The Effect of Egg Temperature on Attentiveness in the Belding's Savannah Sparrow

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Abstract
We manipulated egg temperatures in nests of the Belding's Savannah Sparrow, (Passerculus sandwichensis beldingi), an open-nesting passerine of which only the female incubates. Females increased their attentiveness by 22.0% when we cooled their eggs and decreased their attentiveness by 28.4% when we warmed their eggs. The range of adjustment approximated the upper and lower limits of attentiveness observed for our population during one breeding season; adjustments were primarily made by changing the length of both attentive and inattentive periods. Our results indicate that these birds adjust their attentive time in direct response to fluctuations in egg temperature, independently of diurnal fluctuations in air temperature. They also support the hypothesis that attentiveness is ultimately controlled by an internal rhythm and is only modified by, but not completely explained by, responses to egg temperature.