The Condor 100:654-662 01 he‘ Cooper Omitholog~cal Society 1998

BREEDING ECOLOGY AND BEHAVIOR OF THE HAWAIIAN HAWK ’

CURTICE R. GRIFFIN*, PETER W. C. PATON~ AND THOMAS S. BASKEM Schoolof Forestry, Fisheries, and Wildlife, Universityof Missouri-Columbia,Columbia, MO 65211, e-mail: [email protected]

Abstract. We studied the ecology of the endangeredHawaiian Hawk ( solitarius) Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 on the island of for three breeding seasons.Their breeding strategy is a prime example of a K-selected speciescharacteristic of many in tropical environments:clutch size was one and brood-rearing was among the longest reported for any diurnal raptor. Twenty-eight nests were found in a variety of native and exotic habitats. Incubation lasted 38 days, nestlingsfledged after 59-63 days, and parentscared for fledglings for an average of 30.2 weeks, which was 2.5 to 10 times longer than similar-size temperate zone raptors. Males assisted females with incubation, but only females brooded young. Radio-tagged juveniles remained within 0.63 km of their nests for the first two months after fledging, after which dispersal distances expanded gradually. Avian (45%) and mammalian (54%) prey dominated the diet of nestlings. There was no evidence that avian malaria, introduced predators, or environmental contaminantswere affecting their population. Based upon es- timates of population size, the availability of suitable nesting habitat, and reproductivesuc- cess, we suggestthe speciesbe consideredfor downlisting from endangeredto threatened status. Key words: breeding ecology,Buteo solitarius, clutch size, diet, Hawaiian Hawk, nest success.

INTRODUCTION (Bank0 1980, Scott et al. 1986). The Hawaiian The Hawaiian Hawk or ‘IO (Buteo soliturius) is Hawk is among the most sexually dimorphic of a medium-sized, broad-winged buteonine en- the world’s buteonine raptors (Paton et al. 1994). demic to the Hawaiian Islands. Although this Objectives of the present study were to quantify hawk was believed to be threatened with extinc- nesting habitat, breeding behavior and chronol- tion and included on the Endangered ogy, nest success, diet, and other factors affect- List in 1967, little was known about its popu- ing reproduction. lation status or breeding ecology at the time of METHODS the listing decision. Information about the dis- tribution, abundance, and biology of the Hawai- STUDY AREA ian Hawk is anecdotal (Bank0 1980), with the Our fieldwork was conducted on the island of exception of systematic surveys by Scott et al. Hawaii, the largest island in the Archipelago, (1986) and Hall et al. (1997). of Hawai- 10,456 km2. It is formed by the major volcanoes ian Hawks have been found on Hawaii, Molo- of Kilauea, Mauna Loa, , Hualalai, kai, and (Olson and James 1997), and and Kohala, and comprises about 62% of the there have been eight documented observations land area of the Hawaiian Islands (Armstrong of the species on the islands of Kauai, , and 1973). Although fieldwork took place through- since 1778 (Bank0 1980, Olson 1990). out the island, observations of nests and telem- Since ornithologists have studied the Hawaiian etry studies were concentrated in the northeast- Islands, Hawaiian Hawks have nested only on ern side of the island (Griffin 1985). the island of Hawaii from sea level to 2,600 m Hawaii Island has the greatest diversity of biogeoclimatic zones in the Archipelago because 1Received 5 August 1997. Accepted 13 May 1998. of its large size and recent origin (Armstrong 2 Current address:Department of Forestry and Wild- 1973). As is typical of all Hawaiian islands, life Management and Giaduate Program in-organismic much of the native biota is endemic. However, and Evolutionarv Biology, Universitv of Massachu- most of these indigenous ecosystems have been setts, Holdswor& Hall, b;;nherst,MA-01003. 3 Current address:Department of Natural Resources modified by competition from introduced plants, Science, University of Rhode Island, Kingston, RI overgrazing by introduced ungulates, logging 02881. operations, and urban and agricultural develop-

[6541 HAWAIIAN HAWK BREEDING ECOLOGY 655 ment. Agriculture dominated much of Hawaii Is- viva1 rates (Mayfield 1975), with confidence in- land below 700 m elevation. In 1983, 435 km* tervals developed by Johnson (1979). To calcu- were in sugarcane,66 km2 were in diverse crops late survival probabilities for incubation and (primarily macadamia nuts, papaya and other nestling periods, we used estimatesof 38 and 61 fruit crops, and coffee), and 3,223 km2 were in days, respectively (Griffin 1985). Young hawks pasture (Hawaii Department of Agriculture, Ho- were recorded as fledging successfully only if nolulu, unpubl. data). they were observed after leaving the nest. All intensive observationsof hawks were con- Activities at five nestswere monitored for 696 ducted at low and mid elevations (70-1,740 m) hr from blinds in 1980 and 1981; all nestsexcept Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 where the landscapeis dominated by agricultural the nest at Puu 0’0 Ranch in 1981 (35 hr) were lands, exotic vegetation, and disturbed native watched for more than 120 hr, with observations forests. Hawks were not studied in undisturbed spaced evenly throughout the breeding season. native forests because of difficulty of access. Three nests were monitored in each of the breed- Five nests were observed from blinds including ing seasons, whereas one nest, Puu 0’0, was two in tall ohia () for- observed in both years. Observations of nesting ests with mixed native and exotic understory; activities were made from tree and ground one in an agricultural area within a tree windrow blinds with a spotting scope (15-60X). Blinds separating guava and papaya orchards, and two were located lo-30 m from nests. We attempted in disturbed pasture grasslands with scattered to watch each nest the equivalent of 1 day per large koa (Acacia koa) and ohia trees. week for 8-10 weeks. In 1980, we observed nests in consecutive half-day sessions (i.e., an FIELD PROCEDURES afternoon and the morning the next day, 6-7 hr Fieldwork was conducted 5-6 days per week for per session),whereas in 1981 each nest was ob- 19 months from April 1980 through October served an entire day each week from 05:30 to 1981. Additional fieldwork from October 1981 19:00 with three observersrotating duties every through July 1982 averaged one day per week, 4.5 hr. We attempted to identify prey items to and involved checking nests for activity. species when possible. However, we were un- To find active nests, we first relocated 8 of 11 able to distinguish the three potential (Rattus historic nests found from 1973 to 1979 by a spp.), and prey was sometimes brought to the number of researchers(Griffin 1985). Island res- nest partially consumed making identification idents also showed us several nests after reading difficult. Terminology used to describe courtship about our study in the local newspaper.We also behavior followed Brown and Amadon (1968). systematically searched for new nest territories To monitor hawk behavior, we captured 37 by driving roads in the Puna, Hilo, and North hawks (17 adults, 13 immatures, and 7 juve- Hilo districts of the island. During these road niles) with bal-chatri traps baited with black rats surveys, we stoppedthe vehicle at 1.0~km inter- (Rattus rattus) and 3.6 kg-test monofilament vals and attempted to attract territorial hawks nooses (Berger and Mueller 1959). All captured with playback of an amplified, territorial Ha- birds were banded with U.S. Fish and Wildlife waiian Hawk call over a portable bullhorn for Service metal bands and individually color- 5-10 min (Balding and Dibble 1984). Potential banded with l-3 numbered plastic bands (Na- hawk territories were discovered by watching tional Band and Tag Co., Newport, Kentucky). for soaring hawks and listening for their vocal- Young were banded 3-4 weeks after hatching to izations. For these reasons, 36% of nests found minimize disturbance. were situated within 0.5 km of primary or sec- We radio-tagged three juveniles from three ondary roads. territories in August 1980 to monitor dispersal. During the 1980 breeding season, we found Tail-mounted tags (9 g) were sutured to a single most nests in the mid- to late-nestling stage. In rectrix. Receivers (AVM Instrument Co., Cham- 1981, eight nests were checked prior to egg-lay- paign, Illinois) were used with a 4-element, yagi ing and three were first checked during incuba- null-peak antenna system mounted in a vehicle. tion. In 1982, most nests were checked only for Transmitters operatedin the 150-15 1 MHz band number of young produced; therefore 1982 data and each was identifiable with a unique frequen- were not included in nest successestimates. cy. We attempted to monitor each radio-tagged Nest successwas calculated using daily sur- for at least 8 hr per week, with observation 656 C. R. GRIFFIN, P. W. C. PATON AND T. S. BASKETI periods evenly distributed throughout daylight TABLE 1. Physical characteristicsof Hawaiian Hawk hours. All radio-tagged birds occurred in areas nests on island of Hawaii. with well-developed road systems, so we could Variable i + SE (n) track hawks from a vehicle. In addition, observ- ers were often able to confirm locations by vi- Nest height (m) 9.3 2 0.8 (23) Nest tree height (m) 15.8 ? 0.9 (23) sual sightings. Nest tree dbh (cm) 50.3 2 6.4 (23) For all measurements,we present ff * SE. We Nest diameter (cm) 64.7 2 2.9 (21) used a Mann-Whitney U-test to compare varia- Nest depth (cm) 31.5 2 4.0 (20) tion in breeding chronology between years and Nest cup diameter (cm) 23.9 ? 1.7 (9) Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 as a function of territory elevation, and the log- Nest cup depth (cm) 4.2 ? 0.6 (9) likelihood ratio test (G-test) to determine what affect prior nest successhad on the a bird’s de- cision whether or not to breed in subsequent height, and nests were 3.5-18 m above the years. All statistical analyseswere completed us- ground (Table 1). Most nest trees were fairly ing SAS (1990). Alpha levels of 0.05 were con- large, averaging 50 cm diameter at breast height sidered significant. (dbh), but some were as small as 10.2 cm dbh. Hawaiian Hawks apparently preferred stable RESULTS platforms to construct nests; nine nests were COURTSHIP AND TERRITORIAL DISPLAYS placed on top of bird’s_nest ferns (Asplenium ni- Aerial displays were recorded in all months ex- dus), five in trunk crotches, three on large to cept August and October, and most frequently medium-sized branches (20-50 cm diameter), prior to the breeding seasonin April (27%) and and four on small branches (< 20 cm diameter) May (23%) (n = 26). Males typically performed at their juncture with the trunk. Fifteen nests aerial displays more intensely than females, al- were in native ohia or ohialkoa forests, 8 in for- though females often participated (n = 17). Pairs ests dominated by introduced trees, and 5 were performed mutual soaring, diving, and foot- in pastures. touching. Undulating displays sometimes fol- Nests appeared to be used for several years, lowed mutual soaring displays. Both males and with nesting material added each breeding sea- females performed undulating displays, both in son. Nests averaged over 0.6 m diameter and 0.3 unison (n = 2) and while solitary (n = 5). Males m deep (Table 1). All nests contained nest cups also performed steep downward plunges for lo- that were lined with green leaves. 30 m (n = ll), which Brown and Amadon (1968) referred to as the “Pot-hooks Display.” NESTING CHRONOLOGY AND FREQUENCY OF BREEDING NEST BUILDING AND MAINTENANCE Egg laying occurred from March to June and Nest building was initiated at least two months peaked in late April to early May (n = 20 before egg laying, and adults continued to add clutches, Fig. 1). There was no difference be- nest material during the nestling period. Our ear- tween laying dates in 1980 and 1981 (U,,, 8 = liest record of nest building occurred on 24 Jan- 54, P > O.OS),or between low- (< 1,500 m) and uary. Both sexes frequently added materials to mid-elevation (> 1,500 m) territories (U,,, = 11, old nest structures,and sometimes to more than P > 0.05). Hatching occurred from May to July one nest within the same territory. During the and peaked from late May to late June, and first 3-4 weeks of the nestling period, females fledging took place from early July to mid-Sep- often added green-leafed twigs to the nest cup tember, peaking in mid-August (Fig. 1). (n = 39). Although breeding territories were occupied every breeding season, pairs did not attempt to NEST SITE HABITAT CHARACTERISTICS nest every year. Of six territories that were fol- Twenty-eight nests were found at elevations lowed during three breeding seasons,two pairs from 30-1,740 m in six tree species: 17 nests in laid eggs at the same site all three years; three Ohia, 5 in lama (Diospyros ferra), 3 in koa, and pairs apparently did not attempt to nest in one 1 each in ironwood (Casuarian spp.), mehame year; and one pair nested in only 1980. Another (Antidesma platyphylla), and eucalyptus (Euca- eight pairs were tracked for two breeding sea- lyptus robusta). Nest trees were lo-24 m in sons, of which three pairs attempted to nest both HAWAIIAN HAWK BREEDING ECOLOGY 657

TABLE 2. Numberof light and darkmorph nestlings producedby HawaiianHawk parentsof known color morph.

Adult color morph Number of nestlings Number Male Female of pairs Light Dark Light Light 6 6 0 Light Dark 5 2 3

D&k Light 7 0 7 Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 Dark Dark 1 0 1

broken egg was found below a nest containing a single egg, but the latter may have been a re- placement egg, rather than a two-egg clutch. Mean egg size was 57.0 + 2.7 X 44.0 2 1.2 mm (n = 9), and weighed 56.0 ? 5.7 g (n = 5) for 3-4-week-old eggs. This weight may be slightly lower than newly laid eggs (e.g., a 3- MONTH day-old egg weighed 61.9 g; L. Yoshina, pers. FIGURE 1. Breedingchronology of the Hawaiian comm.). Hawk on the islandof Hawaii. Eggs were abandoned at four nests from 1980-1982, but birds re-nested at only one of these. In this territory, the female laid a replace- years, and five pairs attempted to nest in only ment clutch within 24 days at an alternative nest one year. Nest success the previous breeding site following nest abandonment. This pair left seasondid not affect the probability that hawks the original nest because a logging operation would re-nest at the same site (G, = 1.3, P > cleared much of the habitat in the area. No re- 0.20). nesting was detected at two nests where chicks I 3 weeks old died after falling from the nest. MATE SELECTION Replacement clutches by two captive hawk pairs Hawaiian Hawks have both light and dark color were laid 20 days and 26 days after their first l- morphs. Of 41 pairs we observed, 8 were dark- egg clutches were collected for artificial incu- dark pairs, 11 were light-light pairs, and 22 were bation (L. Yoshina, pers. comm.). light-dark pairs. There was no evidence of as- sortative mating with respect to color morph. INCUBATION, NESTLING, AND Assuming random pairings with respect to color POST-FLEDGLING PERIODS morph, we expected 8.7 dark-dark pairs, 11.9 Incubation lasted approximately 38 days (n = 3 light-light pairs, and 20.4 light-dark pairs, which nests); this estimate was imprecise becauseexact did not differ from our observations (G2 = 0.1, laying dates were uncertain. Their nestling pe- P = 0.9). Frequencies of dark and light offspring riod, the time from hatching to leaving the nest, resulting from different color morph pairings by was 59-63 days in duration (n = 10 nests). adults strongly suggest that the dark morph is Adults were observed delivering prey to juve- produced by a dominant allele for melanism that niles for 25-37 weeks after fledging (.%= 30.3 is not sex linked (Table 2). weeks, n = 4). The minimum period before juveniles dis- CLUTCH SIZE AND EGG CHARACTERISTICS persed from their natal territories ranged from There was no indication that clutch size was 29 to 76 weeks (f = 55.3 + 10.9 weeks, n = more than one egg during our study. Clutch size 4). We were able to radio-track two juveniles was always one for nests we found during the from late August 1980 to early June 1981, and incubation period (n = 10). Of all 30 nesting a third juvenile from 25 August to 23 October attempts recorded during the 3 nesting seasons, 1980 and from 27 February to 16 March 1981. we never saw more than one chick in a nest. A All radio-tagged juveniles remained very close 658 C. R. GRIFFIN, P. W. C. PATON AND T. S. BASKETT

k - - -. Royal Gardens 3.5 - I .x - Thortons’ --c- *-4‘ \ ______Malama-Ki I I *‘

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o- J, 1 1 2 3 4 5 6 7 8 9 10 11 Month FIGURE 2. Dispersal distancesof three radio-tagged,juvenile Hawaiian Hawks in the monthsfollowing fledg- ing. to natal areas during the first two months after in territories in habitat too high to have mos- fledging; none moved farther than 0.63 km from quitoes (Culex quinquefusciatus)(16 of 23 nest the nest (Fig. 2). By the third month and fifth attempts at mid-elevation [ 1,690 to 1,740 m] month after fledging at the Malama-Ki and Roy- were successful)compared to territories that po- al Gardens nests, respectively, the juveniles tentially had large numbers of mosquitoes (4 of were ranging widely within the territories of 7 low elevation [70 to 402 m] nest attemptswere their parents. Both of these juveniles dispersed successful),and found no difference (G, = 0.31, out of their parents’ territories in the sixth month P = 0.55). In addition, we found no difference after fledging. Although the juvenile female at in nest successof hawks with territories in na- Royal Gardens moved as far as 3.7 km from her tive forests (10 of 13 attempts were successful) natal area 6 months after fledging, she was con- compared to sites dominated by exotic, intro- tinually resighted back at the nest begging for duced vegetation (11 of 17 attempts were suc- food for 11 months after fledging. In contrast, cessful; G, = 0.51, P = 0.47). the male juvenile from Malama-Ki dispersed Forty percent of observed nest failures oc- about 2 km from his natal area 6 months after curred when adults abandoned eggs (n = 10). fledging and did not return to his natal area dur- Three of these abandoned eggs contained em- ing the subsequent 4 months that we tracked bryos at advanced stages of development, so him. reasonsfor their abandonment are unclear. How- ever, one egg was infertile. Four other nest fail- NEST SUCCESS ures occurred when 2-3-week-old chicks fell There was no difference in hatching successbe- from nests. tween 1980 and 1981 (two-tailed z-test = 0.39, P = 0.70). Daily survival estimates for eggs in TERRITORY AND MATE FIDELITY the incubation period were 0.991 2 0.009 in 1980 and 0.986 -t 0.009 in 1981. Similarly, Six of eight hawk territories were monitored there was no difference between 1980 and 1981 closely during both the 1980 and 1981 breeding in fledging successestimates (z = 0.71, P = seasonsto determine territory and mate fidelity. 0.48). Nestling daily survival rates were 0.989 At five of these six territories, adult hawks ? 0.008 in 1980 and 0.995 + 0.004 in 1981. paired with the same mate and occupied the In order to examine whether avian malaria af- same territory in both years. The female at the fected chick survival, we compared nest success sixth nest occupied the same territory, but paired HAWAIIAN HAWK BREEDING ECOLOGY 659 with a different male in each of the two breeding cinity of nests. Females generally did not begin seasons. feeding chicks until males left the nest areas. Food caching by females was observed twice. ADULT NEST ATTENTIVENESS AND FEEDING OF YOUNG NESTLING DIET Eggs were incubated about 98% of the time dur- We observed 415 prey items being fed to nest- ing the incubation stage. Females usually con- lings, of which we were able to identify 72.2%. ducted most of the incubation duties, but males Birds (45%) and (54%) accountedfor sometimes assisted. The male at the Keauhou most identified prey taken to nestlings (n = 300 Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 nest incubated SO-77% of the time. However, prey). Adults brought in 13 species of birds, at males rarely incubated in the week before hatch- least 3 speciesof mammals, and 1 invertebrate: ing. 1 Chukar (Akctoris chukur), 2 Japanese Quail During incubation, exchanges occurred with- (Corumix juponica), 2 Wild Turkey (Melegris out inter-sexual aggression.Males with prey ini- gdupuvo) chicks, 1 Pacific Golden Plover (PZu- tiated the changeover by either calling from a viulis j&Zvu), 4 Spotted Doves (Streptopeliu chi- perch within sight of the nest or perching on the nesis), 1 Eurasian Skylark (Aluudu urvensis), 3 nest itself. Females usually took the prey and Melodious Laughing-Thrush (Gurrulux cuno- flew away from the nest to feed. Once the fe- rus), 24 Common Mynah (Acridotheres tristis), male left the nest, the male typically initiated 49 JapaneseWhite-eye (Zosterops juponicus), 2 incubation and remained on the nest until the Northern Cardinal (Cardinalis cardinalis), 10 female returned. House Finch (Curpoducus mexicanus), 1 Hawaii Once eggs hatched, most males did not brood Amakihi (Hemignuthus virens), 3 Apapane nestlings and their visits were limited to food (Himutione singuinea), 32 unidentified birds, 87 deliveries. Females spent 92% of their time rats (Rattus spp.), 66 house mice (Mus muscu- brooding during the first week after hatching. By Zus), 5 small Indian mongoose (Herpestes uu- the second week, females brooded much less ropunctutus), 4 unidentified mammals, and 3 (33% of the time), and brooding activity greatly cockroaches(Bluttodeu). Interestingly, their diet declined thereafter. After the second week post- was dominated by introduced species,with only hatching, female attendancewas increasingly re- three native species of birds observed (Pacific stricted to feeding chicks and shielding them Golden Plover, Hawaii Amakihi, and Apapane). from rain showers. Chicks were protected from rain and brooded overnight through the fourth DISCUSSION week. Although females left chicks unattended The parental care period for the Hawaiian Hawk while they hunted during weeks 2-5, they typi- is among the longest recorded for a buteonine cally remained within sight of nests during this species (Newton 1979). It rivals those of some period. At week 5, females began providing in- tropical whose weight is more than seven creasingly greater amounts of prey at nests,leav- times greater than an adult Hawaiian Hawk ing young unattended for several hours at a time. (Mader 1982). Among 11 species of temperate By week 7, young were left unattended 97% of , incubation averages 33.7 days in dura- the time. tion, fledging takes 43.3 days, and fledged young During the nestling period, prey exchanges receive parental care for 5-12 weeks (data from between adults always occurred at the nest. If Newton 1979). In contrast, tropical buteos take the female was not on the nest when the male much longer to rear their young (incubation = arrived, she flew immediately to it to take the 35.5 days, fledging = 49.1 days, and the parental prey from the male with her bill or talons. Early care period after fledging lasts 8-28 weeks; in the nestling period (weeks l-2) both adults Newton 1979, Santana C. and Temple 1988). often gave soft chirp calls after prey exchanges, The incubation period of the Hawaiian Hawk with the male usually remaining for 15-60 sec. (38 days) is similar to that of other tropical bu- If he remained at the nest much longer or teos, but the nestling period (61 days) and pa- perched within sight of the female, she would rental care period following fledging (25-37 give an aggressive tchew call until he left. As weeks) are longer than those reported for any the nestling period progressed,females appeared other buteos (Newton 1979), with the possible increasingly aggressivetowards males in the vi- exception of the White-tailed Hawk (B. ulbicau- 660 C. R. GRIFFIN. P. W. C. PATON AND T. S. BASKETT

&us) (Mader 1981, 1982). This long parental In 22 historical records of Hawaiian Hawk care period may be due in part to their depen- clutch size or numbers of young in nests from dence on avian prey, which are difficult to cap- 1888 through 1979, 13 nests contained 1 egg or ture. Hawaiian Hawks evolved in an environ- young, 5 nests had 2 young, 3 nests had 3 ment where only avian prey were available be- young, and 1 nest contained “multiple” young cause mammals were introduced by humans to (see Griffin 1985 for summary of these records). the islands, starting 1500 years ago (Berger The discrepancy between frequencies of multi- 1981). The parental care period of two other ple young present in historic records (41%) and tropical raptors that depend on avian prey, the our study (O%, II = 30) is significant (G, = 18.1, Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 Puerto Rican Sharp-shinned Hawk (Accipiter P < O.OOl), although the reasonsfor this differ- striatus venator) and Red-tailed Hawk (Buteo ence are unknown. One possible explanation is jumuicensis), also is significantly longer than that prey was more abundant historically, which that of temperate-zone conspecifics (Delannoy may have promoted larger clutch sizes (Newton and Cruz 1988, Santana C. and Temple 1988). 1979). The apparent year-round residency of Hawai- Re-nesting after loss of eggs or hatchedyoung ian Hawks on territories is conducive to long- is common among accipiters and falcons. How- term pair bonds. Pair fidelity appearsto be high ever, re-nesting is rare among hawks and eagles among Hawaiian Hawks, 5 of 6 (83%) pairs (Newton 1979), except for Harris’ Hawk (Pur- were intact after one year, and similar to other ubuteo unicinctus) and the in tropical birds of prey. For example, in Savanna Venezuela (Mader 1982). Re-nesting also was Hawk (Buteogullus meridionalis) populations, rare among Hawaiian Hawks; so it does not ap- 64% of pairs were intact after a year (n = 11) pear to be a universal adaptation of tropical (Mader 1982). In Hawaii, there is probably little hawks. opportunity to choose a new territory or new We found no evidence of assortative mating mate. Vacancies in breeding territories probably among the polymorphic Hawaiian Hawk, in con- occur infrequently, and the necessary strategy trast to Dunkle’s (1977) research with Swain- for a newcomer would be to establish itself in a son’s Hawks (Buteo swuinsoni). However, territory quickly after the death of an owner. Schmutz and Schmutz (1981) documented ran- Thus, a successful bird would simultaneously dom mating with respect to color morph among acquire both a territory and mate (Newton Ferruginous Hawks (Buteo regalis); they also 1979). Insular Galapagos Hawks (Buteo gulu- suggestedthat one gene determines color morph. pugoensis) have a similar strategy; they remain Both color morphs of the Hawaiian Hawk are in their territories for life (Faaborg 1986), as do widespread acrossthe island (Scott et al. 1986). resident Savanna Hawks in Venezuela (Mader This study was the first attempt to quantify 1982). the breeding ecology and behavior of the endan- In many African raptors, pairs breed only 2 out of 3 years because there is a tendency for gered Hawaiian Hawk. It is therefore important pairs which successfully breed to not nest the to discussfactors that may be affecting the spe- next year (Brown 1970). Hawaiian Hawks ex- cies’ population viability. Breeding and foraging hibit a similar breeding strategy, given that only successdiffers between habitats for many rap- 61.5% of pairs that nested successfully attempt- tors (Newton 1979, Preston 1990). We found no ed to nest the subsequent year (n = 13). The relationship between nest success and habitat, extended postfledging juvenile dependency may although we made only limited observation of promote this sporadic breeding cycle. Interest- hawks breeding in undisturbed native habitats. ingly, pairs with unsuccessful nesting attempts Much of the lowland agricultural areas on the also had a low probability (33%, 12 = 6) of at- island of Hawaii are now unsuitable for Hawai- tempting to nest in the same territory the sub- ian Hawks (Griffin 1985, Scott et al. 1986) sequent year, which suggests a relationship to which only nest in trees. Hence, the extensive territory quality. Newton (1979) reported that sugarcanefields and pasturesfound on the island territory fidelity was greatest for female Euro- have limited value because of the absence of pean Sparrowhawks (Accipiter nisus) in habitats trees. Extensive clearing of lowland forests by rich in food, but they often changed territories Polynesian and European immigrants (Kirch in habitats poor in food. 1982), especially for agriculture, has reduced the HAWAIIAN HAWK BREEDING ECOLOGY 661 quantity of nesting habitat available to hawks in perimentally fed rats killed with fumarin (Men- Hawaii’s forests. denhall and Pank 1980). Endemic passerinesof Hawaii are vulnerable Several potential predators of Hawaiian to avian malaria (Plasmodium relictum capistra- Hawks or their eggs have been introduced to Ha- noae), whose vector is the introduced mosquito waii, including the domestic cat (Felis catus), (Culex quinquefasciatus;Warner 1968, van Rip- Polynesian (Rattus exulans), black rat, Nor- er and van Riper 1985, van Riper et al. 1986). way rat (R. norvegicus), and small Indian mon- Populations of Culex are lower above 1,500 m goose (Scott et al. 1986). Although several of than at lower elevations, and there is a lower these speciesare known to prey on small native Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 incidence of malaria in bird populations above forest birds or seabirdsin Hawaii (Berger 1981), 1,500 m. Although young birds are not prefer- there is no evidence that they are adversely af- entially infected with malaria, they often do fecting the Hawaiian Hawk population. In fact, have much higher parasite levels and suffer Hawaiian Hawks now prey on many of these higher mortality than adults (van Riper et al. introduced predators. 1986). Griffin (1985) analyzed 75 blood samples The Hawaiian Hawk’s population appears to taken from 32 Hawaiian Hawks and no hema- be in a more secure position than previously tozoa were found, indicating the species has thought. They are widely distributed over the is- very high immunogenetic capabilities for avian land, occurring in a broad array of habitat types malaria (C. van Riper, pers. comm.). ranging from macadamia orchardsto all types of Avian pox, caused by strains of Poxvirus av- forests on the island (Griffin 1985, Scott et al. ium, also has been suspectedas a factor in de- 1986, Hall et al. 1997). They are absent only clines of native Hawaiian birds (Warner 1968, from areasthat lack trees (Scott et al. 1986). The van Riper and van Riper 1985, Jenkins et al. species has relatively high breeding success,in 1989). This virus is transmitted directly by con- part because of the prevalence of introduced tact with an infected organism, or via a vector birds and mammals in the hawk’s diet. In con- such as a mosquito (Cavil1 1982). Van Riper and trast to many native birds on the island, the van Riper (1985) diagnosed pox virus in pox- hawk is widely distributed in low elevation hab- like lesions in nine species of native and intro- itats where avian malaria adversely affects most duced Hawaiian passerines. Griffin (1985) ob- species. In addition, there is an absence of en- served pox-like lesions on 2 of 43 captured Ha- vironmental contaminants overtly affecting the waiian Hawks; however, no bacteriological or Hawaiian Hawk. Current population estimates virological samples were collected, and hence range from 1,600 (Hall et al. 1997) to 2,700 these lesions were not confirmed as avian pox. hawks (Griffin 1989) residing on the island of Although organochlorine compounds have Hawaii. This population estimate, coupled with had devastating effects on raptor populations in the information presentedin this study, indicates many parts of the world, there is limited use of that reclassification from endangered to threat- these compounds in Hawaii. None or only trace ened statusmay be warranted (see also Scott et amounts of organochlorine compounds were al. 1986, Hall et al. 1997). However, continued found in one Hawaiian Hawk egg collected in monitoring and the implementation of conser- 1969, and three Hawaiian Hawk eggs and one vation measures to insure the existence of Ha- chick carcasscollected during 1980-1981 (Grif- waiian Hawk nesting and foraging habitat fin 1985). should be pursued before de-listing is consid- Potential for secondary poisoning of Hawai- ered. ian Hawks by rodenticides in Hawaii is believed to be low. Fumarin and zinc phosphidewere the ACKNOWLEDGMENTS primary rodenticides used in Hawaii (D. I? Fel- We are grateful to all who assistedor collaboratedon lows, pers. comm.). There is no direct evidence this project: J. M. Scott, L. E Pank, and D. I? Fellows, of secondary poisoning of Hawaiian Hawks formerly of the U.S. Fish and Wildlife Service; C. J. from these rodenticides, which are used in Ha- Ralph, U.S. Forest Service (USFS); and C. van Riper, waiian macadamia nut orchards and sugarcane formerly of the National Park Service. T A. Burr and M. Morin, formerly of Hawaii Departmentof Land and fields. Effects of anticoagulants on raptors in the Natural Resources,arranged for study permits. Staff of field remain to be assessed;however, no abnor- Hawaii Volcanoes National Park, especially D. Ames malities occurred in Barn Owls (Tyto a&a) ex- and E Kualani, arrangedfor housing and materials.L. \ 662 C. R. GRIFFIN, P. W. C. PATON AND T. S. BASKETT

Pau and E C. Parks assistedin equipmentconstruction. Population status of the endangered Hawaiian R. Banashek,former Young Adult ConservationCorps Hawk. J. Raptor Res. 31:11-15. Director, provided volunteer field assistants.The Pa- JENKINS,C. D., S. A. TEMPLE,C. VAN ROPERIII, AND tuxent Wildlife ResearchCenter and National Fish and W. HANSEN.1989. Disease-relatedaspects of con- Wildlife Health Laboratory provided chemical analy- serving the endangeredHawaiian Crow, p. 77-87. ses and necropsy reports on a carcassand eggs col- In J. E. Cooper [ed.], Disease and threatened lected in Hawaii. J. D. Jacobi,U.S. Geological Survey, birds. Int. Council Bird Preserv. Tech. Publ. 10. Biological ResourcesDivision, Hawaii Field Research JOHNSON,D. H. 1979. Estimating nest success:the Station provided valuable assistance in vegetation Mayfield method and an alternative. Auk 96:65l- analvsesaround hawk nests, L. H. Yoshina (Director, 661. Panaewa Zoo, Hilo, Hawaii) provided information on K~RCH,P. V 1982. The impact of prehistoric Polyne- Downloaded from https://academic.oup.com/condor/article/100/4/654/5126040 by guest on 30 September 2021 captive Hawaiian Hawks, and 3. A. Baldwin (USFS) sians on the Hawaiian ecosystem.Pac. Sci. 36:1- assisted with statistical analysis of data. S. J. Doyle 14. assistedwith field research during 1981-1982. D.-G. MADER,W. J. 1981. Notes on nesting raptors in the Newman provided editorial assistance,and S. S. Clark llanos of Venezuela. Condor 83:48-51. gave technical and secretarialsupport. This publication MADER,W. J. 1982. Ecology and breeding habits of is a contribution from the Missouri Cooperative Fish the Savanna Hawk in the llanos of Venezuela. and Wildlife ResearchUnit and the Missouri Agricul- Condor 84:261-271. tural Experiment Station. MAYFIELD,H. E 1975. Suggestions for calculating nest success.Wilson Bull. 87:456-466. LITERATURE CITED MENDENHALL,V. M., AND L. E PANK. 1980. Secondary poisoning of owls by anticoagulantrodenticides. 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