Abstract
1. 1. Prior to around day 18 of incubation, chicken embryos are apparently poikilothermic. No compensatory increase of metabolic rate is evident when the embryo is gradually cooled. 2. 2. At about day 18 of incubation, a weak metabolic response to egg cooling appears. 3. 3. After external pipping, the metabolic response to gradual cooling is stronger. The embryo need not emerge from the egg for the compensation to occur. 4. 4. We suggest that precocial hatchlings in ovo may exhibit incipient endothermic homeothermy. Full homeothermy may be prevented by the low gas conductance of the eggshell, effectively “throttling” the embryo's heat production capacity. This is a constraint altricial birds probably never experience.
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