BEHAVIORAL
VERDIN
ADAPTATIONS
TO
THE
OF
THE
DESERT
GEORGE T. AUSTIN
L•TT•.E or no free water and great seasonaland daily extremesof
temperature are common features of deserts of the world. Maintenance
of a homeostatic
state undersuchconditions
involvesthe opposingrequirementsof maintaining water balance and thermoregulation.The
harshness
of the desertis well documented.Any trait that lessensheat
stressand reduceswater lossis adaptive. How residentorganismshave
adaptedto, or avoided,these featureshas been a field of fruitful researchin recentyears (see Brown 1968, Farner and King 1971, for
reviews).
Certain desertmammalsare strikinglyspecializedcomparedto nondesert forms in ways that can be correlatedwith environmentalextremes (Dawson 1955; Schmidt-Nielsenet al. 1957, 1967; Schmidt-
Nielsen1958, 1964; Bartholomew
and Hudson1960, 1961; Hudson
1962; Carpenter1966; and others). The generalphysiological
similarity
of desertand nondesertbirds, however,has led to the conclusionthat
birdshaveno specialadaptations
to the desert(Miller 1963).
Yet subtleadaptations
and preadaptations
occurringin the classas a
wholeenablemanyspecies
to inhabitthesehot, arid regions.Preadaptive
featuresof birds are discussed
by Dawsonand Bartholomew(1968).
Severaldata are availablethat suggestsmall physiological
refinements
ratherthan strikingadaptations
are commonamongdesertbirds. These
include
lowerbasalmetabolic
rates,higherevaporative
cooling
efficiencies,
and greatertolerances
for high ambienttemperatures
in desertspecies
(or populations)
compared
with closelyrelatedformsoccurring
in more
roesicenvironments(Salt 1952, Dawson 1954, Hudson and Kimzey
1966,Rising1969,CareyandMorton 1971,Henderson
1971). Breeding
of the Abert's Towhee (Pipilo aberti) and Rufous-winged
Sparrow
(Aimophilacarpalis)in response
to rain ratherthan photoperiod
(Marshall 1963,Ohmart1969) is anotheradaptation
of this type. Several
recentstudiesindicatedthat somedesertspecies
are physiologically
more
efficient in conserving
water than nondesertspecies(Smyth and Bartholomew1966, Willoughby1968, Ohmart and Smith 1970).
Preadaptive and physiologicalfeatures alone appear insufficient to
allowmanyspecies
to adaptto the desertcompletely.Behavioral
adjustments appear to be of major importanceas adaptationsto the desert
judgingfrom the few data available. For example,somespeciesreduce
245
The Auk 93: 245-262. April 1976
246
Gzo•gz T. Auszx•
[Auk, Vol. 93
and shift their activity to cooler microhabitats during midday (Calder
1968, Ricklefsand Hainsworth1968, Ohmart 1969).
This study deals with behavioral features of the Verdin (Auriparus
Jlaviceps),one of the smallerdesertbirds (6.5 to 7.5 g). Becauseof
its size it is faced with the greaterproblemsincurredby surface-volume
relationshipsthan larger species,and studiesof its adaptationsare desirable. The life history of the Verdin was studied by Moore (1965)
and Taylor (1971). Goldstein (1974) made limited measurements
of
oxygen consumptionof the Verdin at various ambient temperatures.
MET]rODS
I studied the ecology of the Vetdin in southern Arizona (Pima County) and
southern Nevada (Clark County). Data recorded on nest placement (Arizona and
Nevada) included entrance orientation, side of tree, height from ground, relationship
to branch structure (forks), and amount of shading at various times of the day.
Entrance orientation and side of tree were measured with a compass and corrected
to true direction. These circular data were treated statistically with the methods of
Batschelet (1965).
Three types of nests were distinguished: breeding, small roost, and large roost.
Breedingnestswere identifiedprimarily by the presenceof eggsand/or young. Other
characteristicsof breeding nestsare a cavity depressedbelow the level of the entrance,
a larger cavity than roosts,and more lining. Large roosts are similar in size but are
often unlined and have a smaller, shallower cavity. Large roosts are generally used
during the winter, but if they remain in good repair the male uses them through
the breeding season. Small roosts are not only reduced in size but are thin-walled
structures with no lining and are generally built in summer by both adults and
young. Nests are described in more detail by Taylor (1971). In order to detect
changes in placement and success,the breeding season was divided into early and
late periods with 1 May as the division date. Nests with the majority of activity
before or after 1 May were designated early and late nests respectively. Eggs are
laid from March through June (rarely July). The first of May is the approximate
midpoint of the breedingseason. About this time, first broods are fledging and nests
for second broods are under construction.
A number of breeding nests were inspected periodically to determine success.
Hatching successis the percentage of eggs laid that hatched, fledging successis the
percentage of eggs laid that fledged, and nestling successis the percentage of young
hatched that fledged. A successfulnest is one that fledges at least one young.
Successdata were treated statistically by Chi-square.
Differential habitat utilization over a range of ambient temperatures was studied
following the procedures of Ricklefs and Hainsworth (1968). Four microhabitats
used by Verdins for foraging were distinguished: exposedportions of the vegetation
above 1.5 m (high exposed), exposedportions of the vegetation below 1.5 m (low
exposed), portions of the vegetation with a mosaic of exposedand shaded vegetation
but predominantly shaded (shade), and those portions receiving little or no sun,
generally near the trunks of trees (deep shade). In winter, only exposedand shaded
microhabitats were distinguished. Microhabitats were simple to delineate on clear
days; on cloudy days the delineation was more subjective. Accuracy on the latter
days was enhancedby limiting my observationsto familiar trees. Thus I could be
April 1976]
247
Verdin Desert Adaptations
LATE
N=206
EARLY
N=182
30
Fig. 1. Entranceorientations
of Verdin broodnests.
reasonably
sure of what portionsof thesetreeswould be exposedor shadedif it
were not cloudy. Observations
on cloudydays when temperatures
were relatively
cool and the four microhabitatswere nearly isothermicserve as a control for comparisonwith clear, hot midday conditions.
Temperatureswere obtained concurrentlyfor each microhabitat. Foraging observations were obtained only on windlessdays to reduce the number of variables
involved in foraging behavior.
Amount of time a foragingVerdin spent in each microhabitatwas timed with a
stopwatch. Data are for sevenpairs of postbreeding(July-August) adult birds in
southern Nevada and about six individuals during winter (December-February) in
Arizona. Data on foraging intensity (expressedas number of perch changesper
minute) were gatheredin both Nevada (sevenpostbreeding
pairs) and Arizona (five
postbreeding
pairs). When a foragingbird was encountered,
the numberof perch
changeswere countedover a stopwatch-timedinterval. Observations
lessthan 2 min
in lengthwere discarded.When a bird stoppedforaging,was out of my sight for
more than 10 sec,or madea flight of over 15 m, observations
were ended. Con-
currentlydeepshadetemperatures
weremeasured.Ratesof nestvisitation(to feed
young) were counted visually.
Statistical significanceis defined throughout at the 0.O5 level.
RESULTS AND D•SCUSS•0N
Nest placementand orientation.--Nestentranceorientations
in Nevada
and Arizona do not differ significantlyand the data are lumped. Early
nestsare orientedpredominantly
in an easterlydirection(mean : 84ø,
significantlydifferent from uniform,Fig. 1). Late nestsare oriented
southwest(mean = 257ø, significantlydifferentfrom uniform,Fig. 1).
Smallroostsare similarlyoriented(mean: 264ø, significantlydifferent
248
G•OROET. AUSTIN
TABLE
[Auk, Vol. 93
1
ENTRAI•CE ORIENTATIONS OF VERDIN N•STS
Entrance
orientation
(degrees)
0-30
40-70
80-110
120-150
160-190
200-230
240-270
280-310
320-350
N
Early
Late
Large
Small
brood
brood
roost
roost
(%)
(%)
(%)
(%)
•t2.6
14.3
29.7
15.9
8.2
4.4
1.6
2.7
10.4
6.3
4.4
4.4
3.9
9.2
20.9
20.9
18.9
11.2
9.2
7.2
8.6
11.2
9.9
9.9
15.1
10.5
18.4
9.7
5.8
2.9
7.8
8.7
14.6
24.2
17.5
8.7
182
Mean orientation
84 ø q- 59 ø
206
152
257 ø q- 61 ø
Uniform
103
264 ø q- 66 ø
from uniform, Table 1). Large roosts are uniformly oriented (Table
1). Early nests are oriented significantlydifferent from late nests and
small roosts. The orientation
of late nests and small roosts do not differ
significantly.
Early nestsare placed on the east side of trees (mean = 86ø, significantly different from uniform, Table 2). Late nests are placed on
the southwestside of trees (mean = 234ø, significantlydifferent from
uniform, Table 2). Roost placementis not significantlydifferent from
uniform (Table 2).
All nest types are placed at similar heights (early brood mean =
189 cm, late brood mean: 185 cm, large roost mean --- 199 cm, small
roost mean = 171 cm). Most late nestsare shadedwhile early nestsare
TABLE
2
SIDE O1' TREE PLACEiV•ENTO1' VERDIN NESTS
Side of
Early
Late
Large
Small
tree
brood
brood
roost
roost
(degrees)
(%)
(%)
(%)
(%)
0-30
12.8
16.9
9.5
8.7
40-70
14.5
4.8
6.8
9.7
80-110
19.0
5.3
10.1
8.7
120-150
160-190
18.4
10.6
6.8
15.5
8.1
12.8
7.8
7.8
200-230
240-270
280-310
320-350
6.7
2.7
5.0
10.1
9.2
12.1
14.5
15.0
9.5
10.1
19.6
13.5
13.6
17.5
14.6
11.7
N
Mean side of tree
179
86 ø q- 67 ø
207
234 ø q- 72 ø
148
Uniform
103
Uniform
April 1976]
Vetdin Desert Adaptations
TABLE
3
PLACEMENT O1ß BREEDING NESTS WITIt
Amount of shade
249
RESPECT TO 8ItADE
Early
Late
brood
brood
(%)
(%)
Deep shade (all day)
1.8
24.0
Deep shade (afternoon)
4.2
50.4
Not
shaded
Number
of nests
94.0
167
25.6
121
exposed (Table 3). Roosts are predominantly (85.8%) placed in forks
of branchesgreater than 0.5 cm in diameter; breedingnestsare generally
(79.2%) placedamongtwigslessthan 0.5 cm in diameter.
The majority of Temperate Zone speciesof birds build nests open
on top; some utilize cavities, which appear to afford more protection
to eggsand nestlings(Nice 1957, Ricklefs 1969). A number of species
constructenclosednests that combinesome of the advantagesof open
nests(relativelyinaccessible
placementnear tips of branches)and cavity
nests (energy conservationand protection from isolation and large
predators). That enclosednests conserveenergy at low temperatures
has been recognized(Kendeigh 1961, Ricklefs and Hainsworth 1969).
For the samereason,the enclosurecould prove disadvantageous
at high
environmentaltemperaturesbecauseof metabolicheat production. Mechanisms that prevent the nest from overheatingmust be present. Thus
nest placement must take into account nest structure and supportive
needsas well as protectionand nest environment. Flexibility and variability are favored to allow for spatial and temporal environmentalvariations.
Adaptations of the nest in respect to placement and environmental
variableshave been studiedlittle. Nest site selectionof Calliope Hummingbirds(Stellula calliope) was suggestedas a major factor in nesting
success(Calder 1971). CactusWrens (Campylorhynchus
brunneicapillus)
build an enclosednest,the entranceof whichis orientedaway from and
into the prevailingwinds during the cool and hot parts of the breeding
seasonrespectively(Ricklefs and Hainsworth 1969). Successof Cactus
Wren nestsorientedinto the wind during the hot part of the breeding
seasonis greater than those oriented otherwise (Austin 1974). This
probably helps moderate the nest temperature. Relation of nesting
successto nest placementis poorly known (Goddard and Board 1967,
Tenaza 1971).
Seasonspecificorientationswere found amongVerdin nests (Fig. 1).
250
GEORGET. AUSTIN
24
24. , MAY
MARCH
IS.
[Auk, Vol. 93
20øJUNE
o
20.
APRIL•
12.
8,
8-
4-
4-
0
I00
200
300
WIND DIRECTION
io
•'5 •o
WIND SPEED (KPH)
0
IOO
200
300
WIND DIRECTION (ø)
5
I'0 1'5 •0
WIND SPEED (KPH)
LAS VEGAS• NEVADA
24-
161284-
0
0
IO0
200
300
WIND DIRECTION (o)
I0
15
20
WIND SPEED (KPH)
0
I00
200
300
WIND DIRECTION(o)
5
I0
15 P'O
WIND SPEED (KPH)
TUCSON) ARIZONA
Fig. 2. Mean wind direction and speed in southern Nevada and southern Arizona
in relation to time of day (Nevada data from U.S. Weather Bureau, McCarran Field,
Las Vegas, Nevada; Arizona data from U.S. Weather Bureau, Tucson International
Airport, Tucson, Arizona adapted from Ricklefs and Hainsworth 1969).
The change in orientation is rather abrupt, occurring in late April
and early May (Table 4). As with the Cactus Wren (Ricklefs and
Hainsworth 1969), wind appearsto be the important factor governing
orientation (Fig. 2). The nest of the Verdin is built with the entrance
avoidingthe wind during the cool part of the season,possiblyto reduce
heat loss by convection. Late nests are oriented into the prevailing
winds (predominantwind directionnot significantlydifferent from nest
entranceorientation,Fig. 2). As wind directionthroughoutthe breeding
seasonremainsroughlythe same (Fig. 2), early and late nestsare oriented
very differently. These orientations may be of importance in moderating
the nest environmentby taking advantageof prevailing wind conditions
as suggestedby Ricklefs and Hainsworth (1969). Orientation into the
wind may increaseconvection,thus removing excessheat from the nest
or enhanceevaporativecoolingof the young and evaporationfrom fecal
sacs (see below). Although Taylor (1971) stated that Verdin brood
nestswere not orientedin any particular direction,he did not separate
early and late nests. Reevaluationof some of his data accordingto
season showed similar orientations to those I found.
April 1976]
Verdin Desert Adaptations
TABLE
251
4
CItANGES IN ENTRANCE ORIENTATION OF VERDIN NESTS•
Entrance
orientation
(degrees)
March
Early
April
Late
April
Early
May
Late
May
Early
June
Late
June
July
0
0-30
4
8
15
1
2
4
1
40-70
7
9
10
1
2
3
1
1
80-110
120-150
160-190
200-230
240-270
1
5
5
0
0
30
16
6
4
1
22
9
7
9
3
5
4
4
12
17
3
3
6
17
18
1
0
5
9
5
0
1
0
0
3
0
0
2
1
0
280-310
0
2
5
14
18
4
2
0
320-350
2
7
12
8
8
3
1
1
24
83
92
66
77
34
9
5
Number
of nests
• As number built during each time period.
The uniform orientation of large roostsis possibly related to lower
wind speedsat night as with the CactusWren (Ricklefs and Hainsworth
1969). Small roost orientation may be related to the time of year they
are built (late May throughJuly). These nestsare built and used at a
time when temperaturesreach their highest levels and little insulation
is needed.
Other variables involved in nest placement are more difficult to interpret. Nests are generally oriented with the entrance facing outward
from the center of the tree (Taylor 1971). Entrance orientation of early
nests is eastwards and nests are placed on the east side of the tree
(Tables 1, 2). Thus nestsare exposedto the warming sun in the cool
morning but are somewhatprotected from the afternoon sun. Similar
placement of late nests seems advantageous. Placement orientation is
similar to entrance orientation (Tables 1, 2), which probably results
from an engineeringlimitation imposed by branch structure that requires facing the nest away from the center of the tree. Late nests are
generallywell shaded,at least during the heat of the day (Table 3).
Such shadingreducesradiant heat load and thus reducesheat gain by
the nest through conductionand convection. Late nests placed on the
west side of the tree are somewhatlower (not statistically different)
than thoseplaced elsewhere.The strong tendency for a southwestentrance orientation (Fig. 1) in late nestsalong with successdata (below)
indicate that entranceorientation is the primary factor governinglate
nest placement.
Roost placement in larger forks is apparently related to support.
Roosts (especiallylarge roosts) are used for at least several months.
Thus placement on a strong support is advantageous. On the other
252
G•ORO•T. AUST•N
TABLE
[Auk, Vol.
5
l•ELATION OF SUCCESSAND ENTRANCE ORIENTATION OF EARLY AND LAT•
VERDIN NESTS1
Early
70-160
% nests found
Hatching success(%)
Late
170-60
220 310
320-210
45.1
80.9
53.9
87.52
46.1
64.4
54.9
91.72
No. eggs
108
68
136
No. nests
28
17
46
55
84.62
63.3
81.5•
44.6
Fledging success(%)
177
No. eggs
104
60
124
168
No. nests
27
15
42
53
92.6"
79.2
94.42
95
27
48
15
Nestlingsuccess(%)
No. hatch
No. nests
% successfulnests
107
34
70.8
106
39
100.0
86.7
76.2
58.5
66.7•
33.0
66.7'ø
26.4
% totally successful
nests
Preferred orientation given first =
100 continuous degrees containing the most nests.
Significantly different from nonpreferredorientation.
hand, breedingnests are used for a little more than a month and protection rather than long-term support is the prime requisite, although
thesenestsmay be usedfor roostingafter the youngfledge.
A few other speciesof birds have been shown to orient their nests
in relation to environmental variables. A predominant east and south
orientation of woodpecker nest entrances was attributed to problems
of nest climate and lighting (Lawrence 1967). Summer nests of larks
(Alaudidae) in Africa are strongly oriented on shaded sides of bushes
while winter nests are less clearly oriented (Maclean 1970). Horned
Lark (Eremophila alpestris), Water Pipit (Antbus spinoletta), and
meadowlark (Sturnella spp.) nests tend to face away from prevailing
winds (Lanyon 1957; Verbeek 1967, 1970). Such orientationswere attributed to protection from cooling winds and rain. Desert Lark (Arntoomanesdesertii) nests are placed in such a way that they are warmed
by the sun in the early morningbut are shadedand cooledby windslater
in the day (Orr 1970).
Although these orientationsare suggestive,the question remains as
to their biological significance.Successdata for southernNevada (2
years) and southern Arizona (2 years) demonstratethe importance of
orientation to success(Table 5). Nearly all criteria of successwere
higher for nests oriented in the predominant directions. This is one of
the few documentationsof a relationshipbetweenbreedingsuccessand
nestplacement(seeGoddardand Board 1967, Austin 1974).
Nest clirnate.--Temperaturesof nestscontainingeggsand nonthermogenicnestlingstend to be correlatedwith ambienttemperaturebut average
April 1976]
Verdin Desert Adaptations
253
40-
35-
ßßAA
ee•
ß..'/
15
5
,
I0
:::;:;:
-(/•, , , i-6iAYS
OLD
,
15
SHADE
20
AMBIENT
25
30
35
40
TEMPERATURE
Fig. 3. Relation of ambient temperature and nest temperature of Verdin nests
containing eggs and nonthermogenicnestlings.
a few degreeswarmer; nests containing thermogenicyoung are considerably warmer than ambient at low temperaturesbut are little above
ambient at higher temperatures(Figs. 3, 4). At shade temperatures
below 20øC nest temperatureaverages8 to 10øC greaterthan ambient,
at temperatures20 to 30øC nest temperatureaverages3 to 5øC greater,
and above30øC nest temperatureaverageslessthan IøC aboveambient.
At low ambient temperatures,nest temperaturecan be maintainedby
the sun, broodingby the female, and by metabolicheat. By brooding
at low ambient temperatures,the female can maintain nestling body
temperatures (Table 6). Brooding ceasesby the 9th or 10th day
(Taylor 1971, this study Table 6), at which time body temperature
control has been attained (Austin MS).
Maintenanceof near homeothermicbody temperaturesby nestlings
allowsthem to be normallyactive. Young in the laboratorywith body
temperatures
below20øC had difficulty raisingtheir headsto gape for
food and swallowing. Also the lack of well-developedhomeothermyof
young less than 10 days of age reducesenergy requirementsneededfor
active maintenanceof high body temperaturesat low ambient temperatures. Thus energyintake can be channeledinto growth.
At high ambient temperatures,nest temperaturescan be moderated
by shade, wind convection,and from evaporation of water from the
young or from fecal sacs retained in the nest cavity. Taylor (1971)
254
GEORGE
T. AUST•N
[Auk, Vol. 93
40'
ß
,<
.
Ae e/
e©
ße•
e
20-
•
5
15
I0
20
0 NEST
WITHYOUNG
25
SHADE AMBIENT
7-12
OAYS OLD
30
55
TEMPERATURE
Fig. 4. Relation of ambient temperature and nest temperature in Verdin nests
containing partially thermogenicand thermogenicnestlings.
reportedthat fecal sacswere removedfrom the nest by adult Verdins,
but Moore (1965) stated that nests are not kept clean. I found that
early nestsare generallyfree of fecal material but it tends to build up
in late nestsand forms a matted masswith the lining in the floor of
the nest. It is not uncommonto find a considerablebuildup of fecal
material at the entranceof late nests even while eggs are being in-
cubated. A similar situationwas found for the CactusWren (Ricklefs
and Hainsworth 1969). The latter further demonstratedthat experiTABLE
6
PERCENT OF TIiViE FEiViALE VERDIN FOUND INCUBATING OR BROODING IN RELATION TO
STAGE OF NESTING CYCLE AND AMBIENT TE35PERATURE•
Ambient temperature
Age (days)
Eggs
0-1
2-3
4-5
6-7
8-9
10-19
Total eggs,9 days
10-20
100 (2)
100 (2)
10o (4)
lOG (5)
100 (2)
75 (4)
0 (9)
95 (19)
20-30
67
71
100
5o
18
11
0
(6)
(7)
(4)
(4)
(11)
(9)
(27)
44 (41)
30-35
30
25
14
0
0
0
0
(10)
(4)
(7)
(6)
(2)
(4)
(15)
15 (33)
• Number of observationsin parentheses,all observationsbetween sunrise and sunset, males do not
incubate.
April 1976]
Verdin Desert Adaptations
255
80
ß
70
ß
ß
ß
ß
ß
õ0
ß
ß
ß
ß
ß
ß oo
ß
0
!
!
40
50
TEMPERATURE
Fig. 5. Percent of time spent by foragingVerdins in exposedmicrohabitatsas a
function of temperature.
mental addition of a small amount of water to wren nests significantly
lowered nest temperature. Evaporation from fecal sacs thus may contribute to nest cooling.
Temperaturerelated behavior.--Time spent foraging in various microTABLE
7
TINiE SPENT FORAGING BY Tt•E VERDIN IN VARIOUS MICROt•ABITATS AS A FUNCTION OF
ANiBIENT TENiPERATURE
Low
T,,
(øC)
20-25
25-30
30--35
35-40
40-45
45-50
Winter
Total time
(sec)
911
6667
5963
2640
15271
7343
7032
exposed
(%)
High
exposed
(%)
Shade
(%)
20.2
22.4
16.4
10.6
28.6
26.0
35.6
27.9
42.8
35.6
40.4
48.1
5.2
0.0
52.9
14.5
4.1
__2
51.4
26.6
47.1
Deep
Diversity of
shade
(%)
microhabitat
use (H') •
8.3
16.1
7.5
13.4
28.9
69.4
__2
1.02
1.03
1.O2
0.89
0.61
0.17
__
• H t = -ZPilogepi (see Lloyd et aI. 1968) maximum H• = 1.10 if all three microhabitats (low
exposed,high exposed,shaded) used equally.
• 0nly exposedand shaded microhabitats were distinguished in winter.
256
GEORGET. Ausxx>t
[Auk, Vol. 93
25'
20-
15-
10-
Fig. 6.
Relation of foraging intensity of the Verdin and ambient temperature.
habitats as a function of ambient temperature is presentedin Table 7.
As ambient temperatureincreases,the percent of time spent in exposed
microhabitats (low and high exposed) decreases(Fig. 5). Deep shade
is the principalmicrohabitatutilized at temperaturesgreaterthan 45øC.
Two indices of foraging intensity demonstratea marked decreaseat
higher temperatures(Figs. 6, 7). The rate of feeding nonthermogenic
young remains constant over a wide temperature range apparently a
responseto their lower energy requirements.Older young are fed more
rapidly at low ambient temperaturesthan at high (Fig. 7) reflecting
a reductionin the intensity of adult foragingbehavior (Fig. 6). The
proportion of time spent in various activities varies considerablywith
ambient temperature (Austin MS). Foraging is the principal activity
at temperaturesbelow 35øC, taking more than 80% of the time. Above
35øC foraging is greatly reduced and long periods of inactivity are
predominant.
By foragingin the shade,the Verdin can continueto forage at high
ambient temperatures and keep heat stress to a minimum. Average
temperature encounteredby the Verdin while foraging was calculated
from microhabitat temperaturesand the proportion of time spent in
each (Fig. 8). These data indicatethat, if possible,the Verdin restricts
April 1976]
Verdin Desert Adaptations
257
25-
YOUNG
ß
ß
ß
ß
ß ß
ß
ß
ß
ß
12-18 DAYS OLD
ß
ß
ßAe ß
DEEP SHADE BLACKBULB TEMPERATURE
Fig. 7. Relation of number of feedingvisits to nestsby Verdins and ambient
temperature (N :
8 nests).
its foragingactivitiesto a mean temperaturebelow 38øC. Otherwisethe
coolestmicrohabitatis usedalmostexclusively.Of interest,and possible
significance,
is that pantingis initiatedby nestlings(in the laboratory)
at ambient temperaturesof 38øC and above (Austin MS). The diversity of microhabitats used at different temperatures decreasesas
temperatureincreases(Table 7) similar to that reported for the Cactus
Wren (Ricklefs and Hainsworth 1969). At temperaturesabove 40øC,
shaded microhabitats are used almost exclusively. A shift to cooler
microhabitatsconcurrentlywith a decreasein foragingintensity (Fig.
6) reducesheat gain from the environmentand metabolic means respectively. The reductionin intensity of activity at higher temperatures
is even more striking than indicated by foraging intensity in Fig. 6.
Foragingactivity is greatly reducedand frequentlyinterruptedat higher
temperatures
by intervalsof no activity, often severalminutesin length.
In winter shadedand exposedmicrohabitatsare used nearly equally
(Table 7). Intensity of foraging at this seasonappearedto be at the
summermaximum (•: 21.7 perchchangesper minute, N = 51).
A few data for cloudy periodsduring midday indicate that changesin
behaviorare responses
to temperaturerather than time of day (Fig. 9).
258
GEORGE
T. AUSTIN
[Auk, Vol. 93
40'
35'
30-
25-
•o
3'0
MINIMUM
BLACKBULB
3'5
4'0
4'5
TEMPERATURE
Fig. 8. Temperature microhabitat of the foraging Verdin as a function of temperature (line representsminimum temperature).
A similar pattern was found in the behavior of Black-tailed Gnatcatchers
(Polioptilamelanura)and CactusWrens (Smith 1967, Ricklefsand
Hainsworth1968). CaptiveRoadrunners
(Geococcyx
californianus)
at
temperatures
belowstresslevelswerehighlyactiveduringmidday,indicatingthat temperature
is a factorin the middayactivity depression
in
thisspecies
(Calder1968,KavanauandRamos1970). Thermoregulatory
problems
weresuggested
for the middayreductionin activityof MangroveSwallows(Iridoprocnealbilinea)in Panama (Ricklefs 1971).
Amountof timespentforaging
by Yellow-billed
Magpies(Pica nuttalli)
was depressed
at high temperature
exceptduringthe breedingseason
(Verbeek1972). Changes
of distributionor activityof insectsand other
prey may alsobe a factor, but it has been shownthat, other factors
beingequal,temperature
is the factorregulating
CactusWren foraging
activity (RicklefsandHainsworth1968).
Little informationis available on comparativebehaviorof birds in
more moderateenvironments.Severalhave suggested
environmentalfactors may be of importancein nest placementand nest entranceorientation (Lanyon 1957; Verbeek1967, 1970; Lawrence1967; Ricklefsand
Hainsworth 1969). A relationshiphas been demonstratedbetween fre-
quencyof activityandmicrohabitat
usageand temperature
amongdesert
birds(Smith1967,Calder1968,Ricklefsand Hainsworth1968,Ohmart
1969). Many birdsdecrease
their activity at middayduringmuchof
the year but this doesnot appearto be temperaturerelated (e.g. see
April 1976]
Verdin Desert Adaptations
259
80
t
ß
70
ß
ß
60
ß
ß
5O
0
ß
40
ß
$0
ß
ß
ß
ß
20
o
o
ß
I0
ß
0
i
0600
I
0800
!
I
I000
1200
1400
1600
TIME OF DAY (POT)
Fig. 9. Percent of time spent by foraging Verdins in exposedmicrohabitats as a
function of time of day (circled points indicate data taken during cloudy periods,
data replotted from Fig. 5).
Morton 1967, Coutlee 1968, Smith et al. 1969). Temperaturerelated
behaviorof desertbirds may be a refinementof this midday depression,
but more of a responseto temperature than to time of day. Data available for the Roadrunner, Cactus Wren, and Verdin seem to indicate
this (Calder 1968, Ricklefs and Hainsworth 1968, Kavanau and Ramos
1970, present study). This reasoningis in agreementwith physiological
data for desert and nondesert birds.
ACKNOWLEDG1V•ENTS
I thank W. K. Taylor for supplying his raw nest orientation data. E. L. Smith
and
P.
Gould
made
available
their
data
on nest
orientation
and
success.
W.
G.
Bradley, G. Clark, R. E. Ricklefs, E. L. Smith, and S. Speich critically reviewed
the manuscript and their suggestionsare gratefully acknowledged. Thanks are also
due to R. Raitt for bringing my attention to Moore's unpublishedMaster's thesis.
SUMMARY
Vetdin breeding nest orientation is seasonspecificwith the entrances
of early and late nestsoriented to avoid and face respectivelythe prevailing winds. Nesting successis greater in nests oriented in the pre-
260
GEoagE T. AUSTIN
I Auk, Vol. 93
dominant directions. Verdins forage at all ambient temperaturesthey
encounterbut greatly reducethe intensityof and amountof time spent
foraging at temperaturesabove 35øC. There is a concurrent shift of
activity from exposedto shaded microhabitats. It is suggestedthat
temperaturerelated behavioris a refinementof the generalmidday depressionof activity but in responseto temperaturerather than to time
of day.
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