Differential response by males and females to manipulation of partner contribution in the great tit (Parus major)

1. In birds with bi-parental care, handicapping is often assumed to decreas e the amount of parental care of the handicapped partner. We discuss how ha ndicapping could alter the shape of the handicapped bird's survival-effort curve (theoretical curve relating the survival of a parent to its effort) a nd show that the optimal response could yield a decrease, no response or ev en an increase in effort of the handicapped bird. 2. Male or female great tits Parus major (L.) were handicapped during the n estling period by clipping a number of feathers in order to study the effec ts on parental care and body condition. 3. Handicapped males significantly decreased their feeding rates, while han dicapped females did not. Condition of handicapped females significantly de teriorated, while condition of handicapped males did not change during the experiment. Females with a handicapped partner fully compensated for their partner's decrease in work rate, while males with a handicapped partner did not show any compensation and even tended to decrease their feeding rates. 4. Using an inverse optimality approach, we reconstructed the theoretical c urve relating the survival of a parent to its effort on the basis of the ex perimental effects. The handicapped male's survival-effort curve appeared t o be slightly steeper than that of handicapped females. This suggests that handicapped males suffer more from an increase in effort than handicapped f emales.