Yellow wagtail Motacilla flava ssp. lacking moult function


For readers from other continents: The Yellow Wagtail Motacilla flava breeds with numerous subspecies throughout the Palearctic and in Alaska. There is a complete postnuptial moult in adults. All subspecies migrating by way of southern Sweden winter in much the same area in W Africa.

Late in the evening on 19.9.98 an adult Yellow Wagtail was caught in the reeds of Foteviken (55.27 N, 13.00 E), SW Scania, Sweden. The bird's plumage struck the eye as being extremely pale and worn, and when it was investigated more closely it became evident, that not a single feather had been moulted. Adult Yellow Wagtails undergo a complete moult between late July and early September and migrate to Africa with completely fresh plumage. In our area it is fairly common that birds belonging to the subspecies flava turn up at roosts visited by migrants with growing primaries 8 and 9, however, and a few late females may even suspend the moult of these two primaries (and one or two secondaries). Still, the synchrony of Yellow Wagtail moult in Sweden is probably more pronounced than in any other long-distance migrant moulting before migration, and the cases of suspension are extremely rare, order of magnitude: one in a thousand. (Cf. Aymí & Jaume (1992), Butll. GCA 9: 11 - 12).

So, the bird was an extreme aberration, but the extremity does not end there. During a sequence of years I have investigated thousands of moulting adults of the subspecies flava, and I am well acquainted with the degree of wear and bleaching in one-year-old remiges in July and August; they may be cut at tips and rather brownish, at times almost sepia-coloured, but they are still quite functional. The flight-feathers of this bird, particularly the tail-feathers, were extremely pointed (only shafts at ends) and more than 50 % of their original surface had been worn away. This bird had no doubt carried its feathers for at least 25 - 26 months, and was on at least its second migration journey to Africa. I do not know if Yellow Wagtail remiges can last for 36 - 37 months, but I doubt that they can, so I assume the bird was in its third calendar year and had been flying with the same (juvenile) feather generation for two full years. It may be added that many late migrants (after 15 September in S. Sweden) in the Yellow Wagtail have foot, bill or plumage defects, most likely caused by grazing cattle and exposure to farming pesticides and herbicides during the nest period.

This note was written by Christer Persson, published on the web on 28.10.98.

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