Using an automatic identification and weighing system, we investigated changes in adult body mass in relation to reproductive behaviour during courtship and incubation in free-living king penguins. Despite stressful nutritional conditions and variability of fast length, the majority of pairs incubated successfully by accumulating large body reserves before fasting, which provided flexibility in fasting strategies. Our data indicate a low body mass during fasting below which males either delayed or stopped breeding: (1) at 12 kg, the male interruped courtship to replenish his body reserves at sea before re-engaging in courtship; and (2) when body mass dropped to 9 kg during incubation, the male deserted the egg. The behavioural decision to go to sea was not controlled by the time spent fasting, but by the amount of body reserves. In deserting males, the depletion of body reserves during incubation before relief by the females was due to a lower stored energy (−2 kg) at the onset of courtship compared with successful males. Unsuccessful males weighed only 12 kg when they started to court, and consequently had no safety margin that allowed them to wait for a delayed female. The unusual depletion of body reserves of male breeders caused only 3% of incubation attempts to fail. Compared with successful pairs, their female partners gained less body mass at sea (−40 g/day) and made a slightly longer foraging trip (+5 days). We suggest that the deserting males had risked breeding with the lowest fasting safety margin possible, rather than breeding later that year. However, the low body mass at desertion did not affect the penguins' survival or their feeding capacity, and therefore did not compromise another breeding attempt during the next season.