Seasonal Variation in Nest Placement of Abert's Towhees

Abstract

Several meteorological variables are known to influence the heat budgets of nesting birds (e.g., Walsberg and King 1978a, b). Such factors include air temperature, incident radiation, wind, and humidity. If the microclimate of the nest is unfavorable, parent birds may become inattentive, exposing eggs or nestlings to excessive heat or cold. They may then desert the nest and their offspring may die. To improve the probability of nesting successfully, many Old World desert birds place their nests so as to receive morning sunlight (Maclean 1976). Several investigators have demonstrated nonrandom orientation of hole entrances of New World cavity-nesting birds (Ricklefs and Hainsworth 1968, Inouye 1976, Inouye et al. 198 1), but few have tested for the significance of nonrandom placement or variation in placement between seasons of open nests. Maclean (1976) indicated that larks in the Kalahari Desert oriented their nests to receive shade in summer and sun in winter. Balda and Bateman (1970) predicted that Pinyon Jays (Gymnorhinus cyanocephalus) in Arizona may place first nests of the season on the sunny sides of ponderosa pines (Pinus ponderosa), but that they may orient later nests more randomly. They showed that these trees in March were 30C warmer on the south side than on the north side, and that the jays selected nest sites to gain more heat: all jay nests, regardless of season, had a significant southward orientation. I report here a second test of Balda and Bateman's (1970) hypothesis, using data for a bird of hotter climates, the Abert's Towhee (Pipilo aberti). This sedentary species inhabits only desert riparian regions of the southwestern United States, whereas the mobile Pinyon Jay occupies a greater elevational and latitudinal range, though breeding chiefly in foothills and lower mountain ranges. Because nests of the Abert's Towhee are typically exposed to higher spring and summer temperatures than those of the Pinyon Jay, a second test of Balda and Bateman's prediction may demonstrate greater seasonal variation in nest placement. The study was conducted in honey mesquite (Prosopis glandulosa) habitat on the Colorado River Indian Reservation 10 km north of Ehrenberg, Arizona. From January to July 1980, I spent 15 h each week (84 5-h searches) looking for nests on, or near a gridded study plot of 20 ha. Field work terminated in July when no new nests were started.