Condor 84:203-207(c; The Cooper OrnithologicalSociety 1982

BODY TEMPERATURESOF INCUBATINGNON-INCUBATINGROADRUNNERS

VERSUSSANDRA L. VEHRENCAMPABSTRACT.-Coretemperatures of free-living Roadrunners (Geococcyx califirnianus) were monitored with temperature-sensitive radio transmitters. Breeding males were found to perform all of the nocturnal incubation and to maintainnormothermic body temperatures at night. Body temperatures of females andnon-breeding males dropped at night to as low as 33°C dependingon the ambient
temperature. Facultative, short-term hypothermia appearsto be a regular energysaving feature of this speciesthat is conspicuouslynot employed by the nocturnal
incubator.

Most birds exhibit a circadian pattern of body the same or different responses.If incubators
temperature, involving a reduction of 1 to 3°C typically maintain high body temperaturesunduring the nighttime hours (Dawson 1954, der conditions when non-incubators show hyAschoff and Pohll970). Lower nocturnal body pothermia, then incubation may be metaboltemperatures appear to be due to reduced mo- ically expensive. Furthermore, the female of
tor activity and other cyclic phenomena of a facultatively hypothermic species may besleep, since nocturnally active birds show a energetically constrained during incubation ifreversal of the pattern, i.e., lower diurnal body she has laid a large clutch of eggsand must
temperatures.Only a handful of speciesexhibit recuperatedepleted reservesvia hypothermia.
true hypothermia, defined operationally as a Thus facultative hypothermia may have imreduction of body temperature below 36°C that portant implications for both clutch size andmay or may not involve torpor (Dawson and the sex that performs the incubation.
The Roadrunner (Geococcyx californianus)
Hudson 1970). Facultative hypothermia is
generally argued to be an energy-saving ad- is a large (350 g) cuckoo that has been reported
aptation in responseto periodic food shortages to lower its body temperature under stressful
and/or excessively low ambient temperatures conditions and to warm up by sunning(Calder and Ring 1974). Species that exhibit (Ohmart and Lasiewski 197 1). Hypothermic
hypothermia include nectar-feeding hum- birds have been found to conservea significant
mingbirds, aerial insectivores such as capri- amount of energy compared to normothermic
mulgids, swallowsand swifts, small passerines birds at the same ambient temperature. The
overwintering in cold climates, and a few trop- Roadrunner is also among the few speciesin
ical speciessuch as anis and colies (Dawson which the male performs most of the incuand Hudson 1970, Calder and Ring 1974). It bation, especially at night (Calder 1967,
has been argued that short-term hypothermia Ohmart 1973, Skutch 1976). In order to obtainshould occur only in small animals due to the a better understanding of the breeding enertheoretically prohibitive effects of thermal in- getics of this unusual bird, I monitored theertia and high rewarming costfor large animals body temperatures of free-living Roadrunners(Calder and Ring 1974), yet vultures are usingradio-telemetry techniqueswhile simulknown to lower body temperatures to 34°C. taneously following breeding activities. I here
Hypothermia is a complex phenomenon for report significant differences in body temperwhich the costs and benefits are still poorly ature for Roadrunners as a function of sex andstageof breeding.
understood.
The constraintsthat hypothermia places on
breeding birds have not been explored. In par- METHODS
ticular, incubation and hypothermia may not I studied a population of Roadrunners at the
be compatible. Incubating hummingbirds have Camp Pendleton Marine Corps Reserve, apgenerally been found to maintain normoth- proximately 15 km north of Oceanside, San
ermic body temperatures at night except dur- Diego County, California. The study site coning unusual cold (Calder 197 1, Vleck 1982). sisted of a 10 km transect running parallel to
Egg temperatures fall well below normal in- the coast and l/2 km inland along old Highway
cubation temperaturesduring theselapsesinto 10 1 between Las Pulgas Road and the San
hypothermia. It is not clear whether non-in- Onofre nuclear power plant. The habitat concubatorsunder similar conditions would show sisted of fairly undisturbed coastal sagescrub

204

SANDRA

L. VEHRENCAMP

and chaparral on the steeper hillsides, andsemi-grazedpastureon the flatter areas.A system of jeep trails (maintained by militarytraining activities) allowed accessto much of
the area by car.
Birds were captured in standardraptor traps(foot-noose traps baited with a mouse), a
home-made trap baited with a mouse, andwith mist-nets set up near nests(Vehrencamp
and Halpenny 1982). The raptor traps caught
many of the non-breeding males. Capture ofbirds at the nest frequently causedthe nest to
be abandoned. The treadle trap, placed along
known foraging routes, more successfullycaptured breederswithout causingnest abandonment. Most birds were sexed by laparotomy.
Nests were found by searchingsystematicallyamong the larger clumps of prickly pear cactus(Opuntia), the preferred nesting site in thisarea.
Temperature-sensitive, miniature radiotransmitters were constructed by inserting a
100 Kohm Yellow Springs Instrument Co.
thermistor in serieswith the 82 Kohm resistorof the standard “Cochran” transmitter described in Bradbury et al. (1979). All radios
operated in the 148 MHz rangeand were monitored with an AVM receiver and a hand-held
Yagi antenna. Two pulse-interval timers wereused,a home-made device which provided forcontinuous recording on a Rustrak strip chart
recorder, and an AVM pulse timer that had
no provision for automatic recording. Ambient temperatures were variously recorded at
1 m above the ground using a Schulteis thermometer, a thermistor-diode attachment to
the home-made pulse timer, records from the
San Onofre power plant, and a Tempscribe
temperature recorder.
Two implantation methods were employed:(1) The transmitter was harnessedexternallyon the back of the Roadrunner, and the thermistor was mounted at the end of long leads,
sealed in silastic tubing and silastic medical
cement, and implanted in the body cavity ofthe bird. The entire packageweighed between
10 and 15 g, representing about 5% of thebird’s weight. This method had the advantagesof (a) allowing a large battery to be used,whichgave a radio life of three to six months, and(b) allowing a whip antenna to be used, whichgave a transmitting range of 1 to 2 km. Thismethod had the disadvantagesof difficult surgery in the field and the need for a correction
factor in the calibration curve following implantation, i.e., there was a significant difference between cloaca1and radio-derived temperatures. (2) The transmitter was further
miniaturized by substitutinga smaller crystal,transistor, and battery, sealedin wax, and en-tirely implanted in the abdominal cavity. This
packageweighed l-2 g and had the advantagesof greater accuracy and easier surgery, but radio life was short (one to two weeks)and transmitting range poor (due to removal of the antenna). The poor transmitting range(1 O-20 m)
made diurnal temperatures very difficult toobtain. Radio attachment and surgery techniques are describedin Vehrencamp and Halpenny (1982).
Each radio was calibrated between 30 and
45°C in a slowly cooling water bath using a
fast-reading Schulteisthermometer. After implanting the radio in the bird, the bird’s cloaca1
temperature was recorded and checkedagainst
the temperature computed from transmitter
calibrations. For the implanted radios, core
body temperature as measured by the radio
averaged 1.O”Clessthan a simultaneous measurement of cloaca1temperature with a Schulteis thermometer; cloaca1 temperature wastypically 41 .O” while core temperature wasusually 40.0”. Similar discrepanciesbetween
cloaca1and intraperitoneal temperatures havebeen recorded in sparrows (Southwick 1973)
and Burrowing Owls (Athene cunicularia;
Coulombe 1970). Slightly higher temperatures
were recorded from birds captured on hot
afternoons (e.g., 42.4” cloacal), and slightly
lower temperatures were recorded from birds
captured early on cool mornings or during
rainstorms (e.g., 38.5” core). Unless otherwise
noted, all temperatures given are core temperatures (TJ. For the externally mounted
units, core body temperature was 3 to 6”higher
than cloaca1temperatures (T,,). This obvious
error was apparently due to a faulty design inthe radio circuit that manifested itself when
the thermistor element was extended from therest of the circuitry on long leads. These calibration curves were therefore linearly corrected usingthe measuredcloaca1temperatureand the known 1” difference between cloaca
and core. The use of a linear correction term
is of course an approximation, but externalcorrected readingsgave resultsidentical to internal radio data.
RESULTSACTIVITIESOF BREEDING

PAIRS

I found eight nests during the two-year study
period. Extensive behavioral observations
were made at two nests where one or bothparents were banded and abandonment did
not occur. My observations agreed well withother published accounts (Bryant 19 16, Bent
1940, Woods 1960, Calder 1967, Ohmart
1973, Whitson 1976, Folse and Arnold 1978).
Clutch size of nests found prior to hatching

ROADRUNNER BODY TEMPERATURESnII
FIGURE 1. ContinuousRustrak recordingof a breeding
male Roadrunner carrying a miniature radio transmitter(Bird l), showing time spent on and off the nest. The
receiving antenna was permanently aimed at the nest, sothat a positive voltage was recorded only when the bird
was on or very near it. This observationtook place during
the early hatching period, when both young nestlingsand
eggswere present.

36

,‘,,

35

k

34
33

205

qo

“L .._.__J

BREEDING MALE

ranged from four to six (mean = 5.0, n = 4).
Incubation commenced with the laying of thefirst egg. The eggs and chicks were covered
almost continuously, except for occasionalabsencesof up to an hour around noon. Both themale and the female incubated during the day.
Figure 1 showsa continuousRustrak recordingof a male’s presence at one nest during theearly hatching period (eggsand nestlingswere
present).Simultaneousobservationsindicatedthat the male did not leave the nest until thefemale came on the nest. Female activity at
the nest is therefore the converseof the male’s
activity. Becausenestlingswere present, both
sexesusually brought food when they came to FIGURE 2. Hourly readings (where possible) of core
the nest, and incubation bouts were somewhat body temperature (TJ and ambient temperature (T,) for
three Roadrunners:A, breedingmale (Bird 1); B, breeding
shorter than in nests with only eggs. Obser- female (Bird 4); and C, non-breeding male (Bird 3).
vations at a second nest in the early stagesof
incubation indicated that the female usually
took two long bouts, one in the morning and removal of the male (internal radio); and 6) a
one in the afternoon, with the male sitting most non-breeding female (external radio).
Examples of daily variations in T, are illusof the rest of the time. Males sat on nests on
12 of 13 nights of observation (five nests).The trated (Fig. 2) for a breeding male (Bird 2), a
one exception occurred when I removed the breeding female (Bird 4), and a non-breeding
male, forcing the female to sit that night; she male (Bird 3). Core body temperatures of thesubsequentlyabandoned the nest. My obser- two breeding males remained nearly at dayvations suggestthat male and female Road- time levels throughout the night. Bird l’s temrunners share diurnal incubation equally, and perature dropped from 4 1.7 to 39.9”C at nightthat nocturnal incubation is entirely per- and Bird 2’s temperature dropped from 4 1.5
to 39.O”C at night. Temperatures of females
formed by the male.
and non-breeding males, on the other hand,
BODY TEMPERATURES OF INCUBATING AND
were frequently much lower at night than durNON-INCUBATING BIRDS
ing the day. All female breeders that were reNine birds were outfitted with temperature- corded either immediately post-laying, during
sensitive radios. Body temperatures were suc- incubation, or during early nestling feeding,
cessfullyrecorded from one to three days and dropped their body temperatures at night wellnights in six of these birds. The sexes and below daytime levels. The lowest temperature
breeding stagesof these six birds were as fol- recorded was 34°C. A non-breeding male also
lows: 1) a normal breedingmale that incubated dropped his body temperature at night to 33°C.
each night (external radio); 2) a breeding male When ambient temperature fell rapidly atthat abandoned his nest and roosted in a dis- dusk, body temperature also fell rapidly; intant tree during the night of recording (internal one case T, fell about 4°C in one hour (Fig.
radio); 3) a non-breeding male (external radio); 2B). Body temperatures rose quickly at dawn.
4) a normal breeding female that did not in- Temperatures began to rise slightly just at
cubate at night (internal radio); 5) a breeding dawn but before the bird became visibly acfemale that was forced to incubate at night by tive. As soon as sunlight touched the ground,206

SANDRA L. VEHRENCAMP

42°C41-40 39 ET
3
g
E
d
c

*
38 -

l

*

.

0 Normal incubating 8

.

.

.

37 -

0 Breeding non-incubating 3.

z*Nocturnal
.

36 -

.

.

.

.

.

.

.

incubating plNormal breeding !flNon- breeding 2

.

.

35 -

.

A Non-breeding p

“.
..

34 -

.

)***

.

.
33

I
6

n

I
8

I

I

I

I

I

I

I

I

IO

12

14

16

18

20

22

24 t “C

Ambient temperature

FIGURE 3. Combined plot of hourly readings of core body temperature (TJ versus ambient temperature (T,) forbreeding and non-breeding male and female Roadrunners,

the birds became active and could often be moregulatory pattern from females during theseen sunning.
incubation period.
One non-breeding female was monitored
late in the seasonand was found not to lower DISCUSSION
her T,. However, ambient temperatures were Nocturnally incubating male Roadrunners
high at this time and her failure to reduce T, maintained distinctly higher body temperasuggestedthat the degree of hypothermia was tures than roosting non-incubating females
related to ambient temperature. A plot of body under the same ambient conditions. The diftemperature versusambient temperature (Fig. ference appeared to be a sexual one and not3) for all birds indicates that T, for all females merely a function of the insulation provided
and non-breeding males varied directly as a by the nest. These findings suggestthat incufunction of T, (Y = .89, y1= 45, P < 0.001 for bation is fairly expensive metabolically. This
all closed-symbol nocturnal points in Fig. 3)
cost can be estimated using published data on
while T, for breeding males remained constant oxygen consumption for normothermic and(r = .32, y2= 27, P > 0.05 for all open-symbol hypothermic Roadrunners (Ohmart and Lanocturnal points in Fig. 3).
siewski 197 1). At an ambient temperature of
Several lines of evidence suggestthat the 10°C a normothermic bird expendsabout 1.5
elevated nocturnal body temperature of in- cm’ g-l hour’ of oxygen, while a hypothermic
cubating males was not merely a consequence bird expends about 1.1 cm’ g-l hour-l. Theof the insulative properties of the nest (Wals- normothermic bird is therefore metabolizing
berg and Ring 1978). Firstly, a female whom about 36% more energy than the hypothermic
I forced to incubate one night by removing the bird. This representsa minimum estimate ofmale lowered her body temperature in a man- the cost of incubation. This calculation may
ner similar to that of a non-incubating female underestimatethe actual cost of incubation for(closed stars in Fig. 3). Secondly, a breeding several reasons.Ohmart and Lasiewski’s normale that abandoned his nest and roosted in mothermic birds were “post-absorptive anda distant tree maintained a high body temper- restingin the dark,” and may have maintained
ature (open circles in Fig. 3). Breeding males slightly lower body temperatures than incutherefore appeared to employ a different ther- bating birds under the same conditions. Fur-

ROADRUNNER BODY TEMPERATURES

ther, a large clutch of eggsmay act as a heatsink, requiring the incubator to expend slightly
more energy than a non-incubator. On theother hand, the nest itself may provide insulcation for the incubator that could reduce its
warming costs relative to a non-incubator(Walsberg and King 1978). Direct measurements of oxygen consumption for incubatorsand non-incubators must be made in order toprecisely quantify the caloric cost of incubation (Biebach 1979, Gessaman and Findell
1979, Mertens 1980, Vleck 1981).
The sexual difference in thermoregulation
may be a correlate of the different physiological conditions of the two sexes at onset of
incubation. Breeding males have conspicuous
fat deposits,and are significantly heavier thannon-breedingmales (354 k 18 g vs. 3 14 k 20
g, P < O.OOl), although breeding and nonbreeding males do not differ statistically inother body measurements(bill, wing, tarsus).
None of the females captured showed conspicuousfat deposits.Laying females were significantly heavier than non-breeding females(3 18 k 2 g vs. 283 IfI 10 g, P < O.Ol), but theyreach pre-breeding weights during early incubation (290 g) and gain even more by the nestling period (325 g). Thus, males are in very
good condition at the onset of incubation,
whereas post-laying females have depleted
their energy reserve and must regain it rapidly
in anticipation of the nestling feeding period.
Females therefore stand to gain more from anenergy-savingthermoregulatory habit such asnocturnal hypothermia, and males are in abetter position to perform the apparently expensive nocturnal incubation.
ACKNOWLEDGMENTS
I thank Liza Halpenny and the many studentsat the University of California, San Diego who aided me in the field.
The Game Unit of the Camp Pendleton Marine Baseprovided logisticalsupportthroughoutthe study.This project
was supportedby N.S.F. grant DEB 78-25230 and by an
Affirmative Action award from the University of California.

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Department of Biology (C-016), Universityof CahforniaSan Diego, La Jolla, California 92093. Received 18 June
1981. Final acceptance11 December 1981.