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Foraging Ecology of the Hawaiian Crow, an Endangered Generalist’

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Foraging Ecology of the Hawaiian Crow, an Endangered Generalist’ TheCondor88:211-219 0 The Cooper Ornithological Society 1986 FORAGING ECOLOGY OF THE HAWAIIAN CROW, AN ENDANGERED GENERALIST’ HOWARD F. SAKAI~ USDA Forest Service,Pacific SouthwestForest and Range Experiment Station, Institute of Pacific Islands Forestry Honolulu, HA 96813 C. JOHN RALPH~ USDA Forest Service,Pacific SouthwestForest and Range Experiment Station, Institute of Pacific Islands Forestry, Honolulu, HA 96813 C. D. JENKINS Department of Wildhfe Ecology,Russell Laboratory, Universityof Wisconsin,Madison, WI 53706 Abstract. The hypothesis that food was limiting the population of the endangeredHawaiian Crow (Corvus hawaiiensis)was investigated by observing its foraging habits. This crow was an omnivore, feeding on a wide variety of items including fruits, invertebrates, flower nectar, mam- mals, plant parts, and passerineeggs and nestlings.It was also an opportunistic feeder, shifting to readily available resources.The crow was adept at finding passerinenests, using this high protein diet of nestlingsand eggsduring the passerinebreeding season.Oha kepau (Clermontia spp.) and olapa (Cheirodendrontrigynum) comprised the bulk of the fruits taken. Arachnida and Isopoda were the predominant invertebrates found in droppings. Crows used the upper half of the canopy of mature trees,especially ohia (Metrosideroscollina) and koa (Acaciakoa), for their daily activities. Although the sample size was small, in the study area food seemed to be reasonably plentiful, and the crows were adaptable. Therefore, other factors probably are restricting the population. Efforts to maintain present habitat of the Hawaiian Crow need to be increased,with emphasis on ensuringa temporally continuous sourceof food. Key words: Hawaiian Crow; Corvus hawaiiensis;Hawaii; feeding strategy. INTRODUCTION pensis)in South Africa (Skead 1952); various The endangered Hawaiian Crow (Corvus ha- Australian crows and ravens, C. tasmanicus, waiiensis)is the largest member of the order C. rnellori,C. OYYU,and C. bennetti(Rowley Passeriformesin Hawaii and is endemic to the and Vestjens 1973); and Carrion Crow (C. co- island of Hawaii. This medium sized (46 to 66 rone)in Britain (Holyoak 1968). Most of these cm long), brownish-black,large-billed crow was crows, in addition to being generalists,are also once common in forests of the Kona and Kau relatively common in their ranges. Food gen- districts (Perkins 1903). Crow numbers began eralists are not usually rare, becausethey can declining in the early 1900s (Munro 1944) and readily adapt to changingconditions. by the end of our study in 1980, only isolated Becausethe Hawaiian Crow is endangered, populationswere found within its former range we hypothesized that if food were limiting the (Fig. 1). Population estimates range from 60 population, the bird would be a specialistcon- to 70 (Marshall 1975) to an estimate made in centratingon a singleor few food sources.Fruits 1978 of 76 + 18 (Scott et al., in press). Many of the ieie (scientific names are found in Table hypotheses for the decline have been ad- 2) vine may have been the most important vanced. One is that limited food supply has food sourcein the 1800s;adults fed their young disrupted rearing of nestlings and fledglings fruits of ieie, and stomachsof adult crows also (Bank0 1976). contained this fruit (Perkins 1903). In addi- Many speciesof crows are generalists,eating tion, crows fed on carrion, young birds, eggs, various foods as they become available, for and various fruits. Crows have been reported example the Common Crow in central New foraging on fruits of introduced plants (Munro York (Hering 1934); the Black Crow (C. ca- 1944) and- more recently-passerine nest- lings, arthropods, and various fruits and flow- ers (Tomich 197 1, Sakai and Ralph 1980, Gif- fin 1983). ’ Received 29 April 1985. Final acceptance12 Novem- This paper reports a study of the feeding ber 1985. habits of the Hawaiian Crow, to determine 2 Presentaddress of first two authors:USDA Forest Ser- vice, Pacific SouthwestForest and Range Experiment Sta- whether the food supply is limiting the pop- tion, Redwood SciencesLaboratory, 1700 Bayview Drive, ulation. Of particular interest were variety of Arcata, CA 95521. foods, preference for feeding perches, and Pill 212 H. F. SAKAI, C. J. RALPH AND C. D. JENKINS the Ranch in the mid 1800s and domestic cattle in the early 1900s; both are common there today. Compared with other known crow habitats, both study sites are relatively stable and are among the least disturbed crow hab- itats remaining. Both sites are montane rain forests with a yearly average rainfall for the past ten years of about 120 to 140 cm, heaviest from July to October. Ohia is the dominant tree species, followed by koa (Table 1). The understory in the Forest Reserve is denserthan on the Ranch. Common understory plants at both sites in- clude tree fern, Hawaiian raspberry,pilo, ohelo, oha kepau, olapa, kolea, and mamaki. MATERIALS AND METHODS Four pairs of crows were studied in the Forest Reserve from May through June 1978, Janu- ary throughJuly 1979, and March to July 1980; and four pairs at the Ranch from March to July 1979. We divided observations into the nonbreeding period (January to mid-March) FIGURE 1. Former known range as of 1980 of the and the breeding period, including nest build- Hawaiian Crow; and study sites, Honaunau Forest Re- ing through fledging (mid March to mid July). serve and McCandless Ranch in the South Kona District, Island of Hawaii. We observed foraging crows by following them on foot, using binoculars, and tape-re- cording their activity. The length of various changesin feeding strategy between breeding activities was later transcribed using a stop- and nonbreeding periods. watch. We classifiedfeeding substratesas twigs (< 1 cm in diameter), small branches (1 to 5 STUDY SITES cm), medium branches (5 to 15 cm); large We studied the crow on the 4,000-ha Hon- branches(> 15 cm); and trunks (main support aunau Forest Reserve (hereafter, “Forest Re- of a tree, with characteristic bark). Distances serve”) and the 24,000-ha McCandless Ranch of hops and flights were measured by esti- (hereafter, “Ranch”) in the South Kona Dis- mating the total distance traveled to the near- trict (Fig. 1). Both areaswere extensively logged est 0.1 m. When a bird was foraging, we al- for koa from the mid 1920s to the late 1950s lowed 5 min to elapse between observations. (Edward Ueda, pers. comm.). The Forest Re- A nonforagingbird was usually observed every serve was grazed by feral cattle until 1959 10 min. We used 1,53 7 observationsof at least (Bryan 1977) but has since remained essen- 10 set, averaging 4 1 sec. tially ungrazed. Wild cattle were introduced to BecauseHawaiian Crows seldom regurgitate TABLE 1. Density, frequency, and height of 12 common tree speciesfound in Honaunau Forest Reserve, Island of Hawaii. <3 m high ~3-15 m high cl.5 cm DBH 27.5 cm DBH >15 m high AVerage Density Frequency Density Frequency Density Frequency height Species (trees/ha) (W (trees/ha) (W (trees/ha) w Cm) Ohia 109.4 35.4 83.7 36.8 95.4 100.0 18.6 Koa 4.5 4.5 3.0 13.7 26.1 19.0 Kolea 24.7 14.8 2z.g 19.9 2.7 7.9 8.0 Kawau 69.9 43.3 56:4 37.4 3.8 13.3 7.0 Pi10 51.4 25.6 66.3 41.1 0.8 3.1 7.0 Manono 23.7 18.1 6.6 0 5.0 Olapa 36.8 43.5 8;:; 40.1 0 : 6.0 5.0 KopikoMamaki 30.82.7 24.12.7 18.54.9 18.54.9 : : - Alani 5.9 5.8 8.5 6.4 0 0 6.0 Oha kepau 19.0 12.6 2.2 2.2 4.0 Hoawa 8.1 7.2 - - : 7.0 HAWAIIAN CROW 213 TABLE 2. Percent’ of plant species and food items on which Hawaiian Crows were observed foraging, between January and July. Substrate foraged upon Med. Sm. branch, Plant Flower Flower branch, lg. branch, Bird Bird Plant species Fruit P=t P=t nectar and twg and trunk nestbng a Akala (Rubushawaiiensis) 1.7 - - - - - - - Aiea (Nothocestrumlongtfolium) 0.1 - - - - - - Alani (Pelea spp.) 0.4 - - - - 0; - - German ivy (Seneciomikanoides) - 0.1 - - - - - - Hai wale (Cyrtandra spp.) 0.1 - - - - - - - Hoawa (Pittosporumhosmerz) 0.4 - - - - - - - Hoi kuahiwi (Smihzx spp.) 0.1 - - - - - - Ieie (Freycinetiaarborea) 0.1 0:1 - - - - - Kanawao (Broussaisiaarguta) 0.1 - - - - - 0.1 - Kawau (Zlex anomala) 0.3 - - - 0.7 2.1 - - Koa (Acacia koa) 0.1 - 0.1 - 4.7 10.6 1.8 - Kolea (Myrsine lessertiana) 0.7 - 0.1 - 0.4 0.6 0.4 0.5 Kopiko (Psychotriaspp.) 0.7 - - - 0.3 - - Mamaki (Pipturusspp.) 1.3 - - 0.1 0”:: - - Mamane (Sophora chrysophylla) 0.1 - 0:1 - - 0.1 - Manono (Gouldia terminalis) 0.5 - - - 0.2 - 0.1 - Ma oi oi (Stenogynespp.) 0.1 - - - - - - - Naio (Myoporum sandwicense) 0.1 - - - - 0.2 - - Oha kepau (Clermontia spp.) 7.7 - - 0.1 0.4 - - - Oha (Cyanea spp.) - - - - - Ohe (Tetraplasandra meiandra) 0”:: - - - or1 0: - - Ohelo (Vaccinium spp.) 0.5 - - - 0.1 - Ohia (Metrosideroscollina) - - 0.1 6.7 9.4 22.8 3.2 0.3 Olapa (Cheirodendrontrigynum) 12.6 - - - 0.6 0.7 - - Olomea (Perrottetiasandwicensis) 0.2 - - - - - - - Painiu (Astelia menziesiana) - 0.1 - - - - - Pi10 (Coprosmaspp.) 1.4 - - - 0.2 0.1 0.1 - Poha (Physalisperuviana) 0.1 - - - - - - - Pukiawe (Styphelia spp.) 1.1 - - - - - - Purple poka (Passifloraedulis) - - - OYl - - - - Southern pokeberry (Phytolaccaoctandra) 0.1 - - - - - - - Thimbleberry (Rubusrosaefolius) 0.2 - - - - - Tree fern (Cibotium glaucum) - 0:1 - - - 0.2 0.1 - ’ Calculated as: 100 x number of observations + total observations. pellets and rarely roost communally like some south grid line, at right anglesto the line. The other corvids, we analyzed adult and nestling distancesof the stationsfrom the grid line were crow droppings to assesstheir food habits determined using randomized numbers up to (Ralph et al.
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