The Rhythm of Incubation from Egg Laying to Hatching in Mountain White-Crowned Sparrows

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Abstract
The rhythm of incubation in mountain white-crowned sparrows developed gradually during the egg laying period and a consistent pattern began with laying of the penultimate egg. Behaviors of incubating females were highly flexible and adaptable to conditions at the nest. During environmental fluctuations females partitioned their activity to provide adequate time for foraging while maintaining egg temperature (Tegg) within normal limits. Abilities of embryos to withstand low and oscillating temperatures allowed females to forage during the coldest hours of the day. Both air temperature (Ta) and exposure of the nest to direct sun influenced the daily rhythm of incubation activities. Mean duration of periods on and off the nest increased with increasing Ta. In some individuals length of periods off decreased with increasing Ta, especially when nests were exposed to sun. Females remained on the nest at such times to prevent heating of embryos to lethal temperatures. There are indications that the onset and cessation of daily incubation activity are cued by photoperiod or light intensity.