Mass losses from the incubation to the nestling stage in individual females were studied during six years in the course of a population study of Pied Flycatchers Ficedula hypoleuca breeding in nestboxes, in a descriptive study of factors contingent on female mass losses, as the male and nest site characteristics. Mass loss was greater for females which were already heavier while incubating. Mass loss was positively related to the number of nestlings in the brood, while it was independent of breeding phenology and female age. Higher mass losses of females were unrelated to female survival, while they were associated to advanced breeding dates the following year in surviving females. Females paired to males in prime condition at the middle of the nestling period had significantly lower mass losses from the incubation stage. Females paired to yearling males lost more mass than those paired to older males, although this effect was only detectable when nesting in high quality sites. Overall, most of our evidence pointed to the facultative adjustment of body mass loss, although a cost component dependent on initial female condition and mate condition could also contribute to mass loss in female Pied Flycatchers.
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