Eggs of intermittently incubating birds are periodically rewarmed by a transient pulse of heat from the parent's brood patch. Estimating the energy cost of rewarming such an egg requires knowledge of the egg's thermal capacity, typically assumed to be the product of the egg's mass and its specific heat, designated here as the gravimetric thermal capacity. When chicken eggs are transiently warmed by an artificial brood patch, the energetic costs of the rewarming indicate that they have thermal capacities about one-third the gravimetric thermal capacity. In this article, I show that birds' eggs warmed locally by a brood patch have effective thermal capacities that differ substantially from the eggs' gravimetric thermal capacities, both in absolute magnitude and in response to varying the temporal properties of the transient pulse of heat. An effective thermal capacity exists because heat from a brood patch flows unevenly through an egg and because of thermal impedance effects on the unsteady component of heat flow into the egg. If these conditions in any way characterize the rewarming of eggs by intermittently incubating birds in nature, intermittent incubation may be considerably less costly in time and energy than has heretofore been assumed.