While the ultimate (extrinsic) factor controlling the annual cycles of birds is the seasonality of the environment, the primary intrinsic (proximate) factor is apparently an endogenous rhythm within the bird. This self-sustaining rhythm tends to ensure that the major processes of migration, breeding and moult occur in the correct sequence each year, and at roughly the right times. The evidence for the existence of an internal rhythm has come largely from studies on captive birds kept for up to several years under rather specific constant daylengths (Gwinner 1968, 1971, 1972, 1981, 1986, Berthold et al. 1974b, Berthold & Terrill 1991). Such birds have no clue from the outside world as to what the date might be. Yet they usually moult and reach breeding and migratory condition in the correct sequence, and at roughly appropriate intervals, with corresponding cycles in body weights, gonad sizes and hormone levels. This finding is taken to imply the existence of some underlying 'endogenous' controlling system. However, in conditions of constant daylength, the cycles do not stick strictly to a year, but tend to drift, getting either shorter (rarely) or longer, hence the term circannual cycles, which typically last 9-13 months.
The existence of internal circannual rhythms, underlying the natural yearly cycles, and persisting for at least two cycles, has now been shown experimentally in more than 20 different bird species, including resident and migratory, temperate and tropical, passerines and non-passerines, as well as in other animals and plants (Table 11.3; Gwinner 1981, 1986, 1996b, Berthold & Terrill 1991). They evidently underlie the control of seasonal activities in a great variety of organisms. Among birds, such rhythms are expressed in gonad development, moult or 'migratory' fat deposition and restlessness. Garden Warblers Sylvia borin and Blackcaps Sylvia atricapillus kept under constant photoperiodic conditions (10 light:14 dark) have shown up to 10 successive moult cycles in eight calendar years, suggesting that, in these species, a circannual rhythm keeps running throughout the entire lifetime, requiring no obvious environmental stimulus. In such experiments, different birds housed in the same room got out of phase with one another, providing further evidence that the rhythms are endogenously controlled and expressed independently of environmental cues. Also, the fact that such cycles are shown by birds that have been hand-reared and kept under
Table 11.3 Evidence for endogenous control of various events in the annual cycles of birds kept under constant photoperiod regimes
Light Maximum Pre- Spring Spring Gonadal Post- Autumn Autumn Weight Source regime duration nuptial fattening restlessness cycle nuptial fattening restlessness cycle (months) moult moult
Dartford Warbler Sylvia undata
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