Timing of molt as a buffer in the avian annual cycle

Birds in seasonal environments must time stages in their annual life cycles precisely. While the fitness costs ofmistimed reproduction and migration are widely recognized, molt has been viewed as a more flexible element. Birds modify thetiming of molt in various ways to keep to overall timetables, but recent studies provide evidence for considerable attendantcosts in fitness. Selective pressure, therefore, affects the timing of molt and its flexibility, favoring timing mechanisms that arefinely tuned to local conditions. We illustrate such timing mechanisms for post-juvenal molt in stonechats (Saxicola torquata).These widely distributed passerines differ in life history traits such as length of breeding season, number of clutches andmigratory behavior. To understand adjustments to temporal environments, we compared the temporal plasticity of captiveAfrican, European, and Siberian stonechats. The subspecies from these regions differed from each other in their reaction normsto photoperiod, in temporal patterns related to the timing requirements of their native habitats. Their circannual rhythms thusencode characteristic features of seasonal activities in a population-specific manner. High heritability in post-juvenal moltcorroborates the interpretation of molt timing as adaptive. We conclude that stonechat populations adapt to temporal environmentsnot by switching between distinct and rigid programs but by adjusting reaction norms to relevant time cues.

Barbara Helm, Eberhard Gwinner, Acta Zoologica Sinica 52(Supplement): 703-706, 2006