In comparing the different annual cycles of birds, one of the most striking findings is the great variation in the sequence and duration of breeding, moult and migration among species, or even among different populations of the same species. This variation shows clearly how the cycles are constrained by features of the birds themselves and adapted to the areas and circumstances in which different populations live. In each population, the cycle is adjusted primarily to the seasonal changes in climate and food to which that population is exposed, migration timing having evolved in concert with the other events that make up the yearly cycle.
In the majority of bird species, breeding, moult and migration occupy short enough periods to be fitted into a calendar year with little or no overlap. In larger species, with longer breeding and moulting periods, overlap between these events is greater, and in the few species in which moult lasts longer than a year, it overlaps with both breeding and migration. Such extensive overlap occurs in some large seabirds, such as albatrosses, and large raptors, such as vultures and
5The testes of male Common Crossbills are active from late autumn to the following summer (Berthold & Gwinner 1978). This allows the birds to breed in late summer or autumn, soon after they have found new areas of ripening cones, or in the winter and following spring. Juveniles may be fertile only a few weeks or months after fledging, and can start breeding before moulting out of juvenile plumage (Berthold & Gwinner 1978, Jardine 1994, Hahn et al. 1997). Parallels to this situation are found among some nomadic species in the deserts of Australia (for example Zebra Finches Taeniopygia guttata move around with partially developed gonads, so they can breed soon after rain has begun, bringing an abundance of grass seed (Immelmann 1963).
eagles. Even in these species, however, moult can be slowed or arrested temporarily during chick-feeding, migration or other difficult periods.
Some parts of the annual cycle are apparently controlled intrinsically, namely the broad time windows of the different processes and the sequences in which they occur. The assumption is that each species has developed the timing and sequence of events that best fit the conditions in which it lives, and has evolved to respond appropriately to the daylength regime to which it is exposed. In this way, the various events occur year after year at appropriate dates. Nevertheless, cycles can be advanced, retarded, lengthened or shortened by changing the experimental photoperiodic regime to which the bird is exposed. This is regarded as response to a time-keeper.
The main uncertainty concerns the relative importance of endogenous control in different species, and the extent to which an internal rhythm can run automatically on constant daylengths. One major source of variation between species is the time that autonomous cycles continue in captive birds in the absence of pho-toperiodic change. In many residents or short-distance migrants from mid-high latitudes, this may be less than one year, but in populations that are resident in the tropics or migrate there for part of the non-breeding period, the cycles may persist much longer - in some populations year after year, throughout a bird's life. In resident temperate zone birds, living year-round at the same latitude, strong endogenous control would seem unnecessary, because they are exposed to the same daylength regime year after year, which could therefore control all processes directly, without the need for an internal rhythm. It is uncertain whether photoperiod acts in any species without an endogenous rhythm (i.e. entirely as a driver rather than as a permisser and synchroniser), but it could only do so if birds could distinguish shortening from lengthening daylengths, otherwise they could give the same responses in autumn and spring, rather than different responses. But in long-distance migrants, which can winter on the equator with little or no daylength change, it is hard to see how individuals could set off for their breeding areas every year at an appropriate date without an internal clock to trigger the return, or at least prevent it from happening until after a certain time has elapsed. Other environmental factors, such as rainfall or food supply, might theoretically act as external cues for migration, but in practice are far too variable from year to year to act as reliable cues to date.
It is also hard to see how northern hemisphere migrants that winter in the southern hemisphere could avoid breeding during the austral summer without some internal clock mechanism to prevent gonad development until a more appropriate date. The same applies more generally in the existence of seasonality in response periods, when long days at one time of year might stimulate breeding, moult or migration, but not at another (so-called photo-refractoriness), and also influence how long these various processes take (Chapter 12). The adaptive value of a temporary suppression of response (refractoriness) is to prevent breeding (or other processes) at seasons when stimulatory daylengths occur, but when it is disadvantageous for other reasons to attempt that activity.
The combination of an endogenous rhythm as a template for seasonal activity, together with daylength as a synchroniser, provides many birds with a basis for seasonal timing that operates well under variable seasonal conditions and movement patterns. This dual system gives reliability, precision and flexibility.
Endogenous rhythms help to buffer short-term environmental influences, and ensure that different events occur within an appropriate time-period, regardless of unusual external conditions. But dependence exclusively on internal control would carry the risk that any slight deviation in the endogenous rhythm would uncouple the bird's activities from the external seasons. Similarly, dependence entirely on photoperiod (or other external cues) would make a bird vulnerable to exceptional events, such as an unavoidably longer than usual stay in wintering or migration areas with a different daylength regime. Only the combination of endogenous rhythm and photoperiodic response provides the bird with a sound basis for seasonal timing, enabling preparation for each event in good time, and promoting the slowing or speeding of successive processes depending on whether they are early or late with respect to prevailing daylengths and other environmental conditions.
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