With increasing latitude, the annual warm season becomes progressively shorter and the cold season longer. Correspondingly, most migratory bird species arrive later in spring, often breed over a shorter period, and leave earlier in autumn at higher latitudes than at lower ones (Chapter 14). Yet in the northern hemisphere, birds from the north of the range do not start breeding when they pass through the southern parts, at a time when individuals that breed in those areas have already started nesting. Nor do southern birds continue migrating northwards with others of their species once they have reached their own particular breeding areas. The implication is that birds nesting at different latitudes have different inherent responses to daylengths, thereby adjusting their annual activity cycles to the latitude at which they breed. Again, this view is amply confirmed by experiments involving the manipulation of photoperiod (for Dark-eyed Junco Junco hye-malis oreganus see Wolfson 1942, for rosy finches Leucosticte see King & Wales 1965, for Chaffinch Fringilla coelebs see Dolnik 1963, for Willow Warbler Phylloscopus tro-chilus see Gwinner 1972, for waterfowl see Murton & Westwood 1977, for Ficedula flycatchers see Gwinner 1990, for Great Tit Parus major see Silverin et al. 1993, for Stonechat Saxicola torquata see Helm 2003).
In general, populations breeding at higher latitudes require longer daylengths before gonad maturation occurs; but their gonads then remain active for a shorter period than in birds that breed at lower latitudes. Birds could adjust their response to different latitudes, under the action of natural selection, in at least three different ways. First, they could alter their rate of response to the same daylength stimulus. Second, they could alter the length of a latent (refractory) period, before a response became possible. Third, they could alter their photosensitive threshold, so that they needed a shorter or longer daylength (or different number of days with daylength above a minimum value) to trigger a response (Wolfson 1959, Marshall 1960, Murton & Westwood 1977). Which of these mechanisms birds use as they extend their breeding ranges into lower or higher latitudes is still uncertain. In addition, different species living together in the same area show differences in photosensitivity that correspond to differences in the timing of events in their respective annual cycles (for different species of pigeons see Lofts et al. 1967).
Even birds from mountain and lowland 100 km apart in the same region may show different circannual rhythms when kept in identical conditions in captivity (Widmer 1999). Nestling Garden Warblers Sylvia borin were collected from both the Upper Rhine Valley (200 m above sea level) and from the Central Swiss Alps (1,500 m), hand-raised and kept under standard conditions for nearly a year.
Under each of two different photoperiod treatments, lowland birds began spring migratory activity significantly earlier than montane birds. From this and other evidence, it was concluded that lowland and montane birds differ in their endog-enously controlled migration programmes.
Similar findings emerged from comparison of the 'reproductive windows' (interval between testes growth and regression) of tropical and temperate Stonechat Saxicola torquata races kept under identical constant photoperiod conditions in captivity (Gwinner 1991, Helm et al. 2005). The tropical birds, which are normally single-brooded, had shorter reproductive windows and longer moults than the temperate zone birds, which normally raise 2-3 broods and then moult fairly quickly. First-generation hybrids between the two races were intermediate in their moults, again indicating genetic control of this process (reproductive windows were not checked). Under natural daylengths, juveniles of Siberian Stonechats Saxicola torquata maura (long-distance migrants) moulted faster, and at an earlier age, than those of European S. t. rubicola (short-distance partial migrants) and African Stonechats S. t. axillaries (non-migratory), and heritability values for moult duration in Siberian birds were high (based on full sibling comparisons) (Helm 2003). It need scarcely be added that the evolution of regional responses of this type depend on 'site-fidelity', in which individuals and their offspring remain in, or return to, the same general areas to breed each year (Chapter 17). Only then can populations evolve specific regionally appropriate responses.
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