Does daylength explain the latitudinal variation in clutch size of Pied Flycatchers Ficedula hypoleuca?

Geographical trends in breeding parameters were studied in the Pied Flycatc her Ficedula hypoleuca in the western Palaearctic. Predictions arising from the hypothesis that daylength and/or energy requirements of the brood expl ain latitudinal clutch size variation were tested. The nestling period decr eased with latitude, but nestling mass on day 13 after hatching did not sho w a trend with latitude. The length of the daily activity period (working d ay) at the time of peak brood demand showed a quadratic relationship with l atitude and did not increase linearly with daylight hours. The present stud y supports the hypothesis that latitudinal clutch size variation is influen ced by the duration of the working day and the energy requirements of the b rood. The balance between the energy requirements of the brood and the pare nts, in relation to the duration of working day and ambient temperature, ar e proposed to explain the latitudinal variation in clutch size in the Pied Flycatcher.