In general, in the northern hemisphere, overlap between breeding and moult increases with latitude (and associated shortening of the favourable season). This is evident in comparisons between closely related species, and between different populations of the same species. For example, breeding and moult overlap extensively in the arctic-nesting Ivory Gull Pagophila nivalis and Glaucous Gull Larus hyperboreus, but much less in more southern temperate-zone gull species, such as Black-headed Gull L. ridibundus and Lesser Black-backed Gull Larus fuscus. In the White-crowned Sparrow Zonotrichia leucophrys, overlap between breeding and moult increases with latitude (and altitude), while moult duration decreases from about 83 days in California to about 47 days in Washington State; that is, by 2.6 days per degree of latitude over a latitudinal span of about 14 degrees (Mewaldt & King 1978). Similar geographical trends in the overlap and duration of moult have been documented in many other species, including Common Starling Sturnus vulgaris (Lundberg & Eriksson 1984), Chaffinch Fringilla coelebs (Dolnik & Blyumental 1967), Willow Warbler Phylloscopus trochilus (Underhill et al. 1992), Blackcap Sylvia atricapilla (Berthold 1995), Dunlin Calidris alpina, and other shore-birds (Cramp & Simmons 1983).
Birds nesting in the high arctic have a very short season in which to raise young. They often have more synchronised and shorter breeding and moulting periods than closely related forms further south, and show more overlap between the different activities. For example, Snow Buntings Plectrophenax nivalis breed, moult and accumulate migratory fat within a period of only 10-12 weeks from mid-June, during which time food is very plentiful. They have one of the shortest nestling periods recorded among passerine birds (about 10 days), and while still feeding their young, the adults start moulting, shedding their flight feathers in such quick succession that for some days they can hardly fly. The whole moult is completed in about 4-5 weeks (Ginn & Melville 1983, Cramp & Perrins 1994). At the same time, they begin to put on weight for migration, leaving the bleakest breeding areas before the end of August. Many other arctic birds can only breed within this short period, postponing their moult until they have reached a more southern staging area or winter quarters. Moreover, if the spring is late, some species cannot breed at all that year, and usually pass earlier than usual to the next stage in their annual cycle, whether moult or migration. Other adaptations to a short arctic season found in some species include pair formation in winter quarters or en route, nest-building and egg-laying immediately after arrival, and accumulation of body reserves for egg production along with migratory fat deposition (as in geese and swans, Chapter 5).
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