The ‘fixed schedule hypothesis’ proposes that parental investment is independent of offspring needs in order to maximize adult survival or to prevent reduced future fecundity. We tested this hypothesis on Tawny Owls Strix aluco by experimental manipulation of brood size. We measured the call frequency of begging chicks, feeding frequency of males, overnight weight gain of nestlings and parental body condition in reduced or enlarged broods, and in control broods. In reduced or enlarged broods nestling begging calls reflected the respective decrease or increase in demand, but the frequency with which the males fed did not differ between the reduced and enlarged broods. Consequently, chicks gained body weight more rapidly in reduced broods, whereas those in enlarged broods grew more slowly. Male body weight did not change during the early nestling period when they delivered food to reduced or enlarged broods, but the condition of females worsened in the enlarged broods. As the males did not increase the feeding frequency in the enlarged broods, the females were under pressure to pass on more of the prey to nestlings begging in these broods. Male Tawny Owls regulated their feeding effort according to a fixed schedule, independently of the chicks' needs, to maintain their body condition. Nevertheless, the females whose condition deteriorated during the feeding of enlarged broods did not stop brooding earlier, nor did they suffer mortality in the breeding period.
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