Abstract 1. 1. Intermittently incubated eggs are alternately warmed by the brood patch during nest visits and allowed to cool during absences from the nest. 2. 2. The thermal energetics of such eggs are not governed by conventional measures of thermal energetics, like thermal conductance or thermal resistance, which only measure the steady-state component of heat flow through an egg. 3. 3. The transient component of heat flow that intermittently incubated eggs experience is limited by by a time-dependent analog of thermal resistance, the thermal impedance. This paper outlines methods for measuring the thermal impedance and other transient-state properties of contact-incubated birds' eggs. Data are presented on these properties for infertile chicken eggs warmed by an artificial brood patch. 4. 4. The infertile chicken egg behaves analogously to a low-pass thermal filter, in which slowly-varying heat inputs modify the temperature of the egg more than rapidly-varying heat inputs. 5. 5. At heat inflow period of 60 min or less, the thermal impedance rises sharply above the thermal resistance. 6. 6. Time constants of contact-incubated chicken eggs are about 20 min, considerably shorter than the 42 min observed in chicken eggs cooling in air. 7. 7. The ‘effective thermal mass’ of a chicken egg, that is the portion of the egg experiencing transient heat flows increases with period of the heat input. As a result, time-varying heat inputs with periods less than 60 min will have little influence on egg temperatures of an egg incubated via a brood patch.