Farner et al.
Moore et al.
+ =examined and response recorded, -=examined and no response recorded.
In addition to the above studies, male Dark-eyed Juncos J. hyemalis kept up to three years on constant dim light showed in that time up to three cycles of gonad growth and regression, spring migratory fattening, moult and spring and autumn restlessness (Holberton & Able 1992). The circannual cycles of individual birds drifted out of phase with one another, and averaged longer than a calendar year.
constant conditions from hatching further implies that they are innate, and do not arise from early experience (Gwinner 1981, 1996b, Berthold 1984b). The control centre of such cycles is unknown, but it almost certainly differs from that of the better-researched circadian (diurnal)2 rhythm, control of which lies within the medial basal hypothalamus (Ball & Balthazart 2003).
Circannual cycles may be reflected in gonad condition alone, in moult alone, in migratory condition alone, or in any combination of these activities. The cycles can thus be viewed as consisting of separate but integrated components involving the different activities (Wingfield 2005). Which of these components are expressed in captive birds depends largely on the constant photoperiod to which the birds are exposed, and perhaps also on the time of year (=internal physiological state of the birds) when the experiment starts. In Starlings Sturnus vulgaris, for example, the range of constant photoperiods under which gonad size and moult are expressed is very narrow, about 11.5-12.5 hours (Schwab 1971, Gwinner 1996b). Outside this range, no proper gonad growth occurs (Dawson 2007). Evidently, photoperiods act as permissive factors, setting limits to the expression of the different components of circannual rhythms; each component may occur only if the experimental photoperiod is close to the range of natural daylengths to which the bird is normally exposed for that component.
In many species, long light periods tend to suppress gonad growth altogether, even over periods of several years (Sansum & King 1976, Wingfield & Silverin 2002), but they seldom suppress moult and fattening (Table 11.3). On specific experimental photoperiods, different activities can thus be eliminated from the annual cycle, separated or overlapped with other activities. For example, prenuptial moult and fat deposition, which normally occur in sequence, can be made to occur simultaneously, while moult or migratory fattening can be made to overlap with any stage of the gonad cycle, or to occur in the absence of a gonad cycle. Such findings are relatively rare, and restricted to particular photoperiods, but they again suggest that the different components of the annual cycle are to some extent independent of one another, and that the chosen photo-regime can affect the phase relations among them (King 1972). The different components are normally kept in phase with one another by strong controlling mechanisms, including the natural photoperiod.
It is also clear that different species react differently to the same constant daylength regime, presumably reflecting their adaptations to different daylength regimes in natural conditions (Table 11.3). For example, under a regime of 9L:15D, Dark-eyed Juncos Junco hyemalis and Fox Sparrows Passerella iliaca showed fat deposition and migratory restlessness (delayed by about two months on the usual spring timing), whereas White-throated Sparrows Zonotrichia albicollis showed no such response, but underwent two pre-nuptial moults at an appropriate interval during the 18-month experimental period (Weise 1962).
Spontaneous endogenous rhythms are most apparent in long-distance migrants, which are normally exposed to varying photoperiodic regimes on migration, and in which the need for some form of endogenous control is greatest (as appreciated
2Kept under constant light, birds show inherent 'circadian' cycles in which periods of activity alternate with periods of rest, as in the natural light-dark cycle. These cycles approximate to 24 hours but, like circannual cycles, often tend to lengthen with time.
long ago by Rowan 1926). They are also apparent in some resident species of equatorial regions, where daylengths are constant year-round. For example, equatorial Stonechats Saxicola torquata axillaris kept caged in constant 11.8L:11.2D conditions in Germany went through up to 12 reproductive-moult cycles in a 10-year period (Gwinner 1996b). However, in temperate zone residents and short-distance migrants caged in constant conditions, the cycles tend to continue for less long, and are more variable among individuals; they seldom proceed for more than one year, and the different events tend to become increasingly out of phase with one another. They are thus less rigid and persistent, as found in some populations of Blackcaps Sylvia atricapilla (Berthold et al. 1972) and in European (as opposed to African) Stonechats Saxicola torquata (Gwinner 1996b).
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