The importance of daylength

Under natural conditions, the endogenous cycles of many birds are kept in phase by seasonal daylength changes,3 and in experimental conditions particular events can be advanced or retarded by appropriate use of an electric light (Farner & Follett 1966, Lofts & Murton 1968, Wolfson 1970). For example, if migratory birds of some species are exposed in late winter to photoperiods longer than natural days, their gonads begin to grow earlier than usual, and they show migratory and reproductive behaviour prematurely (Rowan 1925, 1926, Wolfson 1953, Lofts et al. 1963, King 1972).

The importance of daylength as a time-keeper (Zeitgeber) derives from its reliability. Its seasonal changes are consistent between years, making it the most obvious environmental feature that, at most latitudes, gives a reliable cue to date. The synchronisation of the internal annual cycle to photoperiod has been shown most convincingly in experiments in which birds were exposed to seasonal photoperiodic cycles with periods deviating from 12 months (e.g. six month cycles). As a rule, the birds' biological rhythms then conformed to the altered photoperiodic regime (Gwinner 1986, 1990b). For example, when the normal annual

3Exceptions can be seen in cases like the Sooty Tern Sterna fuscata on Ascension Island, where the period of the cycle is about 9.6 months. This can be interpreted as a free-running endogenous rhythm, in which breeding and moult require a certain time, in an equatorial environment in which ecological conditions for breeding are nearly uniform throughout the year (Ashmole 1963). Selection has favoured maximum reproductive rate, and the cycle has lost (or not evolved) any coupling to environmental synchronisers (Lofts & Murton 1968). The same may apply to various species in equatorial South America where, in a semi-arid area with erratic rainfall at 3°30'N, eight out of ten bird species bred year-round, with the cycles of different individuals out of phase with one another (Miller 1954). In the Rufous-collared Sparrow Zonotrichia capensis, individual cycles averaged six months in duration (four months for breeding and two months for moult), and two complete cycles were manifest each year, 'uncoerced by small variations in photoperiod' and 'only incompletely controlled by the seasonal occurrence of rainfall' (Miller 1959). Outside equatorial regions, the same species has a single breeding period each year, which shortens with increasing latitude, as in other birds. Breeding-moult cycles shorter than one year have been described in other species elsewhere (for the Babbler Stachyris erythroptera and Little Spider-hunter Arachnothera longirostra see Fogden 1972b, for the Bat Hawk Machaeramphus alcinus see Hartley & Hustler 1993). As more studies are done in equatorial regions, other resident bird species may be found to show more than one cycle per year.

cycle of daylength was shortened to six months without altering its amplitude, Garden Warblers Sylvia borin went through four instead of two moult periods within one calendar year, two instead of one gonad cycle, and four instead of two periods of migratory restlessness (Berthold 1996). The same occurred in Sardinian Warblers Sylvia melanocephalus, in which the usual one annual moult occurred twice within one calendar year (six months apart). It also occurred in Stonechats Saxicola torquata, which underwent two gonad and moult cycles in one calendar year (Gwinner & Helm 2003). More remarkably, Dark-eyed Juncos Junco hyemalis, which were exposed to four periods of short (9-hour) days and five periods of long (20-hour) days in one year, showed in this time five periods of gonadal activity, five of fat deposition and two of moult (Wolfson 1954). By shortening the photoperiodic cycle to two months, Starlings Sturnus vulgaris showed up to six gonadal cycles in one calendar year, but on this extreme regime the testes did not fluctuate over the full range and moult was disturbed and incomplete (Gwinner 1996b).

A second line of evidence for the role of the daylength as a time-keeper involves birds from the northern hemisphere that became established in the southern hemisphere by human action. Such birds normally adjusted to the local daylength regime within a year or so of their release. Their annual cycles remained essentially unaltered, except that they were six months out of phase with those in their original home (Aschoff 1955). This finding has been duplicated in many phase-shifting experiments in various captive birds (Gwinner 1986). Similar change occurred in most southern hemisphere birds transported to the northern hemisphere, although some exceptional species (such as Northern Rosella Platycercus venustus and Gouldian Finch Poephila gouldiae) bred in the same calendar months in both hemispheres, their breeding being unexpectedly resistant to change (Baker & Ransom 1938).

Despite the year-to-year consistency of daylength change, at particular dates the weather, food supplies or other conditions vary from year to year. It is therefore advantageous for wild birds to respond, not only to daylength but also to other secondary factors, and modify their activities to suit conditions at the time. Plant growth and invertebrate activity begin much later in cold springs than in warm ones, and any bird that did not respond appropriately to this variation could be seriously disadvantaged. Hence, secondary environmental factors may fine-tune the timing of various events to prevailing conditions: for example, enabling the bird to arrive on its breeding area and start nesting at what is an ecologically suitable time that year (Lack 1954, Wingfield et al. 1993, Wingfield & Jacobs 1999). In general, the effect of daylength (interacting with an endogenous rhythm) could be said to initiate the preparatory processes that precede each event. Then, as development proceeds, other modifying factors, such as food supply, mate and nest-site availability, come to play a greater role (for examples of food effects see Newton 1998a, for mate and social influences see Ashmole 1963, Lewis & Orcutt 1971, for nest-sites see Village 1990 and H. Gwinner et al. 2002).

Daylength has another useful property that is almost certainly used by birds, namely that its annual cycle varies in a consistent manner with latitude. This provides a means by which migrants could identify their latitudinal position with respect to breeding or wintering areas, enabling individuals from the same population to time their migrations in an appropriate manner from different latitudes within the breeding or wintering range. More generally, it provides a basis for selection to encode the seasonal behaviour of populations to latitudinal conditions. This facility is particularly evident in migratory populations (Chapter 12).

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