Geographical variation within species

Even within species, geographical variations in annual cycles are apparent. With increasing latitude, the migrations of many species lengthen, and take up more of the year, while the periods devoted to breeding and moult decline in association with the decreasing length of the favourable season. Again, however, moult shows the greatest variation in its position in the annual cycle. In some species, populations at lower latitudes moult in breeding areas, whereas those from higher latitudes, where the favourable season is shorter, postpone moult for winter quarters. For example, Barn Swallows Hirundo rustica in the most southern breeding populations, which are resident or short-distance migrants, moult during JuneAugust after breeding; whereas those in more northern populations begin moulting in September-October, after they have reached their distant wintering areas. In between, varying proportions of individuals show a split moult, starting in breeding areas, arresting during migration, and resuming in winter quarters (Cramp 1988). Likewise, most European populations of Ringed Plovers Charadrius hiaticula moult rapidly in their breeding areas in August-September, before migrating no further than southern Europe, whereas arctic-nesting ones leave their nesting areas after breeding, and postpone their moult until November-March after reaching their southern African wintering areas (Stresemann & Stresemann 1966). Other geographical variants in the timing and duration of moult occur in other shore-birds species, mainly in association with the latitude of breeding and wintering areas (see Cramp & Simmons 1983, especially for Knot Calidris canutus, Dunlin C. alpina and Little Stint C. minuta). In addition, in species with arrested moults, the position of the split varies to some extent between populations, according to regional variations in the length of the favourable season (Mead & Watmough 1976, Swann & Baillie 1979).

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