Extended Laying Interval of Ultimate Eggs of the Eastern Bluebird

Proximately, clutch size is determined by the termination of the sequential pattern of egg formation and laying. Egg laying is difficult to study, and documentation of detailed patterns is scarce. We used archived video recordings of the Eastern Bluebird (Sialia sialis) to contrast the times of day of laying with variability in egg-laying intervals. Ultimate eggs were laid over a significantly longer interval than previous eggs. The extended interval over which the ultimate egg is laid could be a symptom of physiological constraints in the formation of the ultimate egg, extra resources provided to compensate the ultimate egg, and/or a late egg may become the ultimate egg because being late prevents further ovulation.