Incubating Female Passerines Do Not Let the Egg Temperature Fall below the 'Physiological Zero Temperature' during Their Absences from the Nest

Egg temperature was recorded at 1-5 min intervals during the incubation period in 42 nests of 14 different species. Only the females incubated. The egg temperature therefore declined whenever a female left the nest for a period-off. Regardless of the ambient temperature, however, the females usually returned to their nests and continued incubating before the eggs had cooled down to the suggested 'physiological zero temperature' (25-27°C). For 14 nests (6 species) in which egg temperature was measured at 1 min intervals, the mean minimum egg temperature (MET) during periods-off was 30.4°C. The mean value for all 42 nests, with recording intervals varying from 1-5 min, was 31.2°C. It is suggested that the incubation rhythm adopted is adaptive and evolutionary aspects of the high MET-values are tentatively discussed. During periods of especially low ambient temperature the female either holds the egg temperature at a normal level or temporarily stops incubating altogether. Any intermediate solution seems to be ruled out.