Partial incubation and its function in great tits (Parus major)—an experimental test

In many bird species, females incubate the eggs for short periods before clutch completion. The role of such partial incubation and the effective egg temperatures attained are controversial. Possible functions depend on whether temperatures reach the critical values required for embryogenesis and include the protection of the clutch from predators, brood parasites, and environmental factors; inhibition of bacterial growth on eggs; control of hatching asynchrony; or energy saving by parents. In this study, in a natural population of great tits, we first investigated the occurrence and extent of partial incubation, and second, we evaluated some potential functions by experimental simulation of the observed patterns of partial incubation. We found that egg temperatures during partial incubation were clearly raised above the physiological zero temperature for embryo development. Simulation of partial incubation did not affect any breeding parameters or nestling morphological traits compared with controls. It suggests that partial incubation with temperatures above the minimum temperature for embryo development does not serve to increase the variation in nestling morphology and thus competitiveness, in particular, does not increase hatching asynchrony within brood as one of its currently most discussed functions.