Temperature Regulation in Two Endangered Hawaiian Honeycreepers: the Palila (Psittirostra Bailleui) and the Laysan Finch (Psittirostra Cantans)

TEMPERATURE REGULATION IN TWO ENDANGERED HAWAIIAN HONEYCREEPERS: THE PALILA (PSITTIROSTRA BAILLEUI) AND THE LAYSAN FINCH (PSITTIROSTRA CANTANS)

WESLEYW. WEATHERS1 AND CHARLESVAN RIPER, III 2 •Departmentof AvianSciences, University of California,Davis, California 95616 USA, and 2Departmentof Zoology and Cooperative National Park Resources Studies Unit, Universityof California,Davis, California 95616 USA

ABST•ACT.--Theclosely related, morphologically similar Palila (Psittirostrabailleui) and LaysanFinch (P. cantans)are nativesof thermallydissimilar habitats. The 34.8-gPalila is confined to the cool, montane forests of Hawaii Island, whereas the 31-g Laysan Finch is restrictedto a low, treelessatoll. To studytheir climaticadaptation, we measuredtheir body temperature,oxygen consumption, and evaporativewater loss at stableair temperatures between 3 and 40øC.Bioenergetically, these speciesare most distinct in their responseto heat.The Palila'supper critical temperature (31øC) is 7øClower than the LaysanFinch's, and its heat-straincoefficient is 31% higher(1.76 versus 1.34 mW'g -•'øC •). The Palilais much lessheat tolerantthan the LaysanFinch (which, in its heat tolerance,resembles other passerines).Consequently, the Palila probablyis restrictedphysiologically to cool,mountain forests and is not a suitable candidate for lowland introductions. The Palila's lower critical temperature(17.5øC), thermal conductance (0.612 mW'g •'øC •), and nighttimebasal metabolicrate (BMRp) (12.87 mW/g) were all within 8% of levelspredicted from mass, indicating that its cold toleranceis not unusual for a passerineof its size. The LaysanFinch's BMRc• averaged15.2 mW/g H20/day, 80% of thevalue predicted from mass. Its evaporativewater loss (rhwe)also was lower than expected,2.22 versus 3.31 g HzO/day. Reductions in BMR and rhwe have adaptivevalue for smallbirds living in waterlessenvironments. Received 4 January1982, accepted12 March 1982.

CrYMATtcan affect birds directly (i.e. phys- greaterrole in determiningbird distribution iologically),through its impact on energyand than is generallybelieved. Further thermoreg- water balance, and/or indirectly (i.e. ecologi- ulatory studiesof closelyrelated species from cally), throughits influenceon vegetationand differing climatesare neededto resolvethis is- food availability. Birds exhibit a remarkable sue. degreeof physiologicaladjustment to differing The Laysan Finch (Psittirostracantans and climates, and, except for truly extreme envi- Palila (P. bailleui) are finch-billed members of ronments such as hot, waterless deserts or the the endemic Hawaiian honeycreeperfamily frigid Arctic, they are thought to be limited Drepanididae.Closely rented, they sharemany most often by the effect of climate on vegeta- physical, myological,and osteologicaltraits tion structure and food availability (Dawson (Raikow 1977, Pratt 1979), but occupy quite and Bartholomew1968). Nevertheless,signifi- different habitats.The LaysanFinch is restrict- cant thermoregulatorydifferences do exist in ed to LaysanIsland (25øN), a treelessatoll whose similar speciesfrom dissimilar climates(Daw- highest point is 5 m above sea level. Laysan son 1954, Wallgren 1954, Hudson and Kimzey Island contains a hypersaline lake but no 1966,Rising 1969, Blem 1974, Hinds and Calder standing fresh water. Its landscape is domi- 1973,Hayworth 1980).These findings, together natedby clumpsof bunch grass,in which Lay- with the fact that avian basal metabolic rate san Finchesnest, and intervening patches of (BMR) is climaticallyadaptive (Weathers 1979), bare sand. The Laysan Finch's diet--insects, suggestthat physiologicalcapacity may play a seeds, flower buds, and birds' eggs (Berger

667 The Auk 99: 667-674. October 1982 668 W•ATH•RSAND VAN RIPER [Auk,Vol. 99

1972)--and the open terrain that it inhabits will constrain avian BMR. Furthermore, both suggestthat it is often subjectedto direct solar speciesare currently listed as endangeredby radiation while foraging. the U.S. Fish and Wildlife Service (1976), and The Palila is restricted to Hawaii Island it is imperative that resourcemanagers have (20øN), where it occupiesfairly dry mamane an adequatedata baseupon which to basetheir (Sophorachrysophylla) and mamane-naio(My- preservation strategies. oporumsandwicense) forests on Mauna Kea vol- cano (see van Riper 1980afor a description of METHODS the habitat). Its range has contractedgreatly We studiedfour male Palilasand 12 LaysanFinch- since 1900 (van Riper et al. 1978), and it is cur- es (mixed sexes)during November and December rently restrictedto Mauna Kea'shigher forests, 1980 at the Hawaii Field Research Center (elev. 1,220 from treeline (2,900 m elevation) down-moun- m), Hawaii Volcanoes National Park. The birds had tain to about 2,000 m. Historically, the Palila been held in flight cages at the park for approxi- occurred at somewhat lower elevations (down mately I yr. The roofed cagesprovided protection to 1,220 m, Perkins 1903), but it is essentially from the frequent rains but exposed the birds to a montane bird. It feeds extensivelyon ma- prevailing temperaturesand photoperiods.In body mane seed pods, using its powerful bill to tear mass, Palilas ranged from 33.5 to 40.0 g (mean = open the pods and extractthe seeds(van Riper 36.0 + 1.6 SD), LaysanFinches from 24.4 to 37.9 g (mean = 32.8 + 3.6 SD). 1980b). While feeding, the Palila is often shad- Ratesof oxygenconsumption (•/O2) and evapora- ed from the sun by the mamanetree canopy. tive water loss (rhwe)were determined on postab- Weathers (1977, 1979) pointed out that BMR sorptive birds that had been in darkened4-1 metab- often is reducedin tropical birds that forage in olism chambers for at least I h. Each chamber was the sun but is normal in shade-foragingspecies. equipped with a thermistor probe, perch, hardware Becauselittle of the heat gained by sun-for- cloth floor, and mineral oil trap for urine and feces. aging, lowland, tropical birds can be dissipat- Temperaturecontrol was attainedby submergingthe ed by evaporation(owing to high ambient hu- chamberin a water bath. Ratesof •/O2 were calcu- midity), possessinga reduced BMR increases lated by equation 2 of Hill (1972)from the fractional the time availablefor foraging.Hence, thermal O2 concentrationchange in dry, CO2-freeinlet and outlet airstreams,as measured(to the nearest0.005%) influences---high heat load coupled with re- with an Applied ElectrochemistryS-3A O2 analyzer duced evaporative capacity--act as selective (N-22M sensor).Oxygen concentrationwas moni- forcesfavoring a reduced BMR in these birds. tored with a Heath/Schlumbergerrecorder. Chamber Shade-foragingtropical birds do not confront air flow rate was kept constantat between 470 and exogenousthermal loads and thus do not re- 600 cm3/min(STPD), when air temperature(Ta) was quire reducedBMRs. Nevertheless,BMR is re- less than 35øC, and at 950 cma/min (STPD), when Ta duced in some shade-foragingspecies. For ex- was greater than 35øC. Chamber air-flow rate was ample, MacMillen (1981) found that the BMR measured with upstream Matheson rotameters (603 of four Hawaiian honeycreeperspecies departs tube). Rotameterswere calibratedagainst Brooks Mass from the usualmass dependency (i.e. BMR was Flow Meters (NBS traceable).Because the "perfect" gas laws alone do not correctrotameter flows mea- not proportionalto mass3/4),with shade-for- sured at reduced pressures,we correctedindicated aging species having lower than expected air flow rates to standardconditions (STPD) using BMRs. The selective forces operating in this viscosity factor data provided by Matheson. Body casewere the species' social position in an in- temperature(intestinal) was determined immediate- terspecificdominance hierarchy and their rel- ly after the metabolicrun with a YSI telethermometer ative successat nectar exploitation. (Series511 probe), which was calibratedagainst a Unlike the honeycreepersthat MacMillen NBS thermometer.Rates of rh,•ewere determinedby studied, the Palila and Laysan Finch do not collectingwater vapor from the downstream airline compete for food and are not members of a with a series of three drierite-filled U-tubes. Our initial metabolism measurements were made dominance hierarchy. Becausethey inhabit during the restphase (p) of the circadiancycle. Com- quite different thermal environments and ex- paring the two species' results was confounded, perience different solar heat loads while for- however, by their differing responseto the mea- aging, they provide an opportunityto test fur- surement apparatus. Palilas readily accepted con- ther the hypothesisthat, in the absenceof finement in the metabolismchamber and typically competitive interactions, thermal influences slept during the metabolic determinations. Most October1982] Thermoregulationin Hawaiian Honeycreepers 669

0.21000

Fig. 1. Representativefractional O2 concentrationrecords obtained during metabolismmeasurements. Contrast the recordsfor an active (A) and sleeping (B) Palila with comparableLaysan Finch records(C). FiO2 = O2 concentrationin air upstreamfrom the metabolism chamber; F•O2 = O2 concentrationin downstream air.

LaysanFinches, in contrast,tended to remain awake san Finchesduring the active phase (a) of the cir- and active, even after 9 h in the chambers. This made cadian cycle,when birds are normally awake and/or it difficult to obtain truly basal metabolicrates for active. the Laysan Finches. Figure 1 illustrates the two An unanticipatedproblem arose during this study. species'differing responsesto rest-phasedetermi- The U.S. Fish and Wildlife Serviceabruptly trans- nations. The O2 trace of an active Palila (Fig. 1A), ferred the Palilas to the Honolulu Zoo before we recordedless than 1 h after the bird was placedin completedour studies.This accountsfor the paucity the metabolismchamber, exhibits the widely fluc- of low-temperaturePalila data. tuating O2 concentrationtypical of an activeanimal. In contrast, a flat O• record was typical of sleeping RESULTS Palilas (Fig. lB). Figure 1C presentsstrip-chart recordsfor two LaysanFinches. The left tracerepresents Figure 2 presents values for the Laysan an obviously active animal, the right-hand trace a Finch'smetabolic heat production(/q,,) and seeminglyquiescent bird. Note, however, that the body temperature(To) measuredat different quiescentLaysan Finch recordexhibits considerable temperatures.The Laysan Finch's therrnoneu- "noise," unlike the flat trace of the sleeping Palila (Fig. lB). This "noise" indicatesminor posturalad- tral zone extendedfrom 23.5 to approximately justmentstypical of awakebirds. Obviously,Laysan 36.5øC. Within this zone, Tb increased an av- Fincheswere remainingactive during the "rest" phase erage of 0.75øC.The least squaresregression of the circadiancycle. To avoid this confounding equationfor/•/,,, as a functionof T• below25øC variable,we alteredour protocoland measuredLay- is /•/,,•(mW/g)= 28.46 - 0.54T• (r2= 0.777, 670 WEATttERSAND VAN RIPER [Auk, Vol. 99

48 LaysanFinch o< // 48 • 44 Palilap //

.• 40 1- / .• 40 / I-- 36 • 36

30 30

• 20

E I0

0 I0 20 30 40 0 I0 20 30 40 5O Ambient temperature (øC) Ambient, temperature (øC) Fig. 2. Relationof body temperature(Tb) (above) Fig. 3. Relationof body temperature(T0) (above) andmetabolic heat production (/q,,) (below) of post- andmetabolic heat production (/:/,•) (below) of post- absorptiveLaysan Finches to ambient temperature. absorptivePalilas to ambient temperature.Measure- Measurementsmade during the active phase of the ments made during the rest phase of the circadian circadiancycle. Dashed line representsTb = To.Solid cycle. Dashed line representsTo = T•. Solid lines lineswere fitted to/:/,,• data by themethod of least werefitted to/:/,, data by themethod of least squares. squares.

is not statisticallysignificant. (We described S•.•. = 2.121, S0 = 0.070, n = 19). Note that this above the unfortunate circumstances sur- line, fittedto the low temperature/•/,,data in rounding our limited low temperature Palila Fig. 2, doesnot extrapolate to/:/,, = 0 at T,,= data.) Note, however, that this line does ex- T0.By statistically forcing the line through/:/,• = trapolateto/:/,, = 0 at T(,= Toand that the lack 0 at 41øC,the equation becomes/:/,,= 30.90 - of a significantcorrelation between/:/,, and T• 0.75Ta. The least squares regressionequation results mainly from the scattereddata at 8øC. for/z/,,above 36øC is/z/,, = 1.34Ta- 34.07(r 2 = Consequently,we believe this relation reason- 0.390, n = 6). The correlation coefficient(r) for ably approximatesthe Palila's true metabolic this relationis not statisticallysignificant (P response.The least squaresregression equa- 0.05), however. Consequently, the apparent tion for/:/,,, above31øC is/:/,,, = 1.76Ta- 42.12 upper criticaltemperature (Tuc = 36.5øC)is un- (r •= 0.52, n = 8). The correlation coefficient reliable. Values of/:/,, determined at 38øCare (r) for this relation is statisticallysignificant not significantlyhigher than thosedetermined (P < 0.05). at 35øC(P • 0.10, Mann-Whitney U-test), in- Figure 4 presentsthe relation of evaporative dicating that the LaysanFinch's Tuc probably heat loss (/:/,)--calculatedfroin steady-state exceeds38øC. Becauseof the Laysan Finch's measurementsof evaporativewater loss--to T•. activityduring metabolic determinations, The lines shown in Fig. 4 are the theoretical measurementsat high T•'s involveda largerisk. relationscalculated from body massby Calder We chosenot to risk killing any LaysanFinches and King's (1974) equation 56. At 25øC,Palila rather than to try and refine the T•e estimate. evaporativewater lossaverage 3.92 mg .g-] .h -] Figure3 presentsIZI,,, and To values for Palilas versus2.92 mg.g-•.h -• for the Laysan Finch. measuredat various temperatures.The Palila's Evaporativewater lossof LaysanFinches mea- thermoneutral zone extends from 17.5 to 31.0øC. sured during the day (c0 at 25øC averaged Within this zone, T0 remained constant and 3.83 mg'g •.h 1. averaged39.4øC. The least squaresregression MeasuringH .... /:/•, and To simultaneously equationfor/:/,, asa functionof T• below18øC enablesone to calculatethe rate of dry heat is/z/,,(mW/g) = 23.92- 0.61T•(r 2 = 0.38,n = transfer (h') between the animal and environ- 6). The correlationcoefficient for this relation ment (see King and Farner 1961, Calder and October1982] Thermoregulationin Hawaiian Honeycreepers 671

IO0 2.0 Laysan Finch Laysan Finch 80

60 1.0

o½½ 4o

• 2o

I00 0 • Palila Palila • 8o o o. 60

'" 40

0 0 ' 0 I0 20 :50 40 40 $0 20 I0 0 Ambient temperature (øO) T b - T a Fig. 4. Relation of evaporative heat loss (calcu- Fig. 5. Relation of the dry heat transfercoeffi- lated from steady state water lossmeasurements) to cient, h' (dry conductance),to the difference be- ambient temperature. Curves represent relations tween body and ambient temperature. predictedby Calder and King's (1974)equation 56.

these mid-oceanic islands), the Palila experi- Schmidt-Nielsen 1967). Values of h' for the Pal- ences cooler and drier conditions than those ila and the LaysanFinch are presentedin Fig. encounteredby the LaysanFinch (Table 1). In 5. At low air temperatures(i.e. Tb-- T• • 20øC), terms of continental climates, the mean annual the LaysanFinch lost 35% more heat through air temperatureof LaysanIsland equalsthat of nonevaporative pathways than the Palila Tampa, Florida, whereas that of Puu Laau is (0.51 + 0.05 versus0.69 + 0.10 mW'g-•'øC •). equivalentto that of Columbus,Ohio (Ruffner This reflectsthe LaysanFinch's reduced rate of and Bair 1977). But unlike their continental evaporativewater loss. In both species,h' in- counterparts,these oceanicislands exhibit lit- creasedas T(, approachedT• (i.e. as T• - T• tle seasonal variation in air temperature. approachedzero). In the LaysanFinch, h' ap- Nighttime air temperatureat Puu Laau, for ex- peared to decreasewhen the T• - T• gradient ample, approachesfreezing throughout the reached5øC, indicating a decreasein the rate year, but maximum daytime temperatures of heat lost by radiation, conduction,and con- vection. rarelyexceed 22øC (van Riper 1980a).Thus, the Palila experiencescool temperatures year round, unlike the birds of Columbus, Ohio. DISCUSSION Basedon the climatesto which they are ex- Quantitativelycomparing the Palila and the posed,we expectedthe LaysanFinch's BMR to LaysanFinch's native climatesis difficult be- be about 25% lower than predicted from its causedata for Laysan Island are unavailable• mass and the Palila's BMR to be normal or the nearest weather station is on Midway Is- slightly elevated.The LaysanFinch's daytime land, 265km north and 600 km west of Laysan. BMR was indeed 20% lower than would be Climatic data are available for the Palila's hab- predicted for a 31-g passerine(Table 2). A re- itat (Puu Laau), however, and these are com- ductionin metabolismof this magnitudewould pared with data for Midway Island in Table 1. be expectedin a sun-foraging,lowland, trop- Assumingthe climatesof Midway and Laysan ical bird (Weathers1979) and confirmsour hy- Island are similar (a reasonableassumption for pothesis. Unlike daytime BMR, the Laysan 672 WEATHERSAND VAN RIPER [Auk,Vol. 99

TABLE1. Weather data for Midway Island and Puu doesnot extrapolateto To.Forcing the metab- Laau, Hawaii Island. olism curvethrough T0 increasesthe Laysan Finch's thermal conductance but still leaves Annual temperature Annual conductance22% lower than predicted(Table Elevation (øC)mean precipita- 3). Low conductance,indicating high insula- (m) (range) tion (mm) tion, is consideredadaptive in cold-climate Midway• 3 22.2 1,080 birds. Lower ',han expectedthermal conduc- (9.5-31.5)b tance, however, is not restricted to cold climate Puu Laau½ 2,290 11.2 603e birds: tropical birds with reduced BMRs also (-5-29) 0 havelow thermalconductances (Weathers 1977). Twenty-eight years of record (NOAA 1980). As is obvious from equation 1 (below), when Rangefor 1980. BMR is reducedthere must be a corresponding Data from van Riper (1978,unpubl. obs.). Records for 1973--1975. reduction in thermal conductance if To is to Records for 1965--1974. remain normal. Becausethe LaysanFinch's To is normal, it would require a 20% reductionin conductanceto compensatefor its 20% lower Finch'snighttime BMR is nearly normal (94% BMR. The force-fitmetabolism curve closely of predicted),reflecting the bird's activitydur- matches the expected reduction in conduc- ing nighttime metabolic determinations.The tance. Palila's BMR was 8% less than predicted. Ap- The Palila's thermal conductance does not parently the Palila's cool habitat does not re- deviatefrom the levelpredicted from body mass quire an increasein BMR, such as that seen in (Table3). The predictiveconductance equation many high-latitude birds (Weathers1979). Endotherm thermal adaptation manifests it- is basedlargely on data for temperatespecies. As was the case for BMR, the Palila's thermal self not only through variationsin BMR, but through other thermoregulatoryparameters as niche is not sufficientlycool to require an in- crease in insulation. well, especiallythermal conductance(h) and lower critical temperature (T•e), both of which Lowercritical temperature.--The Palila's T•eis tend to be low in cold-climatespecies (Calder 6øC lower than the Laysan Finch's, a finding and King 1974).Table 3 presentsthese values, seemingly consistentwith the difference in along with other observed thermoregulatory these species'native climates. But because T•c parameters,for the two honeycreeperspecies. is lower in larger birds, or when metabolism Thermalconductance.--The slope of the me- is measured during the rest phase of the tabolism curve on air temperatureequals con- circadian cycle (Calder and King 1974), we ductanceonly when the curve extrapolatesto need to accountfor size and circadian-cycle body temperature(McNab 1980).Although the effects before we can conclude that the differ- Laysan Finch's thermal conductanceseems ence in these species' T•c reflects climatic ad- much lower than expected (Table 3), its curve aptation.

TABLE2. Observedand predictedbasal metabolicrates of Hawaiian honeycreepers.a

Basalmetabolism (mW/g)

Percent- Mass age pre- (g) NA N Observed Predicted b dicted Laysan Finch Daytime 31.0 + 3.08 8 18 15.20 + 1.40 19.09 80 Nighttime 31.6 + 3.05 5 9 13.46 + 1.09 14.34 94 Palila Nighttime 34.8 + 1.41 4 8 12.87 + 0.65 13.97 92

Values are means + standard deviations. NA = number of animals; N = number of observations. Predictedby appropriateequation of Aschoffand Pohl (1970). October1982] Thermoregulationin Hawaiian Honeycreepers 673

TAnrE3. Observedand expected (for passerinebirds) physiological parameters for Psittirostrabailleui and P. cantans.

P. bailleui P. cantans

Observed Expected Observed Expected Thermalconductance [mW(gøC)-•] a 0.612 0.618 0.638(0.751) e 0.957 Heat straincoefficient [mW(gøC) •]u 1.76 1.24 1.34 1.33 Evaporativewater loss(g H20/day)e 3.27 3.38 2.22 3.31 Lower criticaltemperature (øC) a 17.5 17.4 23.5 23.5 Calculatedfrom the relationmW(gøC) -• = 4.784g 0.a6afor cemeasurements and mW(gøC)• = 3.215g 0.46•for p measurementsafter Aschoff (1981). Calculatedfrom the relationmW(gøC) • = 12.5 •o.• from Weathers(1981). Calculatedfrom the relationg H20/day = 1.56 g0.2•7from Crawfordand Lasiewski(1968); expected value at T• = 25øC. Seetext for derivationof expectedvalues. Regressionline forcedthrough/z/,, = 0 at 41øC.

An endotherm's Tt½can be related to its me- (seeWeathers 1981). Consequently,Palilas are tabolism(/q,,) and thermalconductance (h) heat stressedat ambient temperaturesthat cor- throughthe Scholandermodel (see Calder and respondto thermoneutrallevels in mostspecies. King 1974:278) as follows: Additionally,the Palila'sheat-strain coefficient (the metabolism-temperatureslope above T•e) IZI,,= h(To- T,•). (1) is 42% higher than predictedfrom mass(Table Bysubstituting predictive equations for Iq,,, and 3). Thus, not only are Palilasheat stressedat h into the above relation, the expectedT• can moderateTo's, but they expend proportionate- be calculated.For night-restingpasserines, As- ly moreenergy on thermoregulationin the heat choff and Pohl's (1970) equationfor metabolic than other birds of similar size. Lack of heat rate may be convertedto units of mW/g and tolerance correlates with the Palila's cool hab- combined with Aschoff's (1981) equation for itat. Palila'sseldom experience To's above 30øC thermal conductance(converted to units of mW/ (Table1), and they evidentlyhave lost the usu- gøC)to yield al avian toleranceto high temperatures.Ac- cordingly, heat intolerancemay restrict this 36.9 g-0..,7•= 3.21 g-ø.•(T• - T•), (2) speciesto Hawaii's cool, montaneforests. which can be rearrangedas Another Hawaiian honeycreeper,the Ama- kihi (Loxopsvirens), is similarly heat intolerant T,. = T• - 11.50 g0.•87. (3) (MacMillen 1974). Like the Palila, it is a high Similarly, for day-restingpasserines, forest bird and seldom encounters To's above 27øC.Lacking the appropriateselective pres- 45.2 g 0.•0•= 4.78 g ø'4•(T• - Tt•). (4) sure, the Amakihi has either lost or never Rearranging gives evolvedthe usualavian toleranceof high temperature. Indeed, four of six Amakihis died T•. = T• - 9.46 g0.•7. (5) following exposureto 38.9-40.4øCfor 1.5 h Given a passerinebird's T• and body mass(g), (MacMillen 1974),a degreeof heat stresseasily its expectedTt•. can be predictedfrom equation endured by most passerines. 3 or5. In contrast with its montane relatives, the At T,,'sbelow the T•, the Palila'sT• averaged LaysanFinch possessesa more typicallevel of 39.7øC,the Laysan Finch's 40.3øC.Using these heat tolerance.Its T• (38øC)is about7øC higher valuestogether with the body massesgiven in than the Pallia's, and its heat-strain coefficient Table 2 and solving equations3 and 5 for the is 24% lower (Table3). Clearly, the Palila and expectedTt½ reveal that the difference in the Amakihi's heat intolerance reflects their ther- Palila's and Laysan Finch's Tt•. can be com- mallycool habitats rather than a taxonomictrait pletely accountedfor by size and circadianef- of the family Drepanididae. fects (Table 3). ACKNOWLEDGMENTS Responsesto heat.--Palilas are remarkably heat intolerant. Their upper critical tempera- This researchwas supportedby funds from the ture, 31øC,is one of the lowest among birds Instituteof Ecology(University of California,Davis), 674 WEATHERSAND VAN RIPER [Auk, Vol. 99 the National Science Foundation (Grant No. PCM 76- MACMILLEN,Rß E. 1974. Bioenergeticsof Hawaiian 18314), and the National Park Service (Contract CX honeycreepers:the Amakihi (Loxopsvirens) and 8000-7-0009). We thank Debra L. Weathers for her the Anianiau (L. parva). Condor 76: 62-69. assistancewith all aspectsof the study, especially 1981. Nonconformance of standard meta- data collection.Dan Taylor and the staff at Hawaii bolic rate with body mass in Hawaiian honey- Volcanoes National Park created an environment creepers. Oecologia 49: 340-343. conduciveto researchand providedvaluable logistic McNAB, B. K. 1980. On estimatingthermal con- support. ductancein endotherms.Physiol. Zool. 53: 145- 156. LITERATURE CITED NOAA. 1980. Climatologicaldata annual summa- ry, Hawaii and Pacific, vol. 76 (13). Ashville, ASCHOFF,J. 1981. Thermal conductance in mam- North Carolina, Natl. Climate Center. mals and birds: its dependenceon body size and PERKINS,g. C.L. 1903. Vertebrata (Aves). Pp. 368- circadianphase. Comp. Blochem.Physiol. 69A: 611-619. 465 in Fauna Hawaiiensis, vol. 1, part 4 (David Sharp, Ed.). Cambridge, England, The Univ. --, & H. POHL. 1970. Der Ruheumsatz von V6- Press. geln als Funktion der Tageszeitund der K6rper- PRATT,D. 1979. A systematicanalysis of the en- gr6sse.J. Ornithol. 111: 38-47. demic avifauna of the Hawaiian Islands. Un- BERGER,A. J*. 1972. Hawaiian birdlife. Honolulu, published Ph.D. dissertation, Baton Rouge, University Pressof Hawaii. Louisiana State Univ. CALDER,W. A., & J. R. KING. 1974ß Thermal and RAIKOW,R. J. 1977. The origin and evolution of caloric relations of birds. Pp. 259-413 in Avian the Hawaiian honeycreepers (Drepanididae). biology,vol. 4 (D. S. FamerandJ. R. 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