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Behavioral Adaptations of the Verdin to the Desert

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Behavioral Adaptations of the Verdin to the Desert BEHAVIORAL ADAPTATIONS OF THE VERDIN TO 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 homeostaticstate under suchconditions involves the opposingre- quirements of maintaining water balance and thermoregulation. The harshnessof the desertis well documented.Any trait that lessensheat stressand reduceswater loss is adaptive. How residentorganisms have adapted to, or avoided,these featureshas been a field of fruitful re- searchin recentyears (see Brown 1968, Farner and King 1971, for reviews). Certain desertmammals are strikingly specializedcompared to non- desert forms in ways that can be correlatedwith environmentalex- tremes (Dawson 1955; Schmidt-Nielsenet al. 1957, 1967; Schmidt- Nielsen1958, 1964; Bartholomewand Hudson1960, 1961; Hudson 1962; Carpenter1966; and others). The generalphysiological similarity of desertand nondesertbirds, however,has led to the conclusionthat birdshave no specialadaptations to the desert(Miller 1963). Yet subtleadaptations and preadaptationsoccurring in the classas a wholeenable many species to inhabitthese hot, arid regions.Preadaptive featuresof birds are discussedby Dawsonand Bartholomew(1968). Severaldata are availablethat suggestsmall physiologicalrefinements rather than strikingadaptations are commonamong desert birds. These includelower basal metabolic rates, higher evaporative cooling efficiencies, and greatertolerances for high ambienttemperatures in desertspecies (or populations)compared with closelyrelated forms occurring in more roesicenvironments (Salt 1952, Dawson 1954, Hudson and Kimzey 1966,Rising 1969, Carey and Morton 1971,Henderson 1971). Breeding of the Abert's Towhee (Pipilo aberti) and Rufous-wingedSparrow (Aimophilacarpalis) in responseto rain ratherthan photoperiod(Mar- shall 1963,Ohmart 1969) is anotheradaptation of this type. Several recentstudies indicated that somedesert species are physiologicallymore efficient in conservingwater than nondesertspecies (Smyth and Bar- tholomew1966, Willoughby1968, Ohmart and Smith 1970). Preadaptive and physiologicalfeatures alone appear insufficient to allowmany species to adaptto the desertcompletely. Behavioral adjust- ments appear to be of major importanceas adaptationsto the desert judging from the few data available. For example,some species reduce 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 smaller desert birds (6.5 to 7.5 g). Becauseof its size it is faced with the greaterproblems incurred by surface-volume relationshipsthan larger species,and studiesof its adaptationsare de- sirable. The life history of the Verdin was studied by Moore (1965) and Taylor (1971). Goldstein (1974) made limited measurementsof 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. Breedingnests were 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: exposed portions 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] Verdin Desert Adaptations 247 LATE N=206 EARLY N=182 30 Fig. 1. Entranceorientations of Verdin brood nests. reasonablysure of what portionsof thesetrees would be exposedor shadedif it were not cloudy. Observationson cloudy days when temperatureswere relatively cool and the four microhabitatswere nearly isothermicserve as a control for com- parisonwith clear, hot midday conditions. Temperatureswere obtained concurrentlyfor each microhabitat. Foraging obser- vations 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 postbreedingpairs). When a foragingbird was encountered,the number of perch changeswere countedover a stopwatch-timedinterval. Observationsless than 2 min in lengthwere discarded.When a bird stoppedforaging, was out of my sight for more than 10 sec,or made a flight of over 15 m, observationswere ended. Con- currentlydeep shade temperatures were measured. Rates of nestvisitation (to feed young) were counted visually. Statistical significanceis defined throughout at the 0.O5 level. RESULTS AND D•SCUSS•0N Nest placementand orientation.--Nestentrance orientations 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, significantlydifferent from uniform, Fig. 1). Small roostsare similarlyoriented (mean: 264ø, significantlydifferent 248 G•OROET. AUSTIN [Auk, Vol. 93 TABLE 1 ENTRAI•CE ORIENTATIONS OF VERDIN N•STS Entrance Early Late Large Small orientation brood brood roost roost (degrees) (%) (%) (%) (%) 0-30 •t2.6 6.3 9.2 9.7 40-70 14.3 4.4 7.2 5.8 80-110 29.7 4.4 8.6 2.9 120-150 15.9 3.9 11.2 7.8 160-190 8.2 9.2 9.9 8.7 200-230 4.4 20.9 9.9 14.6 240-270 1.6 20.9 15.1 24.2 280-310 2.7 18.9 10.5 17.5 320-350 10.4 11.2 18.4 8.7 N 182 206 152 103 Mean orientation 84 ø q- 59 ø 257 ø q- 61 ø Uniform 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, sig- nificantly 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 nests are 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 18.4 6.8 8.1 7.8 160-190 10.6 15.5 12.8 7.8 200-230 6.7 9.2 9.5 13.6 240-270 2.7 12.1 10.1 17.5 280-310 5.0 14.5 19.6 14.6 320-350 10.1 15.0 13.5 11.7 N 179 207 148 103 Mean side of tree 86 ø q- 67 ø 234 ø q- 72 ø Uniform Uniform April 1976] Vetdin Desert Adaptations 249 TABLE 3 PLACEMENT O1ß BREEDING NESTS WITIt RESPECT TO 8ItADE Early Late brood brood Amount of shade (%) (%) Deep shade (all day) 1.8 24.0 Deep shade (afternoon) 4.2 50.4 Not shaded 94.0 25.6 Number of nests 167 121 exposed (Table 3). Roosts are predominantly (85.8%) placed in forks of branchesgreater than 0.5 cm in diameter; breedingnests are generally (79.2%) placedamong twigs less than 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).
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