Context-dependent costs of incubation in the pied flycatcher

► Behaviours involved in manifesting costs of avian incubation are largely unknown. ► We examined such behaviours in relation to clutch size in pied flycatchers. ► Enlarged clutches received more incubation, at the cost of lowered egg temperature. ► This resulted in a protracted incubation period and reduced nestling production. ► Thus, clutch enlargement may ultimately affect optimal brood size.