Temporal pattern of motor activity of chicks during the early posthatching period

The motor activity of domestic chicks was continuously recorded in running-wheels to ascertain whether it exhibits predictable temporal variations during the first 3-4 days after hatching. As recorded in constant light and several different temperatures, this activity was characterized by a general increment after the first day, by irregular fluctuations over intervals of a few hours, and by regular, near 24-h cycles commencing after an initial period of variable activity. The phase pattern of the cycles of normally hatching chicks correlated consistently with posthatching age. Artificially advancing and delaying emergence from the shell shifted the phases accordingly, thus indicating that the phase pattern, rather than being developmentally scheduled, was determined by initial light exposure and, perhaps, slight temperature change at the time of shell emergence. The cycles, indicative of a circadian rhythm, provide a prospective basis for predicting variations in other aspects of behaviour.