Conflict or Cooperation between Parents in Feeding Nestlings in the Pied Flycatcher Ficedula hypoleuca

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Abstract
A parent bird feeding its young often has the potential to increase its effort depending on how much the other parent is prepared to work. For life-history reasons such as reduced confidence in paternity and higher mating success with age (Houston and Davies 1985), we could expect Pied Flycatcher males to be prepared to work less than females in feeding young. We tested this idea by measuring the feeding rates of males and famales in monogamous pairs; and also by a removal experiment, to see how much more single parents were prepared to work than members of a pair. Unaided males or females raised fewer young, which were also of lower fledging weight, than did pairs. Single males increased their feeding efforts as much as single females, and in monogamous pairs both sexes had similar feeding rates, thus contradicting our expectation. The model at Houston and Davies (1985) assumes that increasing effort affects adult mortality only after the independence of the that increasing effort affects adult mortality only after the independence of the young. In reality, adults might die before that time, and then the joint interest of both parents in staying alive at least until the independence of young, may have led to the stable "agreement" of Pied Flycatcher mates in contributing equally to the feeding of nestlings.