LIMITATION OF REPRODUCTIVE SUCCESS BY FOOD AVAILABILITY AND BREEDING TIME IN PIED FLYCATCHERS

Abstract

Seasonal variation in reproductive success is a common phenomenon in many taxa. In birds, the costs and benefits associated with alternative breeding dates cause variation in clutch size, which is a main cause of seasonal decline in reproductive success. An immediate cause for this might be seasonal variation in the environment, or alternatively, but not mutually exclusively, date-independent differences in the parental/territory quality of early and late parents. The predictions of these hypotheses on seasonal reduction of reproductive success were tested by manipulating the hatching date and the amount of food during the nestling period of Pied Flycatchers (Ficedula hypoleuca). Delay in the hatching date caused lower fledging mass and lower fledging production, suggesting a causal relationship between breeding time and reproductive success. However, if delayed parents were provided with extra food, their success was the same as that of the controls. This suggests that deteriorating food supplies and/or higher energy demands for rearing the brood later in the season are important causes of the lower success of late breeders. Additionally, parental molt was affected by manipulation of both breeding date and food; delayed parents often began molting while still feeding young, but control parents generally did not. Furthermore, fledglings in the delayed group weighed less if their fathers began molting while feeding the young. This implies that parents traded off their onset of molt against the time and energy needed for their brood. These results support the view that timing-related energy constraints on breeding are important causes of seasonal decline in clutch size and reproductive success.