Postjuvenile molt in east African and Central European stonechats (Saxicola torquata axillaris, S.t. rubicula) and its modification by photoperiod.

Twenty-eight stonechats of the European race (Saxicola torquata rubicula) from Austria and thirty-one stonechats of the Central African race (S.t. axillaris) from equatorial Kenya were handraised and subsequently investigated with regard to the temporal pattern of their postjuvenile molt. About one half of the birds of each race were held under their own native photoperiod and the other half under the photoperiodic conditions of the other race. The results demonstrated clear differences in the postjuvenile molt between the two races when birds were kept in the photoperiod under which they normally live. The African birds began to molt earlier and molted longer than their European conspecifics. The time course of postjuvenile molt was affected by photoperiod in both races as molt began and ended earlier under the equatorial photoperiod than under the European photoperiodic simulation. The question why the African birds showed strong photoperiodic reactions under these experimental conditions although in their natural environment they experience only minute photoperiodic variations, is critically evaluated: Three possible explanations are discussed: (1) the photoperiodic reaction may represent a relict from a time when ancestors of the tropical populations still lived in more temperate zones; (2) it may be due to the occasional immigration of conspecifics from populations living further north or south; (3) it may result from effects on a (possibly circadian) submechanism of the system controlling annual cycles which is normally affected by other environmental cues but can also be influenced by photoperiodic variations.