Species which feed on the wing, such as terns and hirundines, commonly replace their flight feathers while on migration, at least where food is abundant (for Black Tern Chlidonias niger see Zenatello et al. 2002, van der Winden 2002). In some tern species, the adults also feed their recently fledged young while on migration, giving overlap between these different activities. Some species of hirundines moult their feathers while breeding or while migrating through favourable terrain, but not when they have a long flight over sea or desert (Elkins & Etheridge 1977, Cramp 1988, Jenni & Winkler 1994). For example, Rough-winged Swallows Stelgidopteryx serripennis in eastern North America moult while migrating south over land, but pause for about two months on the north side of the Gulf of Mexico, crossing when they have finished wing moult (Yuri & Rohwer 1997). In addition, some auk species swim to their wintering areas, enabling the young to set off before they can fly, accompanied and fed by a parent en route. This behaviour facilitates overlap between breeding and migration, but is possible only because the young develop their insulating body plumage and can swim from an early age, migrating through continuously suitable habitat. Because the adults stay with their young, and can escape their main predators (large gulls) by diving, they also moult on migration, shedding and replacing all their flight feathers simultaneously. Some species of auks, such as the Common Guillemot (or Murre) Uria aalge, thus engage in breeding activity, moult and migration at the same time. Other exceptions to the main patterns can be found in the ornithological literature, while others are likely to emerge through future work.
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