Frequency of Arthropods in Stomachs of Tropical Hummingbirds
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436 ShortCommunications [Auk, Vol. 103 Frequencyof Arthropods in Stomachsof Tropical Hummingbirds J. v. gEMSEN,JR., • F. GARY STILES,2 AND PETERE. SCOTT1 tMuseumof Zoologyand Department of Zoologyand Physiology, Louisiana State University, BatonRouge, Louisiana 70803 USA, and 2Escuelade Biologfa,Universidad de CostaRica, Ciudad Universitaria "Rodrigo Facio," Costa Rica Although flower nectar is the most conspicuous per we do not record in detail the kinds of arthro- and energeticallyefficient food sourceof humming- pods consumed,except insofar as this may affect the birds,it is notablydeficient in amino acidsand other frequency of detectablearthropod remains. A de- essential nutrients (Baker and Baker 1975, Hains- tailed study of the arthropod diets and foraging tac- worth and Wolf 1976). Therefore, hummingbirds re- ticsof hummingbirdsis in preparation(Hespenheide quire an additional source of proteins, lipids, and and Stiles unpubl. data). other nutrients. In most or all species,these nutrients The specimensreported here were collected (1) are obtainedby consumingsmall arthropods.Yet ar- from 1980 to 1985 by personnel of the Museum of thropod-foragingby hummingbirdsremains very lit- Zoology,Louisiana State University (LSUMZ) or (2) tle studied comparedwith nectar-foraging(Gass and from 1971 to 1985 by Stiles and his students.Ap- Montgomerie1981, Hespenheide and Stilesunpubl. proximately 70% of all specimenswere collectedin data). The few available time-budget studies of for- Bolivia or Peru, 25% in Costa Rica, and the remainder aging hummingbirds(reviewed by Gassand Mont- in northwestern Ecuador, Venezuela, or Darign, Pan- gomerie 1981) indicate that arthropod-hunting con- ama.Twenty recentspecimens of 15 speciesfrom Ec- stitutesonly 2-12%of total foragingtime exceptwhen uador and Peru depositedin the Academyof Natural nectar is scarceor unavailable.An incubating female Sciences,Phildelphia, also were included. Although Broad-tailed Hummingbird (Selasphorusplatycercus) specimenswere obtained from throughout the year, consumedonly arthropodsfor about 10 days when the samplefrom Peru and Bolivia is mostly from May nectar was unavailable (Montgomerie and Redsell throughAugust, the "dry" season.Most (>95%) birds 1980). Thus, under some field conditions a hum- from group ! above were captured in mist nets, mingbird can satisfy energeticas well as nutritional which were checked from dawn to dusk at 3-4-h needsby feeding exclusivelyon arthropods. intervals;the remainder were shot. Birds from group The contentsof stomachsand crops of collected 2 were collected from mist nets checked at least hour- birds representan obvioussource of dietary infor- ly, or were shot,in roughly equalproportions. Stom- mation. Becausethese data are recordedroutinely on achsand cropsof group ! birds were scoredon spec- specimenscollected by our institutions,the speci- imen labels as containing "insects"(including other menscan provide an indicationof the frequencyand arthropods),as containingunidentifiable material, or generalityof arthropod-foragingin hummingbirds. as being "empty." Group 2 birds were scored in Accordingly,we recordedthe presenceor absenceof greater detail, but the data were convertedto one of arthropodremains in the stomachsand cropsof !,629 the three categoriesused for group ! birds. Although specimensof 140 speciesof hummingbirds(41% of not usually recorded on the labels, stomachsthat the approximately340 speciesin the family Trochil- containedarthropods were almostalways packed with idae). material rather than containing just a trace of parts. Presenceof arthropodsin the digestive tract of a Becausesoft insectsand spiderscan be digestedcom- specimenindicates only that the bird had recently pletely within 3-4 h (Stilesunpubl. data), our sam- fed on arthropods;it tells us nothing aboutthe rel- pling procedure for mist-netted birds is conserva- ative frequencyof arthropodsvs. nectarin the diet. tively biasedagainst detection of arthropods.Also, It is well known that different kinds of foods have many of the stomachcontents of group ! specimens different digestibilities,rates of gut passage,and de- scoredas "unidentifiable" were presumably the re- tectabilities; nectar and arthropods exemplify this mains of soft insectsand spiders. problemin the extreme.Nectar is virtually 100%as- Most (1,279 of 1,629, 79%; see Appendix) hum- similable (Hainsworth 1974, Hainsworth and Wolf mingbird stomachsand crops containedarthropod 1972)and is rarely detectedin the stomachbecause remains. Individuals of 133 (95%) of the 140 species it usuallypasses directly from esophagusto intestine examined had been feeding on arthropods.Of the 7 (the openingsof theseare closelyapposed in hum- speciesfor which arthropodswere not recorded,the mingbird stomachs).Arthropods are extensivelydi- largestsample size was only 5 individuals. gestedin the stomach,following only brief storage Few significantregional differences were detected in the crop.Soft-bodied arthropods, such as most small between samplesof the samespecies; therefore, data spiders,may be digestedrelatively quickly, except from all areaswere pooledin the Appendix.The main for easilyoverlooked poison claws. Many chitinous exceptionswere several speciesof hermits (Phae- insectparts (e.g. wings, head capsules) are easilyde- thorninae: genera Glaucisthrough Eutoxeres).In tectablefor considerablylonger periods.In this pa- stomachsof these speciesarthropods occurred with April 1986] ShortCommunications 437 T^I•LE1. Percentageof individual nonhermit hum- TABLE2. Chi-squarevalues for pairwise compari- mingbirds feeding on arthropods in six humidity- sons of arthropod-feeding frequency between elevation categories.Species found in more than nonhermit hummingbirds in different humidity- one category(see Appendix) were not included. elevation categories.a HM = humid montane, HF = humid foothills, HL = humid lowlands, SL = Percent semihumid lowlands, AH = arid highlands, AL = Humidity-eleva- arthro- No. of No. of arid lowlands. tion category pods birds species HM HF HL SL AH AL Humid montane 86.3 401 26 Humid foothills 79.9 313 32 HM -- 5.24* 5.61' 1.81 1.28 1.89 Humid lowlands 78.4 176 17 HF -- 0.15 3.69 4.21' 4.95* Semihumid lowlands 95.8 24 8 HL -- 4.08* 4.75* 5.50* Arid highlands 92.0 50 11 SL -- 0.38 0.16 Arid lowlands 93.5 46 9 AH -- 0.08 AL -- a* =p < 0.05. significantlylower frequency in SouthAmerican than in CostaRican birds [pooled samplesof all hermits, 60% for South America (n = 231) vs. 78% for Costa ing arthropods.Species were assignedto one or more Rica (n = 138); X2 = 12.26, df = 1, P < 0.001]. We sus- of six humidity-elevation categories.The data on pect that the discrepancybeween South American nonhermitsare summarizedby humidity-elevation and Costa Rican samplesis due to a difference in categoryin Table 1. The null hypothesisthat the specimen-collectingtechniques, which is relevant in probabilityof a stomachcontaining arthropods would the caseof hermitsbecause they take a much higher be the same for birds in the six different habitats was proportion of soft-bodiedspiders than do most non- rejected(X 2 = 17.33,df = 5, P < 0.01).Results of pair- hermits (Hespenheideand Stilesunpubl. data). Non- wise comparisonsbetween birds in different cate- hermits take more hard-bodiedinsects that persist goriesare shown in Table 2. In general, the cooler longer in the gut. The time that netted humming- or drier an area, the more likely a bird was to have birds remainedentrapped prior to collectionwas on arthropodsin its stomach.Arthropods were mostfre- average 3-4 times longer for the South American quent in the stomachsof hummingbirdsof arid or birds.The time of day of collectionmay be another semihumid areas. Within humid areas, montane contributingfactor. Most hermitsare lek speciesthat hummingbirdsmore often had consumedarthropods maycommence lek activitybefore feeding, following than had lowland species(Table 1). These relation- a dawn flight between roost and lek (cf. Stiles and shipsobtained in CostaRican and in South American Wolf 1979).By opening nets at dawn, South Ameri- samples(except that no arid regions exist in Costa can workers might well have captured a number of Rica). Becausethe two samplesare large and were hermits that had not yet fed. By contrast,virtually gatheredindependently, we think the relationships all netting to collect hummingbirds in Costa Rica reflect differencesin hummingbird feeding patterns commenced at least I h after dawn, and it was more between environments. likely that the birds had fed beforebeing collected. The differencesmay reflect variation between en- Because hermits and nonhermits take different vironmentsin availability and quality of nectarand kinds of arthropods,a comparisonof frequencyof arthropods.Dry tropicalforests support relatively few arthropod feeding is problematical.The bias result- species of hummingbird-pollinated flowers com- ing from more rapid digestionof arthropodsin her- paredwith wetter areas(Stiles 1981), and humming- mits' stomachs is minimized in the Costa Rican sam- birds in dry areasmay make relatively greater use of pie. Arthropodsoccurred less frequently in hermits' arthropodsto meet energeticneeds. Nevertheless, the than nonhermits' stomachs(78% for hermits; 85% for energetic importance of good nectar supplies is in- nonhermits,n = 312),but the differencewas signifi- dicated by the facts that dry-forest hummingbird cant only at the 0.10 level (X2 = 3.66, df = 1). The dif- communitiescontain relatively few speciesand show ferencewas