THE CONDOR

The egg temperature (T,_.J of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) was measured throughout incubation and under natural variations in environmental conditions at high altitude. Information on Tegg, air temperature (T,), nest placement and attentiveness patterns was integrated in order to elucidate adaptive forms of incubation behavior. Despite wide daily variations in ambient conditions, incubating females were able to maintain mean T,, between 34 and 38°C. Large oscillations in Tegg (17.843.o”C) occurred when females were foraging, especially during early morning and late afternoon when T,s were low. Apparently low and fluctuating temperatures were not detrimental to embryo development. High Teggs did not occur because females prevented exposure of eggs to solar heating by remaining on the nest during critical periods. Mean T,, was significantly higher in ground than in aerial nests. Habitat availability seemed to be of major importance for selection of nest height. Orientation of nests with respect to the vegetational mass in which they were built was non-random and was probably related to prevailing winds or to sun exposure. To assure hatching, parent birds must regulate the temperature of their eggs under conditions that are often highly variable and thermally stressful. Parental behaviors during incubation must therefore be highly restricted in order to maintain thermal conditions appropriate for embryonic development. Hence, selective forces are focused on such behaviors, and studies of thermal conditions of nests and eggs, coupled with simultaneously gathered data on patterns of parental attentiveness, should enlarge our understanding of avian incubation. In early studies of egg temperature (T,pg) during incubation, measurements were short-term and were not made under conditions representative of natural variation in environmental conditions. These published data tend to portray incubation as a process wherein T,, is rather constant throughout and the same among species (Huggins 194 1, Irving and Krog 1956, Drent 1970, 1975). This image was enforced by data, derived primarily from domestic fowl, showing the importance of a constant optimum Tegg to development and hatching success (Romanoff and Romanoff 1949, Lundy 1969). This concept is changing, stimulated by recent advances in technical expertise as well as the realization that the egg, together with the incubating bird with its nest and clutch, can be viewed as self-contained systems that lend themselves to precise measurement and modelling (Kashkin 196 1, Calder and Booser 1973, Ricklefs 1974, Mertens 1977a, b, Walsberg and King 1978a, b, Carey 1980). Newer studies indicate that embryos of many birds experience wide ranges in temperature during their development and are resistant to chilling but rather susceptible to overheating. Adaptation to chilling is evident in that conductance of the eggshell to water vapor is low in such species (Rahn et al. 1977); although development may be arrested during cold spells, successful hatching occurs (Matthews 1954, Spellerberg 1969, Boersma and Wheelwright 1979, Wheelwright and Boersma 1979, Vleck and Kenagy 1980). Adaptation to heat stress involves marked adjustments in parental attentiveness, which include such responses as shading eggs with body and wings (Howell and Bartholomew 1962) incubating continuously (Russell 1969) or by soaking eggs with water 2 EILEEN ZERBA AND MARTIN L. MORTON carried in breast feathers (Yom-Tov et al. 1978, Howell 1979). Nest location and microclimate may also help to protect embryos from exposure to temperature extremes and reduce thermal stress to the attending parent (Howell and Dawson 1954, Calder 1973, Walsberg and King 1978a, b, Schaefer 1980, Verbeek 198 1, Vleck 198 1). Additional evidence of adaptive reproductive performance should be forthcoming when avian incubation is studied at locations, such as high altitude, where weather conditions fluctuate markedly and unpredictably. For this reason we chose to study the dynamics of incubation in the Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha), a migratory finch that breeds in high mountain meadows of the Sierra Nevada of California. Throughout their nesting season, large daily oscillations in temperature, wind, precipitation, and insolation often occur. Air temperature (T,) is usually near freezing at night, brisk cold winds and storms are common, and solar radiation in the daytime can be intense. At the beginning of their nesting season, most of the ground is often covered with snow, and vegetation is sparse. Females must select nest sites, subsequently protect eggs from freezing at night and overheating during the day, as well as shelter them from wind and precipitation. These conditions undoubtedly provide strong selective pressure for adaptive modifications of parental behaviors as well as embryo biochemistry. MATERIALS AND METHODS