DIET OF BREEDING WHITE-THROATED AND BLACK SWIFTS IN SOUTHERN CALIFORNIA ALLISON D. RUDALEVIGE, DESSlE L. A. UNDERWOOD, and CHARLES T. COLLINS, Department of BiologicalSciences, California State University,Long Beach, California 90840 (current addressof Rudalevige:Biology Department, Universityof California,Riverside, California 92521) ABSTRACT: We analyzed the diet of nestling White-throated(Aeronautes saxatalis) and Black Swifts (Cypseloidesniger) in southern California. White- throatedSwifts fed their nestlingson bolusesof insectsmore taxonomicallydiverse, on average(over 50 arthropodfamilies represented), than did BlackSwifts (seven arthropodfamilies, primarfiy ants). In some casesWhite-throated Swift boluses containedprimarily one species,while other bolusesshowed more variation.In contrast,all BlackSwift samplescontained high numbersof wingedants with few individualsof other taxa. Our resultsprovide new informationon the White-throated Swift'sdiet and supportprevious studies of the BlackSwift. Swiftsare amongthe mostaerial of birds,spending most of the day on the wing in searchof their arthropodprey. Food itemsinclude a wide array of insectsand some ballooningspiders, all gatheredaloft in the air column (Lack and Owen 1955). The food habitsof a numberof speciesof swifts have been recorded(Collins 1968, Hespenheide1975, Lack and Owen 1955, Marfn 1999, Tarburton 1986, 1993), but there is stilllittle informa- tion availablefor others, even for some speciesthat are widespreadand common.Here we providedata on the prey sizeand compositionof food broughtto nestlingsof the White-throated(Aerona u tes saxa talis) and Black (Cypseloidesniger) Swifts in southernCalifornia. The White-throatedSwift is a commonresident that nestswidely in southernCalifornia, while the Black Swift is a local summerresident, migrating south in late August (Garrettand Dunn 1981, Foersterand Collins 1990). METHODS When feedingyoung, swifts of the subfamiliesApodinae and Chaeturinae return to the nest with a bolusof food in their mouths(Collins 1998). This bolus is passedto the nestlings.Each bolus, made up of an array of arthropodsloosely stuck together with saliva,can containonly a few large itemsor severalhundred small ones (Lack and Owen 1955, Collinsunpubl. data).We collectedfive bolusesfrom adultWhite-throated Swifts (subfamily Apodinae)returning to nests in a man-madestructure in Glendale, Los AngelesCounty, California. All collectionswere madebetween 14:00 and 16:00 on 6 June 1997, 6 and 27 July 1999, and 21 June and 28 July 2000, in conjunctionwith bandingand studiesof nestlinggrowth at thissite. We collectedboluses infrequently in order to minimize disturbanceand possibleeffects on the studyof nestlinggrowth (Collinsunpubl. data). The dominantland-cover type within 15 km of the nestsite is urbanwith some areasof chaparral,coastal scrub, and montane hardwood forest (Davis et al. 1998). Western Birds 34:209-215, 2003 209 DIET OF WHITE-THROATED AND BLACK SWIFTS BlackSwifts and other membersof the subfamilyCypseloidinae carry a largermass of food in the esophagus,enabling them to return to provision the nestlingat longer intervals,perhaps only once or twice a day (Collins 1998, Marfn 1999). While we were bandingthe swifts,we took four samplesof this esophagealfood mass on 7 August 2000 from adults returning at dusk to nests at Lawlor Falls, located in the San Jacinto Mountainsnear Idyllwild,Riverside County, California. Foerster (1987)and Marfn (1999) previouslystudied Black Swifts at thissite. The dominantland- cover typeswithin 15 km of this site are chaparral,montane hardwood forest,ponderosa pine (Pinusponderosa), Jeffrey pine (P.jeffreyi), urban, coastalscrub, and annualgrassland (Davis et al. 1998). We storedthe food samplesin 70% ethanolfor later analysis.We sorted the samplesunder a dissectingmicroscope and, usingBorror et al. (1989), identifiedthe prey to order and, when possible,to family. Using an ocular micrometer,we measuredthe bodylength of eachprey item to the nearest 0.1 mm. Standarddeviations are reportedalong with mean values. RESULTS AND DISCUSSION White-throatedSwifts fed on a variety of prey with Hemiptera (bugs, hoppers; 41.86 %), Psocoptera(bark lice; 27.24%), and Diptera (flies; 21.94 %) being the dominant orders represented(Table 1). Over 50 Talkie ! ArthropodComposition of Five Bolusesof White-throated Swift Preya Bolus Orderand Family (or Superfamily) 1 2 3 4 5 Hymenoptera(ants, wasps, bees: 6.58%) Andrenidae -- 2 2 -- 2 Apidae 3 .... Braconidae -- -- 1 -- -- Eupelmidae -- 1 -- -- -- Formicidae -- -- 2 1 -- Halictidae -- -- 2 1 -- Ormyridae -- -- 1 -- -- Pteromalidae -- -- 10 2 -- Torymidae -- -- 1 4 -- SuperfamilyCynipoidea -- -- 1 -- -- Hemiptera(bugs, hoppers: 41.86% ) Achilidae -- 1 -- -- -- Aphididae -- 26 1 23 3 Berytidae -- -- -- 3 -- Cercopidae -- -- 4 -- -- Cicadellidae 1 -- 3 139 2 Lygaeidae -- -- 2 7 -- Miridae -- -- 1 -- -- Psyllidae -- 1 -- -- -- (continued) 210 DIET OF WHITE-THROATED AND BLACK SWIFTS Table 1 (Continued) Bolus Orderand Family(or Superfamily) 1 2 3 4 5 Reduviidae -- -- 4 -- -- Rhopalidae -- -- -- 1 -- Scutelleridae -- -- 1 -- -- SuperfamilyFulgoroidea -- -- 1 -- -- Unknown -- -- 5 -- -- Diptera(flies 21.94% ) Agromyzidae -- 1 -- -- -- Bibionidae -- -- 10 -- -- Bombyliidae .... 5 Calliphoridae -- -- 1 -- -- Cecidomyiidae -- 1 -- -- -- Chloropidae -- -- -- 1 -- Conopidae -- -- 1 -- -- Empididae .... 5 Muscidae -- 1 2 2 -- Oestridae .... 1 Pipunculidae -- 1 1 -- 1 Scenopinidae -- -- 2 -- -- Sciaridae -- -- -- 1 -- Sciomyzidae .... 1 Simuliidae .... 1 Sphaeroceridae -- -- -- 1 -- Stratiomyidae -- -- 3 -- -- Syrphidae -- -- 1 -- 3 Tachinidae -- -- 4 -- 1 Tephritidae -- -- 1 -- -- Therevidae -- -- 32 -- 31 Trixoscelididae .... 1 SectionAcalyptratae -- 1 -- -- -- Unknown -- 2 -- -- -- Coleoptera(beetles: 1.28%) Elateridae -- -- 1 -- -- Melyridae -- -- 1 -- 1 Anobiidae -- -- 2 -- -- SuperfamilyCurculionoidea -- -- 2 -- -- Psocoptera(bark lice: 27.24%) Liposcelidae -- -- 122 22 3 Unknown .... 2 Lepidoptera(butterflies, moths: 0.18%) Unknown .... 1 Thysanura(silverfish: 0.18%) Lepismatidae .... 1 Araneae(spiders: 0.73%) Unknown -- -- 3 -- 1 Total 4 38 231 208 66 aValuesare the numberof individuals(by family)for each bolus.The percentageof arthropod orders from the pooled bolusesis also given. The orders Hemiptera and Homopteraof Borroret al. (1989) are combinedunder the Hemiptera. 211 DIET OF WHITE-THROATED AND BLACK SWIFTS differentfamilies of arthropodswere identifiedfrom among the 547 prey items containedin the 5 boluses(Table 1). The mean number of individuals per boluswas 109.4 + 103.1, range 4 to 231. Taxonomicdiversity also variedfrom bolusto bolus.For example,one boluscontained three individu- als of the familyApidae (bees)and one memberof the family Cicadellidae (leafhoppers),while another containedrepresentatives of 33 familiesin 6 orders.Two bolusescontained many individualsbelonging almost entirely to a singletaxon (in one caseCicadellidae, in the other Psocoptera).In other bolusesthese same taxa were representedonly minimally(Table 1). The sizeof White-throatedSwift food itemsalso varied extensively. Mean bodylength of prey itemsin millimeterswas 3.9 + 2.0, range0.7 to 16.7, in the pooled sampleof 5 boluses(Figure 1). As expected,some boluses containedfewer largeinsects, while othershad many smallerones (Table 1). Large, heavy-bodied,or winglessinsects were hardlyrepresented, as all prey itemswere gatheredin the air columnby flyingadult swifts. However, two individualsof the heavy-bodiedsuperfamily Curculionoidea (weevils) were captured,as wasone smallthysanuran (silverfish), the latterperhaps having been carried aloft by wind currentsalong with a singleballooning spider. Glick(1939) foundthysanurans at altitudesof 330 m, with a singleindividual capturedat over2500 m. Many othersmall, weakly flying insects recorded in these bolusesmay have similarlybeen passivelycarried aloft by air currentsto the altitudesat whichWhite-throated Swifts typically forage. 2O0 18O 160 t40 120 t 00 8O 60 I I I I I I I [ I I ]]I I --I• I• I I-- I 1 2 3 4 5 6 7 8 9 10 1t 12 13 t4 15 t6 17 Pre• Length (mm) FigureI. Distributionof arthropodbody lengths in combinedfood boluses of White- throatedSwifts (n = 5) and BlackSwifts (n = 4), pooledin I-ram sizeclasses. Size classescontain prey items0.,5 mm greaterthan and lessthan the defininglength. 212 DIET OF WHITE-THROATED AND BLACK SWIFTS The BlackSwift samples had a meannumber of preyitems of 51.5 + 9.6 and were much lessdiverse taxonomically than the White-throatedSwift boluses:94% of the prey items were of the family Formicidae(winged reproductive ants), species Solenopsis xyloni and Crernatogaster rnorrnonurn(Table 2). Also representedwere severalhemipterans (bugs, hoppers)and dipterans(flies; Table 2). The meanbody length in millimeters of all of the prey itemsin the pooledsample was 9.5 + 2.2, range 2.7 to 12.8 (Figure 1). Previousstudies of the White-throatedSwift ranked only the relative abundanceof insectorders in the diet and did not quantifythe numbersof eachprey type or provideidentification below the levelof order(Bent 1940, Hespenheide1975). The high taxonomicdiversity of prey items and extremevariation from bolusto bolusagree with datafrom previousstudies of other swifts (Lack and Owen 1955, Collins 1968, 1980, Bull and Beckwith 1993, Tarburton 1986, 1993). It also suggeststhat White- throated Swifts, like most swifts in the subfamilies Chaeturinae and Apodinae,are opportunisticforagers, quick to exploitlocalized abundances of prey, and readilyconsume