Circannual and Circadian Contributions to the Timing of Avian Migration

Abstract

Circadian clocks use mainly daylength as zeitgebers, and there is not clear evidence of other zeitgebers. This is supported by the synchrony of nocturnal activity between cages and departure from natural sites (Gwinner 1968). 'Zeitgebers control period by modulating angular velocity, i.e. the rate at which the oscillation runs during a given fraction of the circannual rhythm.' Two factors are important: 1 property of the stimulus (photoperiod) and 2 phase of the circannual system at which the zeitgeber stimulus occurs. E.g. GTs react differently to photoperiod depending on where they come from (Silverin et al, 1993). Garden warblers wintering at the southern hemisphere experience longer days in winter and this triggers earlier moult and migration than birds wintering at the equator and this makes birds to catch up for their longer journey. Some birds from this S-German population however do not respond properly to southern hemisphere photoperiods and get 'time-trapped': they grow full testicles when experiencing these longer days in winter. Showing different reaction norms of the subspecies of stonechats to photoperiod.