisaggregata and Delphinium barbeyc flower numberswere also highly variable during the study period. Since most of the captureswere at feeders,these data suggestthat in yearswith high floral abundancefeeders are lessattractive, while in years with low floralabundance hummingbirds with nestsor territoriesat greaterdistances increase their use of the feeders.This interpretationis supportedby seasonalvariation in use of feeders,which is highestduring the beginningand end of the seasonwhen floral abundance is lowest.Estimates of hummingbirddensity based on activityat feedersmay thusbe affected by the availability of floral food resources. Key words: Banding; census;floral abundance;Broad-tailed Hummingbird; population size; resources, INTRODUCTION ticides. While these perceptions lack the rigor of Many bird populations vary in size from year to scientific methodology, casual observations by year (e.g., Mulvibill and Lcberman 1987), or even numerous individuals nevertheless suggestthat within years. In some casesthis variation can be these are plausible explanations. Accurate esti- ascribed to particular environmental or demo- mates of hummingbird population size would graphic variables. For example, droughts may permit more careful consideration of the signif- affect reproduction and survivorship through an icance of this variation. effect on food resources(Gibbs and Grant 1987, A variety of techniques can be used to assess Grant and Grant 1989), cold temperatures may population size. Banding offersan advantageover result in decreasedwinter swivorship, or small techniques based on visual sightings, or counts population sizes may result in local extinctions. of song or flight noise that may be conducted However, variation in population size usually more easily, in that individuals are identified and remains unexplained. counted only once. We have carried out long- Hummingbird numbers seem to vary consid- term studies using two methods, banding and erably, as perceived by numbers at feeders. Ap- flight noise, on a resident population of Broad- parent lows are popularly attributed to environ- tailed Hummingbirds (Selusphorus platycercus mental catastropheson the wintering grounds for Swainson). During our 18-year study we have many speciesin Central America, such as vol- observed substantial year-to-year variability in canic eruptions, forest fires, and the use of pes- abundance of Broad-tailed Hummingbirds in Colorado. There is also significant annual vari- ation in the availability of floral resourcesused by these birds. Here we examine the relationship 1Received 14 May 1990. Final acceptance26 De- cember1990. between the abundance of the hummingbirds, as * Rocky Mountain BiologicalLaboratory;P.O. Box indicated .by banding studies, and floral re- 5 19, CrestedButte, CO 8 1224. sources. W91’ 280 D. W. INOUYE, W. A. CALDER AND N. M. WASER TABLE 1. Mean dates of first and last flowering by the four flower species, 1982-1988. Datesof lastflowering werecalculated by addingthe mean lengthof the floweringperiod to the mean dateof first flowering.Data are only shownif floweringoccurred in two or more plots.Data for this table are from a subsetof 23 of the 29 plotsmonitored for anotherstudy (the plotsin whichthese four speciesoccurred). Zpomopsis did not flowerin this subsetof the plotsin 1982 or 1983. 1982 1983 1984 1985 1986 1987 1988 1989 Erythronium grandiflorum 6106 6/14 6/12 6/01 6/01 5/20 5/25 5123 6/17 6/21 6/30 6/09 6/10 6/01 6/04 5/31 Delphinium nelsonii 6/16 6/23 6/25 6/09 603 6/06 6/07 6/03 7108 7/12 7/14 6/26 7/01 6/22 6/20 6/20 Zpomopsisaggregata 7/13 7/7 7105 6/29 6/29 7/04 9/05 8/30 8/28 8/07 8/14 8/21 Delphinium barbeyi 7/27 7/25 7/25 7/11 7/17 7106 7/06 7/06 8/18 8/16 8/21 8/04 8/13 7/27 7/19 7/20 Number of daysof overlan 2 0 36 26 28 21 14 15 MATERIALS AND METHODS most of the growing season (early May to mid- This study was conducted at the Rocky Moun- September), all flowers in the plots were counted. tain Biological Laboratory (RMBL),Crested Plots were scatteredamong habitats including a Butte, CO, at an elevation of approximately 2,900 dry rocky meadow (7 plots), aspenforest (2 plots), m. Broad-tailed Hummingbirds, the only resi- wet meadow (9 plots), dry (but not rocky) mead- dent breeding speciesof hummingbird at RMBL, ow (3 plots), aspen-meadow interface (3 plots) have been studied extensively at RMBL (e.g., and willow-meadow interface (5 plots). For each Calder 1975, 1981, 1984, 1985; Calder and speciesin every plot, the peak number of flowers Booser 1973; Calder et al. 1983; Waser 1978, produced during a given year was determined. 1988; Waser and Inouye 1977; Waser and Price For analyses in this study, the peak numbers for 1983). Hummingbirds were captured and band- each specieswere added acrossplots for each year ed each year at two to three different sites sep- to produce a single value of peak number of flow- arated by about 500 m. Most birds were captured ers. Not all speciesoccurred in all plots; for ex- at feeders at cabins using cages, mist nets, or ample, Ipomopsis was only found in l-7 of the butterfly nets, with some additional mist net cap- plots in any given year. However, representation tures in meadows around RMBL. Although the of speciesin the plots approximated their abun- banding program began in 197 1, it was intensi- dance at the study site. fied beginning in 1979 to the point where we Data from four species of flowers that are think that almost all resident birds were banded; among those most commonly visited by hum- by the end of 1979 and subsequentsummers the mingbirds at the study site were used for this continued netting and trapping yielded primarily analysis. They were Erythronium grandiforum recapturesofbirds already recordedfor that year. (glacier lily; Liliaceae), Delphinium nelsonii (Nel- In addition to data from banding, a potential son’s larkspur; Ranunculaceae), Delphinium bar- correlate of population size was also measured: beyi (tall larkspur; Ranunculaceae), and Ipomop- the numbers of male Broad-tailed Humming- sis aggregata (scarletgilia; Polemoniaceae). Only birds heard during 10 min of listening from a the latter fits the classical characterization of a fixed location in a meadow at RMBL at a stan- hummingbird-pollinated plant (long, red, tubu- dard time of day (noon). Such censuseswere con- lar flowers). Data were not included for other ducted every two to four days throughout each flowersused by hummingbirds, suchas Castilleja summer. Males of this speciesmake a mechan- spp. (Scrophulariaceae), or Aquilegia elegantula ical wing whistle that is audible up to 100 m (Ranunculaceae)because they were relatively rare. away (Miller and Inouye 1983). The four flower speciesused for this study span Data on flower abundance were collected from much of the flowering season (Table 1; Fig. 1). 29 2 x 2-m plots, located within 500-1,000 m of Erythronium, with large, yellow, pendant lily banding sites.Approximately every other day for flowers, is one of the first speciesto flower at the FEEDER CENSUSESOF HUMMINGBIRD POPULATIONS 281 a low of 115 in 1981 to a high of 348 in 1989. From 1980-1987 the number of recaptured (probably resident) birds was more constant than the number of newly captured (probably migrant) birds. The number of birds captured was highest in 1988 and 1989, when the numbers of flowers were lower than in any other year. These were the only two consecutive years of low flower numbers during the study, and this sequencewas apparently responsible for the large numbers of new birds (2 14) banded in 1988, and birds re- FIGURE 1. Flowering curves for the four flowerspe- captured in 1989; many of the recaptures were ciesfor 1986. first banded in 1988. There was also a large num- ber of new hummingbirds in 1985. Data from the census of wing-whistle noises study site, and is also visited by bumblebees and are also shown in Table 2; there was no signifi- other bees. The two blue-flowered Delphinium cant correlation between these census data and species are also visited frequently by bumble- the numbers of banded males (v < 0.100, it = bees, while hummingbirds are the primary vis- 11, P > 0.05), or between the number of flowers itors to Zpomopsis(Waser 1982) although bum- and the number of males heard in the censusof blebees commonly rob the nectar of Zpomopsis flight noises (r = 0.142, P > 0.05). flowers in some years (Inouye 1980). Despite the There was much variation among years in the variation among years in date of first flowering number of flowers produced by each of the four (Table l), the relative sequence of flowering is plant species(Table 3). The reasons for this are generally maintained, and the arrival and nesting not completely understood, although two factors of the birds appear to be synchronized with flow- appear to be important. First, there is a signifi- ering. cant correlation between the number of flowers of both Delphinium speciesproduced each sum- RESULTS mer and the amount of snowfall the previous Table 2 summarizes the numbers of Broad-tailed winter (Inouye 1989); there is no suchcorrelation Hummingbirds captured at RMBL from 1979- for the other two species.Second, every four to 1989.