Collared Flycatcher (Ficedula albicollis) females experimentally were forced to prolong their incubation to address the question whether mass constancy during incubation and subsequent mass loss after hatching is actually related to breeding stage. Compared to unmanipulated control females a week after expected hatching, experimental females did not show any significant mass loss during prolonged incubation, whereas control females that successfully hatched their eggs dropped their mass significantly. Results show that body mass in females is associated with the reproductive stage and may reflect an adaptive strategy. High and stable incubation mass can be a fasting endurance in case of adverse weather conditions when females stay on the nest instead of foraging.