The Condor 95:1016-1023 0 The Cooper Ornithological Society 1993
DIET AND FORAGING BEHAVIOR OF VAUXS’ SWIFTS IN NORTHEASTERN OREGON ’
EVELYN L. BULL U.S.D.A. Forest Service,Pacific NorthwestResearch Station, La Grande, OR 97850
ROY C. BECKWITH U.S.D.A. Forest Service,Pacijc NorthwestResearch Station, Corvallis,OR 97331
Abstract. The diet of Vaux’s Swifts (Chaeturavauxi) during the breedingseason consisted primarily of insects in the orders Homoptera (hoppers, aphids, whiteflies), Diptera (flies), Ephemeroptera (mayflies), and Hymenoptera (ants, parasitic wasps). Diet was determined from 223 food bolusescollected from adults feeding nestlingsand represented24,133 in- dividual insects and spiders. Diet did not differ among five study areas or by time of day. A pair of swifts feeds an average of 5,344 arthropods to their nestlingseach day, and an average of 154,976 arthropods during the nestling growth period. Radio-tagged swifts foraged up to 5.4 km from the nest. Of actual sightingsof radio- tagged birds, 64% were foraging over land, 27% were foraging over water, and 9% were traveling in a straight line. Key words: Chaetura vauxi; diet; arthropods;northeastern Oregon; swift; Vau.xs Swift.
INTRODUCTION determine movements of nesting swifts in rela- The Vaux’s Swift (Chaetura vauxi) is a neotrop- tion to habitat. ical migrant whose numbers have declined sig- METHODS nificantly in Oregon sincethe 1980s (Sharp 1992). This speciesis more common in old-growth for- STUDY AREAS ests than in younger stands (Manuwal and Huff Five study areas, herein referred to as Syrup, 1987, Bull and Hohmann 1992), and the amount Frog, Ukiah, Balm, and Little, were in the Blue of old-growth foresthas decreased since the 1970s. Mountains of northeastern Oregon. Syrup and Certainly their habit of nesting and roosting in Frog were 35 km west-southwest and 41 km large-diameter, hollow trees would explain this southwest of La Grande, Union County, respec- associationwith old growth (Bull 199 1, Bull and tively. Ukiah was 17 km west of Ukiah, Oregon, Cooper 199 1). in Umatilla County. Balm and Little were 38 km There is little published quantitative infor- southeast and 11 km east of Union, Oregon, in mation on the diet of Vaux’s Swifts. Huey (1960) Baker and Union counties, respectively. All the examined the stomach content of one bird and study areas were predominantly coniferous forest found that it fed on Lepidoptera, Hymenoptera, with 2-17% of the area in grassland. Common and Diptera. Davis (1937) determined that leaf- tree species included grand fir (A&es grandis), hoppers appeared to be the primary food that Douglas-fir (Pseudotsugamenziesii), ponderosa swifts fed their young. A collection of Psocoptera pine (Pinusponderosa),and western larch (Larix obtained from swift boluses was published by occidentalis). All areas had streams and ponds Turner (1984). Stomachs of Vaux’s Swifts col- with permanent water that comprised 2-3% of lected in Nicaragua contained small Hymenop- each area. The study areas were selectedon the tera in June (Howell 1957). basis of swift abundance. Our objectives were to (1) determine diet of DIET Vaux’s Swifts during nesting in northeastern Or- egon, (2) compare diet among five study areas, We searched for nest trees used by Vaux’s Swifts (3) compare diet at different times of the day, (4) in June and July in 1991 and 1992 in all five relate diet to habitat within study areas, and (5) study areas. Nests were located by watching for 1 hr trees that swifts had previously used for nesting (Bull and Cooper 199 1) and that we pre- I Received22 April 1993. Accepted13 July 1993. sumed might be suitable for nesting because they
[1016] VAUXS’ SWIFT DIET 1017 appeared to be hollow. Swifts flying in or out of averagenumber of arthropods in each bolus and a tree indicated that the tree was being used as multiplying by the average number of visits to a nest site. the nest in a day. We assumed that each visit Once a nest tree was located, we checked it was to deliver food to the young. We then mul- weekly for 1 hr to determine when the swifts tiplied by the duration of the nesting period to startedfeeding young. When young were present, determine number of arthropods fed during this we collected diet samples by catching the adult period. Average number of visits/hr to the nest in a net just before it entered the nest cavity. was determined by recording activity for 9 hr at When the adult returned to the nest with a food each of three nests with nestlings (Bull and Col- bolus for the young, we simply collected the food lins, in press). bolus to get a food sample. We compared swift diet among five study areas We used two methods to trap adults. We sus- and at different times of the day (morning, af- pended a mist net (2 x 5 m and 6-cm mesh) ternoon, and evening) in each study area and for between two trees so it hung in front of the nest all areas combined by using a two-way analysis entrance. We also used a mist net on a frame (55 of variance. Significancewas defined as P 5 0.05. x 55 cm) suspended from two shelf brackets mounted above the nest entrance. The frame was TELEMETRY raised and lowered with strings. A tarp (5 x 6 We gatheredinformation on movements of nest- m) was placed under the net to catch the food ing swifts by following swifts in two study areas. bolus when the bird was caught because they We radio-taggedfour adult swifts associatedwith commonly expelled the bolus when they flew into three nests in the Ukiah area and five swifts as- the net. sociatedwith three nestsin the Frog area. A0.7-g In 199 1, when we were developing this tech- transmitter was glued to the back of each swift, nique, we attempted to collect one diet sample and the swifts were located daily during 10 days from each of five nests daily for one week. In between 29 July and 9 August. 1992, we attempted to collect three diet samples, We determined how much time swifts spent representingdifferent times of the day, from each foraging above the nest stand versus foraging at of 15 nests each week. Time periods were mom- a distance. Swift presencein the nest stand was ing (07:00-12:00), afternoon (12:01-17:00), and monitored by standing at the nest trees at Frog evening (17:Ol-2 1:OO). Samples collected from and Ukiah for 6 hr on three different days and one nest usually were collected on different days recording if the radio-tagged birds were close-by to reduce stressto the birds. Samples were not or distant. A bird with a “booming” transmitter alwaysobtained when scheduledbecause the birds signal, where we could not detect a direction, was learned to avoid the net or did not return to the classified as within the nest stand (within 0.2 nest tree when we were present. We attempted km). On at least 12 occasions,the shiny antenna to catch a bird at a particular nest for up to 2 hr on a bird was actually seen, and the signal was and then abandoned our efforts so young could booming. A bird with no signal or a weaker sig- be fed. Diet samples were collected in July, Au- nal, where we could discern a direction, was clas- gust, and September. sified as distant. These distances and categories The arthropods in the food boluses were ini- determined from the volume of the transmitter tially separated by order and then identified to signal are supported by other telemetry studies family. Except for the Diptera (flies) and Hy- we have conducted in the same areas (Bull and menoptera (ants, wasps,and sawflies),some fam- Holthausen 1993). ilies represented by one or two specimens were Locations 20.4 km from the nest were ex- classified as “others” in their particular order. tremely difficult to discern, so we attempted one The Araneida (spiders) were listed as such and location per bird per day. If we obtained two not separatedfurther. To determine the size-range locations for one bird, they were at least 1 hr of the arthropods comprising the diet, the body apart. To get locations throughout the day, we length of representativesof each order were mea- worked during 08:00-14:00 on half the days and sured under a microscope equipped with an oc- during 14:00-20:00 on the other half. ular micrometer. We used actual sightingsof the birds and very We also determined the number of arthropods near approximations based on the volume of the fed to the young in a day by determining the signal. We found birds by getting at least two 1018 EVELYN L. BULL AND ROY C. BECKWITH
TABLE 1. Number and percentageof arthropodsin arthropods in 1992. It seems that the Vaux’s Vaux’s Swiftboluses collected in northeasternOregon Swift gathersmany arthropods that are airborne by year. during the nesting period. The spiderswere prob- ably “ballooning” on silk threadswhen they were 1991 1992 Arthropods n % n % caught. The dominant arthropod taxa were similar in Homoptera 2,955 61 7,399 38 199 1 and 1992 (Table 1). The Homoptera (hop- Diptera 891 19 5,732 30 Ephemeroptera 514 11 3,728 19 pers, aphids, and whiteflies), primarily three spe- Hymenoptera 216 4 1,505 8 cies of Cicadellidae (leafhoppers), are the most Lepidoptera 80 2 115
TABLE 2. Number of prey items (individuals/family) found in 223 food bolusesof Vaux’s Swifts in northeastern Oregon.
Familv 1991 1992 Family 1991 1992
Homoptera Phoridae 25 95 Cicadellidae 2,740 6,709 Sphaeroceridae 19 75 Aphidae 155 426 Therevidae 21 71 Psyllidae 1 160 Lauxaniidae 3 70 Cercopidae 21 79 Tephritidae 5 70 Fulgoridae 14 18 Ceratopogoni- Others 24 dae 3 63 Pipunculidae 5 62 Coleoptera Agromyzidae 5 53 Scolytidae 56 326 Sarcophagidae 19 53 Curculionidae 3 4 Drosophilidae 3 47 Elateridae - 3 Dolichopidae 26 45 Stauhvlinidae - 4 Bombvliidae 7 42 Buprestidae 1 - Piophilidae 2 27 Others 5 56 Tipulidae 7 25 Hymenoptera Empididae 235 1,101 Muscidae 108 609 Formicidae 111 1,110 Tachinidae 70 595 Braconidae 198 75 Ephydridae 25 409 Ichneumonidae 8 Bibionidae 2 388 Pteromalidae 11 :: Sciaridae 26 325 Eulophidae 2 7 Anthomyiidae 34 286 Dryinidae 1 Chironomidae 4 255 Encyrtidae - Chamaemyi- Eupelmidae - idae 3 24 Avhididae 4 Heleomyzidae 1 24 Torymidae - Milichiidae 5 23 Eurytomidae Lonchaeidae 8 17 Diprionidae 1 Platypezidae 15 Chrysididae - Scenopinidae : 14 Cynipidae - Cecidomyiidae 1 11 Eucharitidae - Chaoboridae - 10 Halictidae - Otitidae - 6 Figitidae - Dryomyzidae 5 Meeasnilidae 1 Lonchopteridae s 5 Pamphiliidae - Pallopteridae 1 2 Vespidae - 1 Psvchodidae - 2 Proctotrupidae 1 - Rdagionidae 4 2 Stephanidae 1 - Acroceridae 2 1 Diptera Asilidae 1 Chloropidae 28 208 Calliphoridae :, 1 Sepsidae 33 130 Culicidae - 1 Mycetophilidae 27 121 Opomyzidae 1 Stratiomyiidae 24 120 Tabanidae : 1 Syrphidae 70 118 Scatophagidae 2 - Simuliidae 5 103 Xylophagidae 1 -
The miscellaneous arthropods, 27 and 95 in 199 1 However, the higher percentage of Homoptera and 1992, respectively, were composed of small at Ukiah and of Diptera at Little indicated either numbers of Trichoptera (caddisflies), Plecoptera an apparent difference in prey selection by the (stoneflies), and broken unknowns that would swifts or more likely, a local abundance of their require more time for proper identification than prey (Fig. 1). could be justified considering their small com- The ephemerids (mayflies) were not always ponent of the diet. present in swift boluses; however, they com- The diets among study areas were not statis- prised more than 75% of each of 18 boluses.This tically different (F = 0.47, df = 4, 24, P = 0.13). high percentageprobably correlates with timing 1020 EVELYN L. BULL AND ROY C. BECKWITH
q Hymenoptera n Diptera q Homoptera ffi Ephemeroptera
100,
80
Balm Frog Little Syrup Ukiah Study Area FIGURE 1. Composition (%) of major arthropod taxa in the diet of Vaux’s Swifts in five study areas in northeasternOregon, 1992. of adult emergence of this aquatic taxon. In a length of the arthropods included in the diet is similar way, individual bolusesalso showedpeaks shown in Table 4. This range and mean size of of occurrence of ants during swarming of re- prey items is similar to that reportedfor the Short- productives; one such bolus contained 99% tailed Swift (Chaeturu brachyura) (Collins 1968) winged adult ants. and Vaux’s Swifts in Venezuela (Collins, un- The average number of arthropods per bolus publ.). ranged from 96.6 at Frog to 146.4 at Ukiah (Ta- We observed an average of 3.6 visits/hr to ble 3). The greater average at Ukiah probably nests during 27.7 hr of observation at nests. We resulted from the frequency and small size of the observed swifts foraging as early as 06:OOand as lealhoppers and aphids in their diet. The number late as 21:O0. Activity depended on weather, of arthropods per bolus was reduced as the fre- however, so we conservatively estimated that quency of larger arthropods increased. The body swifts foraged for 13 hr/day. This meant that swifts made 47 daily trips to the nest and assum- ing that they fed young each visit, they fed 5,344 TABLE 3. Mean number of arthropods per bolus of arthropods/day to nestlings,using the mean from the Vaux’s Swift collectedin northeasternOregon, 1992. Seven of the 174 bolusescontained only partial sam- Table 3. If this foraging rate is consistent ples so are not included here. throughout the nestling period of 27-32 days (29 used in calculation), 154,976 arthropods are fed Nests (n) Boluses (n) (2) Arthropods SE to the nestlings at each nest. The presenceof the various arthropod taxa in Balm 4 46 108.5 11.49 Frog 3 42 96.6 45.71 a bolus varied by time of day (Fig. 2) although Little 1 3: 106.2 32.71 the difference was not statistically significant (F Syrup 3 111.0 10.15 = 0.47, df = 2, 12, P = 0.64). The apparent Ukiah 4 38 146.4 16.41 difference among times probably reflects the ac- Total 15 167 115.4 5.72 tivity patterns of their prey. At specific times, VAUX’S SWIFT DIET 1021 individual swifts also appeared to concentrate TABLE 4. Overall body length (mm) of major groups their feeding activity to take advantage of the of arthropods collected from Vaux’s Swift boluses in northeasternOregon, 1992. readily available food sources,such as swarming ants and emerging mayflies. This behavior also Number Mean has been observed for other Chaetura swifts (Fi- Arthropods measured length SE Range scher 1958, Collins 1968). Homoptera 100 4.08 0.21 1.41-9.65 FORAGING BEHAVIOR Diptera 140 5.89 0.26 2.11-13.13 Ephemeroptera 30 6.82 0.11 5.76-7.91 Swifts spent the majority of their time in or over Hymenoptera 85 3.80 0.16 2.30-9.11 the tree stands adjacent to the nest. We moni- Coleoptera 69 3.96 0.09 2.24-6.30 tored three birds for 18.5 hr, three birds for 17 Other insects 26 4.81 0.52 2.64-10.99 Spiders 30 2.75 0.16 1.65-5.15 hr, and three birds for 5 hr, and found they spent TotaP 480 4.60 0.52 1.47-13.13 an average of 53% of their time in proximity 1 Mean of means (n = 7). (~0.4 km) of the nest stand. Outside the nest stand (~0.4 km away), we obtained 112 telemetry locations on nine birds. km from the nest, were over mixed conifer for- The swifts were actually seen in 40% of these ests, 30% over water, and 18% over grasslands. observations. For the remainder, we could not Half the sightings over forest at Frog were over see the bird because our vision was obstructed old growth, and the remainder over younger by the tree canopy, but we judged the bird to be stands. Frog contained 11 ponds, 14.5 km of within 200 m. Birds were foraging0.4-1 km from stream (total 2% water), 16% of the area in grass- the nest at 60% of the locations, 1.1-2 km at lands, and 82% in forest within 3.5 km of the 2 l%, 2.1-3 km at 7%, and 3.1-5.4 km at 11%. nests. Ponds and streams were preferentially se- At Frog, 52% of the visual locations over 0.4 lected for foraging, and forests were used less
H Morning qAfternoon 0 Evening
60
Homop. Dipt Ephem. Hymen. Coleop. Aran Major Arthropods FIGURE 2. Percentageof major arthropods found in Vaux’s Swift boluses at different daily times, 1992; Homop. = Homoptera, Dipt. = Diptera, Emphem. = Ephemeroptera, Hymen. = Hymenoptera, Coleop. = Coleoptera, Aran. = Araneida. 1022 EVELYN L. BULL AND ROY C. BECKWITH than available. Foraging over grasslandsequalled is important that nest stands have a high density their availability. of insects so the travel time and energy expen- At Ukiah, 78% of the locationswere over mixed diture between foraging areas and the nest are conifer forestsand 22% over water. Of the forest minimized. Information is needed on insect den- locations, 28% were in old growth and the re- sity over and within different forest types, suc- mainder in younger stands. Ukiah contained 11 cessional stages,and silvicultural systemsto ac- ponds, 4 km of stream (2% of area), 1% of the curately assessthe effects of land management area in grasslands,and 97% in forest. Again for- activities on Vaux’s Swifts. aging over water was favored. In conclusion, the data indicate that size and composition of prey govern the composition of DISCUSSION boluses. The Vaux’s Swift feeds on rather small The Vaux’s Swift feeds on a high diversity and prey (< 13 mm) and is an extremely diverse feed- a great number of insects during the nestling pe- er that takes advantage of the variety of arthro- riod basedon the content of food boluses.Radio- pods that are airborne at the time the birds are tagged swifts in both Frog and Ukiah foraged nesting. In our study areas, they concentrated on more over water than expected based on avail- the speciessuch as leathoppers, true flies, may- able area. We observed the swifts foraging over flies, flying ants, and bark beetles. water 27% of the time, yet only 2% of the study areashad open water. Many aquatic insects(e.g., ACKNOWLEDGMENTS many true flies, mayflies, stoneflies, caddisflies) We thank B. E. Carter, H. D. Cooper, R. Guse, J. E. were likely collected near water, as well as the Hohmann, C. Hunter, C. Kolmorgan, K. I. Kronner, T. Millay, S. Sheldon, P. E. Talbot, B. C. Wales, and insects associatedwith the plants growing in the B. R. Yonker for their assistancewith field work. C. vicinity. The Empididae (dance flies) are often T. Collins and T. R. Torgersenreviewed an earlier draft found flying in large groups near water sources of the manuscript. One nest was on Boise Cascade, or moist areas and would be easy prey for the Corp. land. We also thank J. Digiulio for identifying adult swifts (Borror and DeLong 197 1). During the Diptera and Hymenoptera to family. Funding was provided by the USDA Forest Service, Pacific North- emergence periods mayflies would be a locally west ResearchStation. abundant prey item in aquatic areas (Borror and DeLong 1971). LITERATURE CITED Homoptera comprised the majority ofthe diet. The leafhoppers feed on a variety of plants, es- BORROR,D. J., AND D. M. DELONG. 1971. An intro- pecially grassesand other forbs, that are common duction to the study of insects.Holt, Rinehart and Winston, New York. throughout all the study areasin grasslands,open BULL, E. L. 1991. Summer roosts and roosting be- pine stands, logged areas, and on a variety of havior of Vaux’s Swifts in old-growth forests. plants in riparian zones (Franklin and Dymess Northwestern Nat. 72:78-82. 1973). BULL, E. L., AND C. T. COLLINS. In press. Nesting chronology, molt, and ectoparasites of Vaux’s The majority of foraging occurred over mixed Swifts in northeasternOregon. Avocetta. conifer forests.Insects collected above or within BULL,E. L., AND H. D. COOPER. 1991. Vaux’s Swift these canopies likely included flies, ants, para- nests in hollow trees. Western Birds 22:85-9 1. sitic wasps,leafhoppers, bark beetles,and moths. BULL, E. L., AND J. E. HOHMANN. 1992. The asso- The Muscidae (house flies, face flies, stable flies) ciation between Vaux’s Swifts and old growth for- estsin northeasternOregon. Western Birds 24:38- breed in decaying vegetation and animal dung 42. (Borror and DeLong 197 1); therefore,flying adults BULL, E. L., AND R. S. HOLTHAUSEN.1993. Habitat would be common throughout all the areas be- use and managementof Pileated Woodpeckersin cause cattle were present. The Tachinidae are northeastern Oregon. J. Wildl. Manage. 57:335- flies that are common parasites on herbivorous 345. COLLINS,C. T. 1968. The comparative biology of two Lepidoptera and Hymenoptera larvae. The speciesof swifts in Trinidad, West Indies. Bull. abundance of various butterflies and moths, in- Florida State Mus. 11:257-320. cluding the western spruce budworm, can pro- DAVIS, W. B. 1937. A Vaux Swift and its young. vide numerous hosts for these flies. Condor 39~222-223. FISCHER,R. B. 1958. The breeding biology of the We found swifts foraging up to 5.4 km from Chimney Swift ChaeturapeZagica vegetation of Oregon and Washington. U.S. For. MORSE,R. A., AND F. M. LAIGO. 1969. The Philip- Serv. Gen. Tech. Rep. PNW-8. pine Spine-tail Swift, Chaeturudubia McGreggor, GEORGE,W. G. 1971. Foliage-gleaningby Chimney as a honey bee predator. Philippine Entomolgist Swifts (Chaeturupelagica). Auk 88: 177. 1:138-143. HOWELL,T. R. 1957. Birds of a second-growthrain SHARP,B. E. 1992. Neotropical migrants on National forest area of Nicaragua. Condor 59:73-l 11. Forestsin the Pacific Northwest: a compilation of HUEY, L. M. 1960. Notes on Vaux and Chimney existing information. U.S. For. Serv., Portland, Swifts. Condor 62:483. OR. MANUWAL,D. A., AND M. H. HUFF. 1987. Spring TURNER,B. D. 1984. Psocopterafrom Venezuela- and winter bird populationsin a Douglas-fir forest a collection of psocids from the food boluses of sere. J. Wildl. Manage. 51:586-595. swifts. Ent. Stand. 15:209-2 13.