RESPONSES TO HIGH TEMPERATURE IN NESTLING DOUBLE-CRESTED AND PELAGIC CORMORANTS
ROBERT C. L^$IEWSKI AND GREGORY K. SNYDER
ADULTand nestlingcormorants are often subjectto overheatingfrom insolationat the nest. Their generallydark plumage,exposed nest sites, and reradiation from surroundingrocks aggravate the thermal stress. Young nestlingsmust be shieldedfrom the sun by their parents. Older nestlingsand adults compensatefor heat gain throughbehavioral adjust- mentsand modulationof evaporativecooling by pantingand gular flutter- ing. This study was undertakento examinesome of the responsesto high temperaturein nestlingsof two speciesof cormorants,the Double-crested Cormorant,Phalacrocorax auritus, and the PelagicCormorant, P. pelagicus. The evaporative cooling responsesin birds have been studied in some detail in recent years (see Bartholomewet al., 1962; Lasiewskiet al., 1966; Bartholomewet al., 1968; Calder and Schmidt-Nielsen,1968, for more detailed discussions),although much still remains to be learned.
MATERIALS AND •VIETI-IODS The nestling cormorantsused in this study (four Phalacrocoraxpelagicus and four P. auritus) were captured from nests on rocky islands off the northwest coast of Washington. As their dates of hatchingwere not known it was impossibleto provide exact ages. From comparisonsof feather developmentwith descriptionsin the literature (Bent, 1922; Palmer, 1962), we judged that the pe'lagicuschicks were approximately 5, 5, 6, and 6 weeksold, while'the auritus chickswere 3.5, 3.5, 4.5, and 6 weeksof age upon capture. The nestlingswere taken to the laboratoriesat Friday Harbor, Washington on the day of capture and housed in three 4' X 4' X 4' chicken wire cages. The cageswere equippedwith plywood platforms for the birds to sit on and coveredon top and two sides to shield birds from wind and rain. The cormorants were fed whole and cut fish twice daily and the cageswere washedafter feedings.By the end of the first day in captivity, they learned to acceptfish presentedat the end of a stick and within 3 days after capture, all birds were eating fish thrown into cages. The aur•tus weighedfrom 1,64o to 1,870 g upon capture and 4 weeks later weighed between 2,14o and 2,61o g. They were quite tame and easily handied, voracious, rarely fought over food, and showed little aggression towards the handlers. The pelagicus weighed from 750 to 1,180 g when captured. They remained nervous throughout,were extremelyexcitable, fought constantlyover food, and were aggressive toward any person or object moving near them. Although we handled them rarely and they gainedsome weight, they did not adjust to captivity, and none survived more than 2 weeks. Birds were exposed to controlled temperature conditions by placing them in a 9.5 ft ø incubator that could be regulatedat a desiredtemperature ñ 0.5øC. Humidity was maintained between 11-15 mm Hg water vapor pressure by drawing room air through the chamber at variable rates using a vacuum cleaner and variable trans-
529 The Auk, 86: 529-540. July, 1969 530 LAS•EWSK•^N•) SN¾•)•R [Auk, Vol. 86 former. Chamber humidity was monitored with a HygrodynamicsUniversal Indi- cator usedin conjunctionwith wide range humidity sensors(-4- 1.5 per cent RH). A 30-gauge copper-constantanthermocouple was inserted at least 4 cm into the cloaca,and securedwith two surgicalclips to a retrix, and with dental wax to ventral body feathers eaudad to the cloaca. We assumethat cloacal temperature is equivalent to core body temperature (T•0. Ambient temperature (TA) and Tn were monitored by attaching thermocouplesto a Honeywell Elektronik 16 potentiometric recorder. Temperatures of evaporating surfaceswere obtained in two ways: 1) fine 40-gauge thermocoupleswere sewninto placeso that the bimetallicjunction restedon the surface, 2) a 30-gauge thermocouplewas pressedto the surfaces of buccal and gular regions or wall of trachea, or insertedthrough an 18-gaugeneedle into the interclavicular air sac and then pressedagainst sac walls. The chamber was lighted by a 15-watt incandescent bulb, and the bird was observed through a double glass port from a dimly lighted room. Breathing rates were timed with a stopwatch while gular flutter rates were determined with a Strobotac (General Radio Co.). All cloacal temperatures, breathing, and gular flutter rates reported were obtained from birds that had maintained a stable cloacal temperature (+ 0.1øC) for at least 15 minutes. All birds studied were presumablypostabsorptive, as their last meal was no lessthan 14 hours prior to onset of experimentation.
RESULTS Description of gular flutter.--Wh'en P. auritus are not heat stressed they usually have their mouths closed (Figure 1, pose 1) and breathe slowly (10 12fmin). The hyoid is held in the floor of the buccal cavity in the sameplane as the mandibles. The glottis is.positioned between the two horns of the hyoid just anterior to the hinge of the jaw. Under these conditions the major sites of evaporation will be the buccal cavity and trachea, since without gular flutter little or no air movement occurs in the pharynx and esophagusposterior to the glottis. When exposedto low heat loadsDouble-crested Cormorants open their mouthsslightly and commencegular fluttering (Figure 1, pose 2). The glottis and hyoid are depressed1-1.5 cm ventral to. the lower line of the mandibleand the hyoid is flared laterally. Th'is createsan open chamber boundedby the buccal and pharyngealsurfaces and thoseof the anterior portion of the esophagus.The tidal air from respiratorymovements passes over the buccal, pharyngeal, and tracheal surfaces. Independent from respiratoryair movements,a rapid, repetitive bowing of the hyoid (10- 1lfsec) bringsadditional external a/r (plus somerespiratory air) acrossthe moist membranesof the mouth•, pharynx• and esophagus.There may be some increase in respiratory rate during low heat loads, although we observedgular flutter in somebirds that were breathing at resting levels. When auritus are subjectedto high heat loads, the hyoid is depressed 3.5-4.0 cm below the lower margin of the mandible and the glottis is located approximatelyone cm below the hyoid. This depressionand flaring of the hyoid createsa considerablylarger cavity than that found duringlow heat loads,expanding the buccaland pharyngealareas as well July, 1969] Nestling Cormorant Temperatures 531
Figure 1. Typical attitudes assumed by cormorants subjected to different heat loads. Pose 1 shows a Double-c•ested Cormorant at rest and not heat stressed. Pose 2 representsa Double-crestedCormorant subjectedto low heat loads soon after the onset of low amplitude gular flutter. Poses3 and 3a illustrate Double-crestedCormorants, exposedto high heat loads, engagedin a maximal evaporative cooling effort. Pose 3a is a dorsopostetiorview showinglateral flaring of hyoid and esophagusduring high amplitudegular flutter. The PelagicCormorant in Pose4 is subjectedto high heat load, but the area moved by gular flutter is considerably less than that in Double-crested Cormorants. as exposinga greaterportion of the esophagus(Figure 1, poses3, 3a). The hyoidmovements (and resultantvibrations of esophagealmembranes) movelarger amounts of air acrossthe moistsurfaces than duringlower heatloads even though gular flutter rate increases only slightly (see Figure 532 L^S•EWSK• ^•D S•¾DER [Auk, Vol. 86
IOOO
8oo
6oo
4OO Phalacrocoraxpelagicus 6-7 weeks
2OO
ß Flutter rate o Breathing rate • 8o • 60
4O
o o
o o o o o 20 0 8 o• • o o o
o o oø o o 1039 40 41 42 43 44 BODY TEMPERATURE øC
Figure 2. The relation of rates of gular flutter and breathing to body cloacal) temperature in 6- to 7-week-old Pelagic Cormorant nestlings.
3). The amplitudeof hyoid movementlaterally is 1-1.5 cm, which moves the upper 15 cm of th'e esophagus(almost all of the thin-walled area of the esophagus).The rate of respirationincreases with increasingbody temperature(see Figures2 and 3). Qualitativelypelagicus behaved similarly to auritus at comparabletem- peratures, although the extent of the esophagealarea fluttered and the amplitudeof flutter were not as great even during maximumheat stress (Figure 1, pose 4). Rates o/ gular /lutter and breathing.--In nestlingsof both species studied,the ratesof gular flutter were relatively independentof heat stress and T•, while breathingrates increasedwith T•,. Rates of gular flutter range from 790-920/min in pelagicus(Figure 2), and showeda slight increasewith increasingTm Breathing rates of restingpelagicus nestlings were 11-12/min and increasedwith T, to 68 breaths/minat T], of 43.8øC. No differences were observed between 4.5- to 5.5- and 7.5- to 9-week-old auritus with respectto gular flutter or breathing rates. Gular flutter rates July, 1969] Nestling Cormorant Temperatures 533
IOOO
8oo
6oo
Pholocrocorox ouritus 4oo 4.5-5 5 weeks
200
• IOO ß Flutter rate o Breathing rate •_ 80
d 6o 8
4O
8 8 o o
2O o
o
10 39 40 41 42 43 44 BODY TEMPERATURE *C
Figure 3. The relation of rates of gular flutter and breathing to body (cloacal) temperature in 4.5- to 5.5-week-old Double-crested Cormorant nestlings. ranged from 615-677/min and increasedslightly with T]• (Figure 3). Breathing rates varied from 11 12/min in the resting birds to 120/min when T• reached 42.6øC. Sitesof evaporation.--WhenTv. equalsT^, there is no net gain or lossof heat via avenuesof radiation, conductionand convection. If a bird is to maintaina constantT, undersuch conditions, it must evaporatesufficient water to dissipatean amount of heat equivalent to the metabolic heat production.Sites of evaporationmay have temperaturesappreciably below that of the coreTg (makingit possibleto identify theseareas). Measuring surfacetemperatures does not, however,permit quantificationof evapora- tion sincethe temperatures.attained by the evaporativesurfaces will be influenced by rates of blood flow through the area as well as rates of evaporation. Figure 4 presentsrepresentative temperatures recorded from potential evaporativesurfaces of two 6.5-week-oldauritus nestlingswith T• = T•. Surface temperaturesof 0.9 to 5.1øC lower than Tg were re- corded in buccal and pharyngeal regions, tracheal walls, and that portion 534 LA$IEWSKIAND SNYDER [Auk, Vol. 86
t'r'o-•h•'o• X••67-40 2 ,, /
Intercloviculorsoc/ //// I wall 41.5ø• / / • / TA=TB =41.5 • •igu•e •, •ep•esent•tive ev•po•tive su•ce temperaturesf•om two 6,•-wee•-o]d •oub]e-c•ested •o•momnts, •mbient temperature w•s held •t body temperature (41.•C) during thesemeasurements. of the esophagusmoved during gular flutter, implicating these areas as sites of evaporation. In these studies the interclavicularsac wall tem- peraturesrecorded did not differ from T•. Body temperatures.--PelagicCormorant nestlings 6-7 weeksold were unable to maintain constant body temperatures during heat stress as indicatedby the fact that T• was generallyhigher or equal to T^ over the range studied (Figure 5). It is uncertainwhether the inability of these nestlingsto regulatebody temperaturein the face of h'eatstress was due to their physiologicalcharacteristics or to the nervousnessof individualsof this species,or both. Nestling auritus were studiedat two stagesof development;4.5 to 5.5 weeks and 7.5 to 9 weeks after hatching. Nestlings of both age classes maintained relatively constant T• below T.• of 40øC (Figure 6). At Tx above 40øC, body temperaturesof the 4.5- to 5.5-week-oldchicks were near that of the environmentalthough one bird was able to regulate July, 1969] Nestling Cormorant Temperatures 535
Pholocrocorox pefogicus
6-7 weeks
•42
40 42 44
AMBIENT TEMPERATURE *C
Figure 5. The relationship between body (cloacal) temperature and ambient tem- perature in 6- to 7-week-old Pelagic Cormorant nestlings. Water vapor pressuremain- tained between 11-15 mm Itg. its T• 1.0 to 1.8øCbelow Ta for almost50 minutes. As the auritusnestlings grew and matured, they showedenhanced abilities to maintain TB below high environmentaltemperatures (Figure 6). When, for example,one 9-week-oldchick was exposedto a Ta of 45øC, it was able to maintain a T• below42 øC for over 1 hour without any apparentstress.
DISCUSSION Modulation of evaporative cooling.--Severalpatterns of evaporative coolingresponses in birds have thus far emerged: panting alone, synchro- nized panting and gular flutter, and panting and gular flutter at different rates. Both cormorantswe studiedflutter at a high and relatively constant rate and pant at a slowerrate that increasesgradually with Te. This pattern was establishedearlier in Pelecaniformesby Bartholomewet al. (1968) in the Double-crestedCormorant and Brown Pelican, Pelecanus occidentalis,and by Calder and Schmidt-Nielsen(1968) in the White Pelican,Pelecanus erythrorhynchos. The occurrenceof rates of gular flutter that are relatively independent of heat load have been describedpreviously in the Common Nighthawk, Chordeilesminor (Lasiewskiand Dawson,1964), Poor-will,Phalaenoptilus nuttallii (Lasiewski and Bartholomew,1966), Domestic Pigeon, Columba 536 L^s•ws•c• ^•D S•¾D•R [Auk, Vol. 86
44
Pholocrocorox ourifus
ß 45- 55 weeks o 75-9 weeks
42
../
o o a240 o o
TA=TB o
<36 40 42 44
AMBIENT TEMPERATURE øC
Figure 6. The relationship between body (cloacal) temperature and ambient tem- perature in two age classes oœ Double-crested Cormorant nestlings. Water Yapor pressure maintained between ll-15 mm Hg. livia (Calder and Schmidt-Nielsen,1966), Roadrunner,Geococcyx cali- Jornianus(Calder and Schmidt-Nielsen,1967) and the Double-crested Cormorant, Brown Pelican, and Mourning Dove, Zenaidura macroura (Bartholomewet al., 1968). The relative independenceof gular flutter frequenciesfrom heat loadsis presumablyrelated to. the fact that the gular regions are driven at their resonant frequenciesthereby reducing the metabolic cost of evaporative cooling. Crawford (1962) demonstrated that dogspant at the natural resonantfrequencies of their thoracic cages and suggestedthat use of the natural frequency was advantageousin permitting evaporativecooling at a minimal metabolic cost. Bartholomewet al. (1962), Lasiewskiand Dawson (196.4), MacMillen and Trost (1967), and Calder and Schmidt-Nielsen(19'67) have docu- mentedthe low energycost of gular flutter. The musclesdriving the h'yoid during gular flutter are relatively small in comparisonto the metabolically active massof a bird, and we can assume,a priori, that the energy re- quired for gular flutter is low in comparisonto total metabolism. Further- more, loweringand flaring of the hyoid in cormorantsrequires relaxation of someassociated muscles, and this may partially offset the cost of gular flutter. The cormorantswe studied exhibited respiratory rates that increased July, 1969] NestlingCormorant Temperatures 537
gradually with increasingT•. This correlationis similar to that demon- stratedin two speciesthat pant, the HouseSparrow, Passer domesticus (Kendeigh,1944) and the Tawny Frogmouth,Podargus strigoides (La- siewskiand Bartholomew,1966) as well as in severalspecies that combine gularflutter and panting: the DomesticFowl, Gallusdomesticus (Frankel et al., 1962), the Brown Pelican and Double-crestedCormorant (Bar- tholomewet al., 1968), and the White Pelican (Calder and Schmidt- Nielsen, 1968). The pattern of a gradualincrease in breathingrate con- trastswith the morerapid shift to highpanting frequencies found in dogs (Hemingway,19'38; Crawford, 1962), in DomesticPigeons and Road- runners(Calder and Schmidt-Nielsen,1967), and in Ostriches,Struthio camelus(Crawford and Schmidt-Nielsen,1967). The resting rates of respirationin both speciesof cormorantswere approximately10-12fmin, whichcompares favorably with breathingrates predictedfor nonpasserinebirds of this size by Calder (1968). Double- crestedCormorants had higherbreathing rates at comparable.hyperthermic body temperaturesthan did PelagicCormorants, which may partially ex- plain their superiorability at thermoregulatingunder heat stress. The gular flutter responsepermits evaporationfrom surfacesnot normallyexposed to the tidal air of respiratorymovements (i.e. variable portionsof the pharynxand esophagus).Evaporation from gular flutter in cormorantsis modulatedprimarily by controllingthe volumesof air movedper flutter. It is difficult at presentto assesseither the relative contributionsof panting and gular flutter to evaporativecooling, or the relativemetabolic costs of pantingand gular flutter. Sitesof evaporation.--Theimportance o.f gular flutter and respirationas mechanismsof evaporativecooling in Double-crestedand Pelagic Cor- morantsis corroboratedby recordso.f surfacetemperatures lower than cloacaltemperatures (see Figure 4) in the buccal,pharyngeal, tracheal, and esophagealregions. Kallir (1930) demonstratedtemperatures of evaporatingsurfaces in birdswhich were belowT•, as did Wilsonet al. (1952). Hutchinson(1955) concludedthat the temperaturesof the evap- orative surfaces of heat stressed Domestic Fowl must be 2 to 4 ø below that of the body. Lasiewskiand Bartholomew(1966) demonstrated gular temperaturesof 1.5 to 3øC lower than T• during gular flutter in the Poor-will,and showedthat as TX increased,the. gular surfacetem- peraturesdescreased. Calder and Schmidt-Nielsen(1968) and Schmidt- Nielsenet al. (1969) recordedlower evaporativesurface temperatures in the White Pelicanand Ostrich,respectively. Measurements on the Ostrich revealedthat the buccaland trachealsurfaces and possiblythose of the postthoracicand interclavicularair sacswere important sites of evaporation. Our data on the cormorantsdid not reveal lowered temperatureson the 538 L^S•EWSK•^m) S•,T•)E}• [Auk, Vol. 86 walls of the interclavicularair sac, althoughthis cannot be used as evidenceagainst the role of the air sacsin evaporativecooling in thesebirds. Body temperatures.--Inour studies,th'e auritus nestlings were better able to thermoregulateduring heat stressthan pelagicusnestlings. This specific differenceis in part due to the larger sizeof auritus (and subsequentlower surface-to-volumeratio) and the greater amountsof air moved during flutter and panting, although we cannot evaluate the importanceof the nervousnessof the pelagicus. Cormorant chicks are altricial at hatching and almost certainly show little capacity for thermoregulation.The 4.5- to 5.5-week-oldauritus nestlingswere near adult weightswhen studied,and yet showedlimited thermoregulatoryability during heat stress. At this stage, the bodiesof the nestlingswere completelycovered with down, althoughwing and tail featherswere developed. As they gained weight and replacedthe body down featherswith juvenalplumage, the nestlingsdemonstrated enhanced physiologicalcapacities to. regulate Tg during heat stress. Bartholomew (1966) has shownthe ontogeneticdevelopment of homeothermiccapacities during heat stressin anoth'erPelecaniform bird, the Masked Booby,SuIa dactylatra. The behavior of adult Pelecaniformbirds in brooding and shieldingtheir young from thermal stressis critical to the survival of younger nestlings. Occasionally young White Pelicans die from overheating without the attendant care of adults (Bartholomewet al., 19'53). During periodsof intense insolation, the reluctance of adult Brown Pelicans "to leave their young was very conspicuous"(Bartholomew and Dawson, 1954). Similar chick-adultresponses have been noted in the Masked Booby (Bartholomew, 1966), and Double-crestedCormorant (Bent, 1922; Palmer, 1962). Ad- vanced nestlingsof many Pelecaniformsshow considerablemotility and may seekshade or enter the water to relieveheat stress. Nestlings of both speciesof cormorantswe studied show elevatedbody temperature(hyperthermia) when exposedto high ambient temperatures. This seemsto be a generalavian responseto heat stress,i.e. almost all speciesof birdsstudied in this respectbecome hyperth'ermic. Hyperthermia during heat stressmay be important in the conservationof water. When Tx exceedsTm birds must expendwater for evaporationat a rate roughly proportionalto heat gain. Hyperthermiareduces heat gain by reducing the thermal gradientbetween the body and environment,thereby lowering necessaryevaporative water losses. Our study suggeststhat when Double-crestedand Pelagic Cormorant nestlings,taken from the samearea, are exposedto h'eatstress, the Double- crested Cormorants exhibit superior thermoregulatory abilities. This finding is in accord with the following considerations. Double-crested July, 1969] NestlingCormorant Temperatures 539
Cormorantsare largerthan PelagicCormorants and have a lowersurface- to-volumeratio. Their anatomyand size permit exposure of a largergular, pharyngeal,and esophagealsurface for evaporativecooling and they pant at higher frequencies.Double-crested Cormorants have a more southern distribution and tend to. feed closer to shore. Double-crested Cormorants generallynest in moreexposed situations, while Pelagic Cormorants nest in remoteor precipitoussites.
ACKNOWLEDGMENTS We thank RobertFernaid, Director of Friday Harbor Laboratories,University of Washington,and the personnelat the Laboratoriesfor their assistance.We appreciate the adviceand aid givenby Martin Codyof the Universityof California,Los Angeles. This researchwas supported by NSF Grant GB 5347and a UniversityResearch Grant to R. C. Lasiewski.
SUMMARY Double-crestedand Pelagic Cormorant nestlings pant andgular flutter whenheat stressed.Nestlings of both speciesgular flutter at high and relativelyconstant rates and pant at slowerrates that increasegradually with'body temperature. Gular flutter supplements respiratory evaporation. Evaporationduring gular flutter in cormorantsis modulatedprimarily by controllingthe volumeof air movedper flutter. Surfacetemperatures of 0.9 to 5.1øClower than body temperaturewere recordedin buccaland pharyngealregions, tracheal walls, and that portion of the esophagus movedduring gular flutter, implicating these areas as importantsites. of evaporativecooling. Double-crested Cormorant nestlings are able to expose larger gular, pharyngeal,and esophagealsurfaces for evaporationthan PelagicCormorant nestlings, and showsuperior thermoregulatory ability during heat stress. At ambienttemperatures below 40øC, nestlingsof both speciesmain- tainedrelatively constant body temperatures.At ambienttemperatures above40øC, Pelagic Cormorant nestlings 6 to 7 weeksold had bodytem- peraturesgenerally higher than or equalto ambienttemperatures. When exposedto ambienttemperatures above 40øC, 4.5- to 5.5-week-oldDouble- crestedCormorant nestlings had body temperatures equal to environmental temperatures,while 7.5- to 9-week-oldnestlings of this speciesmaintained relatively constantbody temperatures.
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