Timing of Postjuvenal Molt in African (Saxicola torquata axillaris) and European (Saxicola torquata rubicola) Stonechats: Effects of Genetic and Environmental Factors

Data on the timing of postjuvenal body molt of 322 Stonechats (Saxicola tor- quata) were examined by multifactorial residual maximum-likelihood analysis for effects of external and genetic factors. The Stonechats, which belonged to the European subspecies (S. t. rubicola), the African subspecies (S. t. axillaris), and their F1 and F2 hybrids, were exposed to different photoperiodic conditions. The birds differed conspicuously in the timing and duration of their postjuvenal molt. These differences were significantly related to photo- periodic conditions, genotypic group, and the interaction of these factors. European birds generally molted earlier and faster than African birds, and hybrids showed intermediate patterns. Both subspecies started molt earlier under shorter photoperiods but took longer to complete it. African Stonechats had a weaker response to photoperiod than the European subspecies. In addition, molt timing was negatively related to hatching date, with chicks hatched late in the season molting at a younger age than those hatched earlier. Finally, the two subspecies differed in their molt timing when exposed to short days at an early age. The earlier a European bird was placed under short days, the earlier it molted; the reverse was true in African Stonechats. From a small data base, we estimated heritabilities (h2) through full-sib analyses and offspring-parent regressions. In both subspecies, the timing of molt showed high genetic variation, especially at its onset. Within photoperiodic groups, h2 at molt onset was close to unity but decreased during the course of molt. The two Stonechat subspecies timed their postjuvenal molts in a qualitatively similar manner but showed dif- ferences that may reflect differences in the selection pressures of their respective environ-