Incubation is an important component of avian parental care and slight changes in incubation temperature can affect offspring phenotype. Although many extrinsic and intrinsic factors may generate variation in incubation temperature, they remain underexplored under natural conditions. Using a robust data set encompassing 55 nests, 22 816 behavioral observations, and > 1 million paired ambient and egg temperatures, we describe the relationships among abiotic factors, female incubation behavior, incubation temperature, and incubation period for tree swallows Tachycineta bicolor. We report a large amount of individual variation in incubation behaviors and average incubation temperatures for our study population. The average on-bout incubation temperature was 34.1 degrees C, with daily egg temperatures ranging from 18.0-39.2 degrees C. Females modulated the number of times they left the nest and the amount of time they stayed off the nest according to interactions between precipitation and temperature patterns. Models generated from our observations predicted that the number of female off-bouts was the lowest under warm and dry conditions while more off-bouts were taken under cold and dry or warm and wet conditions. During cold and dry conditions, females stayed off their nest approximate to 4 times longer than under warm and dry conditions. However, this pattern was reversed under periods of rainfall; females tended to take shorter off-bouts when it was rainy and cold compared to longer off-bouts during warmer rain events. Furthermore, variation in female behavior was associated with differences in overall incubation temperature such that females that maintained greater incubation constancy produced higher incubation temperatures at a given ambient temperature than those that displayed lower incubation constancy. Our results provide perspective on the timing of breeding, as some of the advantages of breeding early may be countered by cooler, early season temperatures and precipitation that cause reproducing females to favor self-maintenance at a potential cost to optimal incubation temperatures for offspring development.
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