University of Groningen Timing of postjuvenal molt in African (Saxicola torquata axillaris) and European (Saxicola

-Data on the timing of postjuvenal body molt of 322 Stonechats (Saxicola torquata) 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 photoperiodic 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 differences that may reflect differences in the selection pressures of their respective environments. Received 26 March 1998, accepted 6 November 1998. WE ANALYZED POSTJUVENAL MOLT in Stonechats (Saxicola torquata) from two different populations to assess the influence of environmental factors on the timing of molt (see Gwinner et al. 1983, Gwinner and Neuger 1985). Postjuvenal molt is the first molt in Stonechats and corresponds to the first prebasic molt in the terminology of Humphrey and Parkes (1959). Here, we are concerned only with body molt, although Stonechats previously were found to renew at least some of their secondaries during postjuvenal molt (Gwinner et al. 1983). An appropriately timed molt is an important component of the overall seasonal organization (Gwinner 1986, Jenni and Winkler 1994). The deterioration of climatic conditions toward the end of breeding exerts high pressure to accelerate molt, which may be opposed by energetic limitations (Jenni and Winkler 1994). Molt costs that depend on the rate of molt (Lindstrom et I Address correspondence to this author. E-mail: gwinner@erl.ornithol.mpg.de al. 1993, Klaassen 1995) may favor slower replacement of feathers. Ideally, molt should not overlap with energetically expensive activities, such as somatic growth and migration. Behavioral observations suggest that costs of molt are high in Stonechats, which catch their prey in short, precise flights (Konig 1996, Flinks and Kolb 1997). To preserve maneuverability and to reduce daily energy expenditure, Stonechats should maximize the time taken for molt within the range allowed by external conditions. The two subspecies of Stonechats considered here live in very different environments. Central European Stonechats (S. t. rubicola L. 1776) are short-distance migrants that winter in the Mediterranean region (Cramp 1988, Glutz von Blotzheim and Bauer 1988). They produce two or three clutches of about five eggs each season. When new young hatch, siblings from previous clutches are driven out of the territory, where they molt in aggregations of juveniles (Konig 1996). Young from late clutches must prepare for migration much faster than their older sib-