The Regulation of Egg Temperatures and Attentiveness Patterns in the Dusky Flycatcher (Empidonax oberholseri)

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Abstract
Egg temperatures (Tegg) and attentiveness were measured in E. oberholseri breeding at high altitude in the Sierra Nevada of California [USA]. Eggs were tended in daytime during the laying period but were above the assumed threshold temperature for development (28.degree. C) for only 17.6% of the time. Mean Tegg increased during the laying period and for the first few days of incubation. This may have been related to brood patch development. Full-time incubation began at night following the laying of the penultimate (6 cases) or prepenultimate egg (3 cases). Once incubation began, there was little diurnal variation in mean Tegg. Eggs were above developmental threshold for 92.2% of the time during the active day and 99.9% of the time during night rest. Mean Tegg for 5 nests for the full period of incubation was 37.62.degree. C. Trends in Tegg indicated that body temperature (Tb) of incubating females decreased at night when ambient temperature (Ta) was between 12.degree. C and 4.degree. C. Shivering thermogenesis probably was employed at Tas below 4.degree. C. During storms, despite heavy hail and rain, females endured on the nest for long periods. If the storm continued, they eventually left to feed, thus exposing eggs to possible damage. When potent thermolytic conditions were coupled with food shortage, as during large storms or prolonged episodes of cold, windy weather, incubating females neglected their eggs in order to extend foraging time. They also allowed eggs to cool at night by withdrawing their brood patch. This apparently increased their ability to defend Tb. Attentiveness varied diurnally, being highest at midday. Duration of foraging trips varied little with time of day or Ta, but attentive bouts were longest at midday. The rate of foraging bouts was highest at the beginning and end of the active day. Unlike many species, total attentiveness in E. oberholseri was correlated positively with Ta. This may be a general pattern in open-nesting birds at high altitude that has evolved because of the need to protect eggs from damage by solar radiation. Repeated excursions of eggs to low temperatures or outright periods of neglect during cold weather did not seem to affect embryo survival.