Transient-State Thermal Properties of Contact-Incubated Chicken Eggs

Abstract

Chicken eggs subjected to a sinusoidally varying input of heat from a brood patch experience sinusoidally varying egg temperatures. As the embryo matures, steady and transient egg temperatures become more uniform, declining about 0.8° C at the brood patch and increasing from 0. 4° to 0. 8° C in the more peripheral regions of the egg. The increasing homogeneity of egg temperature is reversed by killing the embryo. Steady resistance of the egg declines from 41.9° C W⁻¹ at the start of incubation to an average of 33.8° C W⁻¹ by the end and is independent of the period of variation of brood patch heat output. Capacitative reactance of the egg declines about 10% through incubation in concert with an average 10% mass loss by the egg. Capacitative reactance is strongly dependent on the period of the heat input, increasing from 27.8° C W⁻¹ at a period of 120 min to 151° C W⁻¹ at a period of 20 min. Thermal impedance declines roughly 12% throughout the incubation period, which is attributable in similar parts to increasing circulation within the egg and to the declining mass of the egg. The time constant for the warming of a contact-incubated egg declines from 13.1 min at the start of incubation to 11.4 min at the end. There is no evidence of hysteresis in the heating and cooling rates of the egg. The time constant for temperature change of a contact-incubated chicken egg is considerably shorter than that of an egg cooling in air. The importance of these results for understanding the energetics of intermittent incubation is discussed.