ADVANTAGES OF HATCHING SYNCHRONY IN THE PIED FLYCATCHER FICEDULA-HYPOLEUCA

Many bird species begin incubation before the clutch is complete. whic h leads to asynchronous hatching of the young. Several hypotheses have been proposed to explain this hatching pattern, most of them assuming that asynchronous hatching is of adaptive value. Pied Flycatcher broo ds were induced to hatch either synchronously or asynchronously in ord er to compare the breeding success resulting from these two hatching p atterns. Experimental maximization of hatching synchrony led to an inc rease in the production of young. Synchronously hatched young also had longer tarsi and higher fledging weights than young from asynchronous broods. Synchronous hatching was favourable in both years of study, e ven though one was cold and rainy and the other warm and dry. Survivor s in asynchronous broods in which some mortality had occurred were not in better condition than young in synchronous and unreduced broods, i n contrast to what the brood reduction hypothesis predicts. Thus, this study does not support Lack's (1947, 1954) original presentation of t his hypothesis as an explanation for the presence of the semi-asynchro nous batching pattern occurring naturally in Pied Flycatchers. However , the raising of experimentally synchronous broods might induce a larg er parental effort perhaps affecting parental survival probability or subsequent fecundity. It is not possible to decide whether the absence of complete synchrony in the Pied Flycatcher is due to hormonal const raints, or whether some factor, such as predation on the nest contents (eggs, young or adults), may also make hatching asynchrony adaptive.