Different populations of a species wintering in the same area

Different species wintering in the same area, and hence subject to the same daylength regime, may start their migrations at different dates, weeks or sometimes months apart, depending on the distance they have to travel and the dates their breeding areas become fit for occupation (for shorebirds see Piersma et al. 1990a). Such differences are also found in different races (or populations) of the same species wintering in the same area, as shown for White-crowned Sparrows Zonotrichia leucophrys in California (Blanchard 1941), and for Yellow Wagtails Motacilla flava in tropical Africa (Curry-Lindahl 1958, 1963, Wood 1992). Although exposed to the same winter conditions, the members of the various races differ in the dates at which their gonads develop, and at which they accumulate fat and depart for breeding areas (Blanchard 1941, Curry-Lindahl 1963, Fry et al. 1972). In general, in these northern hemisphere species, races that breed furthest south are first to leave, and those that breed furthest north are last. Thus, in the

Yellow Wagtail, the first to leave in spring is the southern race M. f. feldegg, then M. f. lutea, followed by M. f. flava and M. f. flavissima, and finally M. f. thunbergi. These various races arrive in their breeding areas in the same sequence, spanning the period March-June, from south to north. Inherent differences in endogenous rhythms could explain such population differences, as could inherent differences in the threshold daylengths required to trigger departure. Either mechanism could account for how birds of different races can leave their shared wintering area in appropriate sequence and reach their respective breeding areas (at different latitudes) at appropriate dates. However, the Yellow Wagtails are the more remarkable because several races winter together on the equator, where daylength is constant year round. In these birds, endogenous control seems essential, with different races responding differently, according to where they breed, and setting their 'internal clocks' before they reach the equator, so as to leave at appropriate dates some weeks or months later.

The idea of endogenous influence is supported by the observation that the onset of spring migratory restlessness in caged warblers in Europe coincides with the spring departure of conspecifics from their equatorial wintering grounds (e.g. Gwinner 1968). Captive migrants spontaneously resume spring migratory activity after a winter rest, even when kept under constant daylengths. Moreover, different species or different races of the same species kept under identical captive conditions reach breeding, moulting or migratory condition at different dates appropriate to the latitude at which they breed (for Phylloscopus warblers see Gwinner 1972). It seems that the seasonal timing of winter events in equatorial or trans-equatorial migrants is mainly accomplished by the involvement of an endogenous timing mechanism, as discussed in Chapter 11. Among populations wintering in the temperate zone, endogenous control would seem less important than in equatorial regions, because in the same area in the temperate zone, different wintering populations could respond to different threshold daylengths, so that they left in appropriate sequence. However, an endogenous influence, or an ability to separate increasing from decreasing days, would still be necessary in the temperate zone to prevent such birds from returning northward under the same daylengths in autumn that stimulate northward migration in spring.

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