Food supplementation of parents before hatching of the young prolongs the nestling period in European pied flycatchers (Ficedula hypoleuca)

Fledging at older ages has been posited as benefiting offspring survival, whereas shortening the duration of the nestling period may reduce parental costs. Improving parental condition through food supplementation before hatching of the young may reduce parental foraging costs and thereby delay the inducement of fledging while not affecting nestlings directly. Here I show experimentally that broods of supplemented parents fledged at older ages while breeding success and nestling growth variables were not affected. Furthermore, measurements of nestlings about to fledge that were not included in the experiment show that they attain lengths of wing and tail that are 80% and 56% of adult values, respectively, with each day of delay in fledging improving flight capacity at fledging.