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1998 Breeding Ecology and Behavior of the Curtis R. Griffin

Peter W. C. Paton University of Rhode Island, [email protected]

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Citation/Publisher Attribution Griffin,., C Peter W. C. Paton, & Baskett, T. (1998). Breeding Ecology and Behavior of the Hawaiian Hawk. The Condor, 100(4), 654-662. doi:10.2307/1369746 Available at: http://dx.doi.org/10.2307/1369746

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Breeding Ecology and Behavior of the Hawaiian Hawk Author(s): Curtice R. Griffin, Peter W. C. Paton and Thomas S. Baskett Reviewed work(s): Source: The Condor, Vol. 100, No. 4 (Nov., 1998), pp. 654-662 Published by: University of California Press on behalf of the Cooper Ornithological Society Stable URL: http://www.jstor.org/stable/1369746 . Accessed: 16/07/2012 15:11

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http://www.jstor.org The Condor 100:654-662 ? The Cooper Ornithological Society 1998

BREEDING ECOLOGY AND BEHAVIOR OF THE HAWAIIAN HAWK'

CURTICER. GRIFFIN2,PETER W. C. PATON3AND THOMASS. BASKETT School of Forestry,Fisheries, and Wildlife,University of Missouri-Columbia,Columbia, MO 65211, e-mail: [email protected]

Abstract. We studiedthe ecology of the endangeredHawaiian Hawk ( solitarius) on the island of for three breeding seasons. Their breeding strategy is a prime exampleof a K-selectedspecies characteristicof manybirds in tropicalenvironments: clutch size was one and brood-rearingwas among the longest reportedfor any diurnalraptor. Twenty-eightnests were found in a varietyof native and exotic habitats.Incubation lasted 38 days, nestlings fledged after59-63 days, and parentscared for fledglingsfor an average of 30.2 weeks, which was 2.5 to 10 times longer than similar-sizetemperate zone raptors. Males assisted females with incubation,but only females brooded young. Radio-tagged juveniles remainedwithin 0.63 km of their nests for the first two months after fledging, after which dispersal distances expandedgradually. Avian (45%) and mammalian(54%) prey dominatedthe diet of nestlings. Therewas no evidence that avian malaria,introduced predators,or environmentalcontaminants were affecting their population.Based upon es- timatesof populationsize, the availabilityof suitablenesting habitat,and reproductivesuc- cess, we suggest the be consideredfor downlistingfrom endangeredto threatened status. Key words: breedingecology, Buteo solitarius,clutch size, diet, Hawaiian Hawk, nest success.

INTRODUCTION (Banko 1980, Scott et al. 1986). The Hawaiian Hawk is the most of The HawaiianHawk or Ilo (Buteo solitarius) is among sexually dimorphic 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 threatenedwith extinc- nesting habitat, breeding behavior and chronol- tion and included on the EndangeredSpecies ogy, nest success, diet, and other factors affect- List in 1967, little was known about its popu- ing reproduction. lation status or at the time of breeding ecology METHODS the listing decision. Informationabout the dis- tribution,abundance, and biology of the Hawai- STUDY AREA ian Hawk is anecdotal (Banko 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 documentedobservations land area of the Hawaiian Islands (Armstrong of the species on the islands of Kauai,, and 1973). Although fieldwork took place through- since 1778 (Banko 1980, Olson 1990). out the island, observations of nests and telem- Since ornithologistshave 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 of its size and recent 1 Received 5 August 1997. Accepted 13 May 1998. large origin (Armstrong 2 Currentaddress: Department of Forestryand Wild- 1973). As is typical of all Hawaiian islands, life Managementand GraduateProgram in Organismic much of the native biota is endemic. However, and of Massachu- EvolutionaryBiology, University most of these indigenous ecosystems have been setts, HoldsworthHall, Amherst,MA 01003. modified from introduced 3 Currentaddress: Department of NaturalResources by competition plants, Science, University of Rhode Island, Kingston, RI overgrazing by introduced ungulates, logging 02881. operations, and urban and agricultural develop- [654] HAWAIIAN HAWK BREEDING ECOLOGY 655 ment. Agriculturedominated much of HawaiiIs- vival rates (Mayfield 1975), with confidencein- land below 700 m elevation. In 1983, 435 km2 tervals developed by Johnson (1979). To calcu- were in sugarcane,66 km2were 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 Departmentof Agriculture,Ho- were recorded as fledging successfully only if nolulu, unpubl.data). they were observed after leaving the nest. All intensive observationsof hawkswere con- Activities at five nests were monitoredfor 696 ducted at low and mid elevations (70-1,740 m) hr from blinds in 1980 and 1981; all nests except where the landscapeis dominatedby agricultural the nest at Puu O'O Ranchin 1981 (35 hr) were lands, exotic vegetation, and disturbed native watchedfor more than 120 hr, with observations forests. Hawks were not studied in undisturbed spaced evenly throughoutthe breeding season. native forests because of difficulty of access. Threenests were monitoredin 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. Observationsof nesting ests with mixed native and exotic understory; activities were made from tree and ground one in an agriculturalarea within a tree windrow blinds with a spotting scope (15-60x). Blinds separatingguava and papaya orchards,and two were located 10-30 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 FIELDPROCEDURES afternoonand the morningthe next day, 6-7 hr Fieldworkwas conducted5-6 days per week for per session), whereasin 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 throughJuly 1982 averaged one day per week, 4.5 hr. We attemptedto identify prey items to and involved checking nests for activity. species when possible. However, we were un- To find active nests, we first relocated8 of 11 able to distinguishthe threepotential (Rattus historic nests found from 1973 to 1979 by a spp.), and prey was sometimes brought to the numberof researchers(Griffin 1985). Islandres- nest partially consumed making identification idents also showed us several nests afterreading difficult. Terminologyused to describecourtship about our study in the local newspaper.We also behaviorfollowed Brown and Amadon (1968). systematicallysearched 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-chatritraps baited with black rats surveys, we stoppedthe vehicle at 1.0-km inter- (Rattus rattus) and 3.6 kg-test monofilament vals and attemptedto attract territorialhawks nooses (Berger and Mueller 1959). All captured with playback of an amplified, territorialHa- 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 1-3 numberedplastic bands (Na- hawk territorieswere discovered by watching tional Band and Tag Co., Newport, Kentucky). for soaring hawks and listening for their vocal- Young were banded3-4 weeks afterhatching to izations. For these reasons, 36% of nests found minimize disturbance. were situatedwithin 0.5 km of primaryor sec- We radio-tagged three juveniles from three ondary roads. territoriesin August 1980 to monitordispersal. During the 1980 breeding season, we found Tail-mountedtags (9 g) were suturedto a single most nests in the mid- to late-nestlingstage. In rectrix.Receivers (AVM InstrumentCo., Cham- 1981, eight nests were checked priorto egg-lay- paign, Illinois) were used with a 4-element, yagi ing and three were first checked during incuba- null-peakantenna system mountedin a vehicle. tion. In 1982, most nests were checked only for Transmittersoperated in the 150-151 MHz band numberof young produced;therefore 1982 data and each was identifiablewith a uniquefrequen- were not included in nest success estimates. cy. We attemptedto monitor each radio-tagged Nest success was calculated using daily sur- for at least 8 hr per week, with observation 656 C. R. GRIFFIN, P. W. C. PATON ANDT. S. BASKETT periods evenly distributedthroughout daylight TABLE 1. Physicalcharacteristics of HawaiianHawk nestson islandof Hawaii. hours. All radio-taggedbirds occurredin areas with road so we could well-developed systems, Variable • ? SE trackhawks from a vehicle. In addition,observ- (n) ers were often able to confirm locations vi- Nest height (m) 9.3 ? 0.8 (23) by ? sual Nest tree height (m) 15.8 0.9 (23) sightings. Nest tree dbh (cm) 50.3 ? 6.4 (23) For all measurements,we present?• SE. We Nest diameter(cm) 64.7 ? 2.9 (21) used a Mann-WhitneyU-test to compare_ varia- Nest depth(cm) 31.5 ? 4.0 (20) tion in breeding chronology between years and Nest cupdiameter (cm) 23.9 ? 1.7(9) Nest 4.2 ? 0.6 as a function of territoryelevation, and the log- cupdepth (cm) (9) likelihood ratio test (G-test) to determinewhat affect priornest success had 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 statisticalanalyses were completedus- ground (Table 1). Most nest trees were fairly ing SAS (1990). Alpha levels of 0.05 were con- large, averaging50 cm diameterat breastheight sidered significant. (dbh), but some were as small as 10.2 cm dbh. Hawaiian Hawks stable RESULTS apparently preferred platforms to construct nests; nine nests were COURTSHIPAND TERRITORIAL DISPLAYS placed on top of bird's-nestferns (Aspleniumni- Aerial displays were recordedin 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 breedingseason in April (27%) and and four on small branches(< 20 cm diameter) May (23%) (n = 26). Males typicallyperformed at their juncture with the trunk. Fifteen nests aerial displays more intensely than females, al- were in native ohia or ohia/koaforests, 8 in for- thoughfemales often participated(n = 17). Pairs ests dominatedby introducedtrees, and 5 were performed mutual soaring, diving, and foot- in pastures. touching. Undulating displays sometimes fol- Nests appearedto be used for several years, lowed mutual soaring displays. Both males and with nesting materialadded each breeding sea- females performedundulating displays, both in son. Nests averagedover 0.6 m diameterand 0.3 unison (n = 2) and while solitary(n = 5). Males m deep (Table 1). All nests containednest cups also performedsteep downwardplunges for 10- that were lined with green leaves. 30 m (n = 11), which Brown and Amadon referredto as the "Pot-hooks NESTINGCHRONOLOGY AND FREQUENCY OF (1968) Display." BREEDING NESTBUILDING 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 materialduring the nestling period.Our ear- tween laying dates in 1980 and 1981 (Ul2, 8 liest recordof nest building occurredon 24 Jan- 54, P > 0.05), or between low- (< 1,500 m) and uary. Both sexes frequentlyadded materialsto mid-elevation(> 1,500 m) territories(U4,4 = 11, old nest structures,and sometimes to more than P > 0.05). Hatchingoccurred 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-leafedtwigs to the nest cup tember,peaking in mid-August(Fig. 1). (n = 39). Although breeding territorieswere occupied every breeding season, pairs did not attemptto NESTSITE HABITAT CHARACTERISTICS nest every year. Of six territoriesthat 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 (Diospyrosferra), 3 in koa, and pairs apparentlydid not attemptto nest in one 1 each in ironwood (Casuarian spp.), mehame year; and one pair nested in only 1980. Another (Antidesmaplatyphylla), and eucalyptus(Euca- eight pairs were tracked for two breeding sea- lyptus robusta). Nest trees were 10-24 m in sons, of which threepairs attemptedto nest both HAWAIIANHAWK BREEDING ECOLOGY 657

5- TABLE2. Numberof lightand dark morph nestlings EGG LAYING producedby HawaiianHawk parents of knowncolor morph.

Adult color morph Number of nestlings Number Male Female of pairs Light Dark Light Light 6 6 0 Light Dark 5 2 3 w z4-? HALCHING Dark Light 7 0 7 3- Dark Dark 1 0 1 m E O -r--

81 broken egg was found below a nest containing FLEDGING a single egg, but the lattermay have been a re- 4. NMONTHO = 16 placementegg, ratherthan a two-egg clutch. Mean egg size was 57.0 ? 2.7 X 44.0 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. FIGURE1. Breedingchronology of the Hawaiian comm.). Hawkon 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 attemptedto nest in only ment clutch within 24 days at an alternativenest one year. Nest success the previous breeding site following nest abandonment.This pair left season did not affect the probabilitythat hawks the original nest because a logging operation would re-nest at the same site (G1 = 1.3, P > cleared much of the habitatin the area. No re- 0.20). nesting was detected at two nests where chicks 3 weeks old died after from the nest. MATESELECTION < falling Replacementclutches by two captive hawkpairs HawaiianHawks have both light and darkcolor were laid 20 days and 26 days after their first 1- morphs. Of 41 pairs we observed, 8 were dark- egg clutches were collected for artificialincu- darkpairs, 11 were light-lightpairs, and 22 were bation (L. Yoshina,pers. comm.). light-darkpairs. There was no evidence of as- sortative mating with respect to color morph. INCUBATION,NESTLING, AND POST-FLEDGLINGPERIODS Assuming randompairings with respect to color morph, we expected 8.7 dark-darkpairs, 11.9 Incubationlasted approximately38 days (n = 3 light-lightpairs, and 20.4 light-darkpairs, which nests); this estimatewas imprecisebecause exact did not differ from our observations(G2 = 0.1, laying dates were uncertain.Their nestling pe- P = 0.9). Frequenciesof darkand light offspring riod, the time from hatchingto leaving the nest, resultingfrom differentcolor morphpairings 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- producedby a dominantallele for melanismthat niles for 25-37 weeks after fledging (2 = 30.3 is not sex linked (Table 2). weeks, n = 4). The minimum period before juveniles dis- AND EGG CLUTCHSIZE CHARACTERISTICS persed from their natal territoriesranged from There was no indication that clutch size was 29 to 76 weeks (2 = 55.3 + 10.9 weeks, n = more than one egg duringour study. Clutchsize 4). We were able to radio-tracktwo 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 thirdjuvenile from 25 August to 23 October attemptsrecorded during the 3 nesting seasons, 1980 and from 27 Februaryto 16 March 1981. we never saw more than one chick in a nest. A All radio-taggedjuveniles remainedvery close 658 C. R. GRIFFIN,P. W. C. PATON ANDT. S. BASKETT

4

RoyalGardens 3.5 . Thorton's ------Malama-Ki I 3 I I 02.5 I I

o I

0.5

1 2 3 4 5 6 7 8 9 10 11 Month FIGURE2. Dispersaldistances of threeradio-tagged, juvenile Hawaiian Hawks in themonths following fledg- ing. to natal areas during the first two months after in territoriesin habitat too high to have mos- fledging;none moved fartherthan 0.63 km from quitoes (Culex quinquefasciatus)(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 afterfledging at the Malama-Kiand Roy- were successful) comparedto territoriesthat po- al Gardens nests, respectively, the juveniles tentially had large numbersof mosquitoes(4 of were ranging widely within the territoriesof 7 low elevation [70 to 402 m] nest attemptswere their parents.Both of these juveniles dispersed successful), and found no difference(G1 = 0.31, out of theirparents' territories in the sixth month P = 0.55). In addition,we found no difference after fledging. Although the juvenile female at in nest success of hawks with territoriesin na- Royal Gardensmoved as far as 3.7 km from her tive forests (10 of 13 attemptswere 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 attemptswere 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 abandonedeggs (n = 10). to his natal area fledging and did not return dur- Three of these abandonedeggs contained em- the 4 months that we tracked ing subsequent bryos at advanced stages of development, so him. reasonsfor their abandonmentare unclear.How- NESTSUCCESS ever, one egg was infertile.Four othernest fail- ures occurred when 2-3-week-old chicks fell There was no differencein hatchingsuccess be- from nests. tween 1980 and 1981 (two-tailedz-test = 0.39, P = 0.70). Daily survival estimates for eggs in TERRITORYAND MATE FIDELITY the incubation period were 0.991 ? 0.009 in 1980 and 0.986 ? 0.009 in 1981. Similarly, Six of eight hawk territorieswere monitored there was no differencebetween 1980 and 1981 closely duringboth the 1980 and 1981 breeding in fledging success estimates (z = 0.71, P = seasons to determineterritory 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 orderto examine whetheravian malariaaf- same territoryin both years. The female at the fected chick survival, we comparednest success sixth nest occupiedthe same territory,but paired HAWAIIAN HAWK BREEDING ECOLOGY 659 with a differentmale 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. ADULTNEST ATTENTIVENESS AND FEEDING OF YOUNG NESTLINGDIET Eggs were incubatedabout 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 incubationduties, but males Birds (45%) and (54%) accountedfor sometimes assisted. The male at the Keauhou most identifiedprey taken to nestlings (n = 300 nest incubated 50-77% of the time. However, prey). Adults broughtin 13 species of birds, at males rarelyincubated in the week before hatch- least 3 species of mammals,and 1 invertebrate: ing. 1 Chukar(Alectoris chukar), 2 Japanese Quail During incubation,exchanges occurredwith- (Coturnixjaponica), 2 Wild Turkey (Melegris out inter-sexualaggression. Males with prey ini- gallapavo) chicks, 1 Pacific Golden Plover (Plu- tiated the changeover by either calling from a vialis fulva), 4 Spotted Doves (Streptopeliachi- perch within sight of the nest or perchingon the nesis), 1 EurasianSkylark (Alauda arvensis), 3 nest itself. Females usually took the prey and Melodious Laughing-Thrush(Garrulax cano- flew away from the nest to feed. Once the fe- rus), 24 Common Mynah (Acridotherestristis), male left the nest, the male typically initiated 49 JapaneseWhite-eye (Zosteropsjaponicus), 2 incubation and remained on the nest until the Northern Cardinal (Cardinalis cardinalis), 10 female returned. House Finch (Carpodacusmexicanus), 1 Hawaii Once eggs hatched,most males did not brood Amakihi (Hemignathus virens), 3 Apapane nestlings and their visits were limited to food (Himationesinguinea), 32 unidentifiedbirds, 87 deliveries. Females spent 92% of their time rats (Rattus spp.), 66 house mice (Mus muscu- broodingduring the firstweek afterhatching. By lus), 5 small Indian mongoose (Herpestes au- the second week, females brooded much less ropunctatus), 4 unidentified mammals, and 3 (33% of the time), and broodingactivity greatly cockroaches(Blattodea). Interestingly, their diet declined thereafter.After the second week post- was dominatedby introducedspecies, with only hatching,female attendancewas increasinglyre- 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 protectedfrom rain and brooded overnight through the fourth DISCUSSION week. Although females left chicks unattended The parentalcare periodfor the HawaiianHawk while they huntedduring weeks 2-5, they typi- is among the longest recorded for a buteonine cally remainedwithin sight of nests during this species (Newton 1979). It rivals those of some period. At week 5, females began providingin- tropicaleagles whose weight is more than seven creasinglygreater amounts of prey at nests, leav- times greater than an adult Hawaiian Hawk ing young unattendedfor severalhours at a time. (Mader 1982). Among 11 species of temperate By week 7, young were left unattended97% of , incubationaverages 33.7 days in dura- the time. tion, fledgingtakes 43.3 days, and fledgedyoung During the nestling period, prey exchanges receive parentalcare 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 1-2) both adults Newton 1979, Santana C. and Temple 1988). often gave soft chirp calls afterprey exchanges, The incubation period of the Hawaiian Hawk with the male usually remainingfor 15-60 sec. (38 days) is similar to that of other tropicalbu- 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 reportedfor any the nestling periodprogressed, females appeared other buteos (Newton 1979), with the possible increasinglyaggressive towardsmales in the vi- exception of the White-tailedHawk (B. albicau- 660 C. R. GRIFFIN, P. W. C. PATON ANDT. S. BASKETT datus) (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 numbersof young in nests from dence on avian prey, which are difficult to cap- 1888 through1979, 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 introducedby humans to (see Griffin 1985 for summaryof these records). the islands, starting 1500 years ago (Berger The discrepancybetween frequencies of multi- 1981). The parental care period of two other ple young presentin historic records (41%) and that on avian the = = tropical raptors depend prey, our study (0%, n 30) is significant (G1 18.1, Puerto Rican Sharp-shinnedHawk (Accipiter P < 0.001), althoughthe reasons for this differ- striatus venator) and Red-tailed Hawk (Buteo ence are unknown. One possible explanationis jamaicensis), also is significantly longer than that prey was more abundanthistorically, which that of temperate-zoneconspecifics (Delannoy may have promotedlarger clutch sizes (Newton and Cruz 1988, SantanaC. and Temple 1988). 1979). The apparentyear-round residency of Hawai- Re-nestingafter loss of eggs or hatchedyoung ian Hawks on territoriesis conducive to long- is common among accipitersand falcons. How- term bonds. Pair to be pair fidelity appears high ever, re-nestingis rare among hawks and Hawaiian 5 of 6 among Hawks, (83%) pairs (Newton 1979), except for Harris' Hawk (Par- were intact after one and similar to other year, abuteo unicinctus) and the in birds of For in Savanna tropical prey. example, Venezuela (Mader 1982). Re-nesting also was Hawk ( meridionalis) populations, rare among HawaiianHawks; so it does not ap- 64% of were intact after a = pairs year (n 11) pear to be a universal adaptationof tropical (Mader1982). In Hawaii, thereis probablylittle hawks. opportunityto choose a new territory or new We found no evidence of assortative mate. Vacanciesin territories mating breeding probably the HawaiianHawk, in con- and the among polymorphic occur infrequently, necessary strategy trast to Dunkle's research with Swain- for a newcomerwould be to establishitself in a (1977) son's Hawks after the death of an owner. (Buteo swainsoni). However, territoryquickly Schmutz and Schmutz documentedran- a successful bird would (1981) Thus, simultaneously dom with to color both a and mate mating respect morphamong acquire territory (Newton Hawks also Insular Hawks Ferruginous (Buteo regalis); they 1979). Galapagos (Buteo gala- that one determinescolor have a similar remain suggested gene morph. pagoensis) strategy;they Both color of the Hawaiian Hawk are in their territoriesfor life as do morphs (Faaborg1986), across the island et al. resident Savanna Hawks in Venezuela (Mader widespread (Scott 1986). This was the first to 1982). study attempt quantify the and behaviorof the endan- In many African raptors,pairs breed only 2 breedingecology HawaiianHawk. It is therefore out of 3 years because there is a tendency for gered important to discuss factors that be the pairs which successfully breed to not nest the may affecting spe- cies' and next year (Brown 1970). Hawaiian Hawks ex- populationviability. Breeding foraging success differs between habitatsfor hibit a similarbreeding strategy, given that only many rap- tors Preston We found no 61.5% of pairs that nested successfully attempt- (Newton 1979, 1990). between nest success and ed to nest the subsequentyear (n = 13). The relationship habitat, made extended postfledgingjuvenile dependencymay although we only limited observation of promote this sporadic breeding cycle. Interest- hawks breeding in undisturbednative habitats. ingly, pairs with unsuccessful nesting attempts Much of the lowland agriculturalareas on the also had a low probability(33%, n = 6) of at- island of Hawaii are now unsuitablefor Hawai- tempting to nest in the same territorythe sub- ian Hawks (Griffin 1985, Scott et al. 1986), sequent year, which suggests a relationshipto which only nest in trees. Hence, the extensive territory quality. Newton (1979) reported that sugarcanefields and pasturesfound on the island territoryfidelity 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 habitatspoor in food. 1982), especially for agriculture,has reducedthe HAWAIIANHAWK BREEDINGECOLOGY 661 quantityof nesting habitatavailable to hawks in perimentallyfed 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 theireggs have been introducedto Ha- noae), whose vector is the introducedmosquito waii, including the domestic cat (Felis catus), (Culex quinquefasciatus;Warner 1968, van Rip- Polynesian (Rattusexulans), black rat, Nor- er and van Riper 1985, van Riper et al. 1986). way rat (R. norvegicus),and small Indianmon- Populationsof 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 species are known to prey on small native incidence of malariain bird populationsabove 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 HawaiianHawk 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. introducedpredators. 1986). Griffin(1985) analyzed75 blood samples The Hawaiian Hawk's populationappears 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 distributedover the is- very high immunogeneticcapabilities for avian land, occurringin a broad arrayof habitattypes malaria(C. van Riper,pers. comm.). rangingfrom macadamiaorchards to all types of Avian pox, caused by strainsof Poxvirusav- forests on the island (Griffin 1985, Scott et al. ium, also has been suspected as a factor in de- 1986, Hall et al. 1997). They are absent only clines of native Hawaiian birds (Warner1968, 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 transmitteddirectly by con- part because of the prevalence of introduced tact with an infected organism, or via a vector birds and mammalsin the hawk's diet. In con- such as a mosquito(Cavill 1982). Van Riperand trast to many native birds on the island, the van Riper (1985) diagnosed pox virus in pox- hawk is widely distributedin low elevation hab- like lesions in nine species of native and intro- itats where avian malariaadversely 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 capturedHa- vironmentalcontaminants overtly affecting the waiian Hawks; however, no bacteriological or Hawaiian Hawk. Currentpopulation estimates virological samples were collected, and hence range from 1,600 (Hall et al. 1997) to 2,700 these lesions were not confirmedas avian pox. hawks (Griffin 1989) residing on the island of Although organochlorine compounds have Hawaii. This populationestimate, coupled with had devastatingeffects on raptorpopulations in the informationpresented in this study,indicates many parts of the world, there is limited use of that reclassificationfrom endangeredto threat- these compoundsin Hawaii. None or only trace ened status may 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 implementationof 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 rodenticidesin Hawaii is believed to be low. Fumarinand zinc phosphidewere the ACKNOWLEDGMENTS rodenticidesused in Hawaii Fel- primary (D. P. We aregrateful to all who assistedor collaboratedon lows, pers. comm.). There is no direct evidence thisproject: J. M. Scott,L. E Pank,and D. P.Fellows, of secondary poisoning of Hawaiian Hawks formerlyof 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 macadamianut orchardsand formerlyof the NationalPark Service. T. A. Burrand sugarcane Landand fields. Effects of on in the M. Morin,formerly of HawaiiDepartment of anticoagulants raptors NaturalResources, arranged for studypermits. Staff of field remain to be assessed; however, no abnor- Hawaii Volcanoes National Park,especially D. Ames malities occurredin Barn Owls (Tyto alba) ex- and E Kualani,arranged for housing and materials.L. 662 C. R. GRIFFIN,P. W. C. PATON ANDT. S. BASKETT

Pau and F C. Parksassisted in equipmentconstruction. Population status of the endangered Hawaiian R. Banashek,former Young Adult Conservation Corps Hawk. J. RaptorRes. 31:11-15. Director,provided volunteerfield assistants.The Pa- JENKINS,C. D., S. A. TEMPLE,C. VANRIPER III, AND tuxentWildlife Research Center and NationalFish and W. HANSEN.1989. Disease-relatedaspects of con- Wildlife Health Laboratoryprovided chemical analy- serving the endangeredHawaiian Crow, p. 77-87. ses and necropsy reportson a carcass and eggs col- In J. E. Cooper [ed.], Disease and threatened lected in Hawaii.J. D. Jacobi,U.S. GeologicalSurvey, birds. Int. Council Bird Preserv.Tech. Publ. 10. Biological ResourcesDivision, HawaiiField Research JOHNSON, D. H. 1979. Estimatingnest success: the Station provided valuable assistance in vegetation Mayfieldmethod and an alternative.Auk 96:651- analyses aroundhawk nests, L, H. Yoshina(Director, 661. PanaewaZoo, Hilo, Hawaii) providedinformation on KIRCH,P. V. 1982. The impact of prehistoricPolyne- captive HawaiianHawks, and J. A. Baldwin (USFS) sians on the Hawaiianecosystem. Pac. Sci. 36:1- assisted with statisticalanalysis of data. S. J. Doyle 14. assisted with field researchduring 1981-1982. D. G. MADER, W. J. 1981. Notes on nesting raptorsin the Newmanprovided editorial assistance, and S. S. Clark llanos of Venezuela.Condor 83:48-51. gave technicaland secretarialsupport. This publication MADER,W. J. 1982. Ecology and breedinghabits of is a contributionfrom the Missouri CooperativeFish the Savanna Hawk in the llanos of Venezuela. and Wildlife ResearchUnit and the MissouriAgricul- Condor84:261-271. turalExperiment Station. MAYFIELD, H. E 1975. Suggestions for calculating nest success. Wilson Bull. 87:456-466. LITERATURE CITED MENDENHALL,V. M., AND L. F PANK. 1980. Secondary poisoning of owls by anticoagulantrodenticides. ARMSTRONG,R. W. 1973. Atlas of Hawaii. Univ. Press Wildl. Soc. Bull. 8:311-315. of Hawaii, Honolulu. NEWTON,I. 1979. Populationecology of raptors.Bu- BALDING,T., ANDE. DIBBLE.1984. Responses of Red- teo Books, Vermillion,SD. tailed, Red-shouldered,and Broad-wingedHawks OLSON,S. L. 1990. The supposedCalifornia record of to high volumeplayback recordings. Passenger Pi- a HawaiianHawk, Buteo solitarius.'Elepaio 50: geon 46:71-75. 1-2. BANKO,W. E. 1980. History of endemic Hawaiian OLSON, S. L., AND H. E JAMES. 1997. Prehistoricstatus birds. Part I. Population histories-species ac- and distributionof the HawaiianHawk (Buteoso- counts.Forest birds: Hawaiian Hawk ('Io). CPSU/ litarius), with the first record from Kauai. UH Avian History Rep. 6A, Dept. Botany, Univ. Bishop Mus. Occ. Papers49:65-69. Hawaii, Honolulu. PATON,P. W. C, E J. MESSINA,AND C. R. GRIFFIN. BERGER,A. J. 1981. Hawaiianbirdlife. Univ. Press of 1994. A phylogeneticapproach to reversed size Hawaii, Honolulu. dimorphismin diurnalraptors. Oikos 71:492-498. BERGER,D. D., ANDH. C. MUELLER.1959. The bal- PRESTON,C. R. 1990. 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