VARIATION IN THE ONSET OF INCUBATION IN THE PIED FLYCATCHER (FICEDULA-HYPOLEUCA) - FITNESS CONSEQUENCES AND CONSTRAINTS

Reproductive correlates and fitness consequences (survival, recruitmen t) of variation in the incubation onset of pied flycatchers (Ficedula hypoleuca) were studied across 4 years in a southern European populati on, where between 36 and 89% of the females started to incubate with t he penultimate egg or earlier. Females started to incubate up to 3 day s before completing their clutches, this being accompanied by hatching spread. Females who advanced incubation before completing the clutch tended to be in better condition and to lay larger clutches than those that postponed incubation until the clutch was complete. Although lay ing date had no effect on onset of incubation, the effect of clutch si ze was much more marked when the influence of laying date on clutch si ze was controlled. Females who started to incubate earlier fledged mor e young than females delaying incubation but their higher fledgling su ccess was unrelated to recruitment. Females starting to incubate with the last egg had fewer fledglings but of larger size, higher mass, and in better condition than females who advanced the incubation. Female survival to the next breeding season did not differ between those that started incubation before clutch completion and those that did not do so, but an advanced onset of incubation by females was related to hig her mortality of their pair mates. Females that increased the size of the clutch with respect to the previous year tended to incubate their eggs earlier, while those whose clutch size did not change or was lowe r started incubation at about the same stage they did the year before. This trend was apparently influenced by improvements in body conditio n. Clutch size may constrain onset of incubation through environmental or genetic correlations, which are likely to be mediated by hormones. It is proposed that, in this population, an early onset of incubation in pied flycatchers does not affect reproductive fitness in most circ umstances.