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moult

moult

Figure 11.1 (continued) Sequence 5: Examples: Kentish Plover Charadrius alexandrinus, Spotted Redshank Tringa erythropus, some individuals of Curlew Sandpiper Calidris ferruginea, Red-necked Stint Calidris ruficollis and Solitary Sandpiper Tringa solitaria. Sequence 6: Examples: Wilson's Phalarope Phalaropus tricolor, Spotted Sandpiper Actitis macularia, and other populations of shorebirds. Sequence 7: Examples with two complete moults per year: Willow Warbler Phylloscopus trochilus, Bobolink Dolichonyx oryzivorus, Sharp-tailed Sparrow Ammospiza caudacuta; examples with one complete and one partial moult per year: Rose-breasted Grosbeak Pheucticus ludovicianus, Melodious Warbler Hippolais polyglotta. Sequence 8: Examples: Lanceolated Warbler Locustella lanceolata and some other Locustella warblers, Great Knot Calidris tenuirostris, Curlew Sandpiper Calidris ferruginea, Western Sandpiper Calidris mauri.

timing is less crucial than that of other events. Moult is necessary in birds because feathers wear and deteriorate, becoming less effective for flight and insulation. In general, residents and short-distance migrants moult in summer after breeding (residents more slowly, sequence 1), while long-distance migrants moult in late summer in the breeding area (as in sequence 1), in autumn at a migratory staging area (sequence 2) or in winter (sequence 3), depending on population (Pienkowski et al. 1976, Bensch et al 1991, Jenni & Winkler 1994). In many other migratory species, the moult is split, occurring partly in one area and partly in another, separated by migration. It can be split between breeding area and wintering area (sequence 4), between breeding area and a staging area (sequence 5), or between a staging and wintering area (sequence 6). The autumn staging area where moult occurs may lay hundreds or thousands of kilometres from the breeding area. The moult is normally arrested during the migration itself, so that the bird can fly with a full set of flight feathers, some new and others old. The bird resumes the second phase of moult wherever it left off in the first phase. In patterns 5 and 6, a split moult is associated with a split migration, which is halted while moult occurs. In other (mostly large) species, split moults are associated with breeding (as moult stops temporarily during chick feeding), or with periods of winter food shortage (for examination through modelling of moult scheduling, see Holmgren & Hedenstrom 1995).

While some migratory species have a single split moult, replacing their feathers once, but in two bouts, others have two separate moults, replacing the same feathers twice in one year. One moult occurs either before or after autumn migration (the so-called post-nuptial or pre-basic moult), and the other before or during spring migration (the pre-nuptial or pre-alternate moult) (sequences 7 and 8). In a small proportion of species that moult twice each year, such as the Willow Warbler Phylloscopus trochilus and Bobolink Dolichonyx oryzivorus, both moults are complete, involving the replacement of both wing and body feathers. But in most twice-yearly moulting species, the autumn moult is complete and the spring moult is partial, involving the replacement of the body feathers only (and sometimes a few tertial, secondary or tail feathers). In some species with two moults per year, both plumages are the same, but in other species at pre-nuptial moult the males or both sexes don a special breeding plumage.1 Spring body moult occurs in many species of passerines, shorebirds and others, and usually overlaps with migration. In many species of diving and dabbling ducks, the second (pre-nuptial) moult (mainly body feathers) follows a few weeks after the first (post-nuptial) moult (complete),

1ln many scolopacid waders, the pre-nuptial moult is a double moult, involving the production over part of the body of two generations of feathers in quick succession. The first such plumage is distinguishable from the basic winter plumage but still cryptic, and the second 'supplementary' plumage comprises the showy breeding garb (Stresemann & Stresemann 1966, Jukema & Piersma 2000). In many species, these body moults begin in winter quarters and continue through migration.

Other bird species acquire a special breeding dress by abrasion, when the dull tips of the body feathers wear off to expose the richer colours below. This is how the male Eurasian Linnet Carduelis cannabina acquires its red breast and forehead, the male Brambling Fringilla montifringilla its black head, and the Snow Bunting Plectrophenax nivalis its striking black-and-white breeding plumage. ln some species, the bill and other unfeathered areas also change colour in the breeding season, under the action of gonadal hormones. Thus, in both the male Brambling and the male House Sparrow Passer domesticus, the beak changes from pale to black for the breeding season.

beginning in the sequence body - wing - body. In consequence, drakes are in dull 'eclipse' plumage for only a few weeks each year, and in bright breeding plumage for most of the year (Cramp & Simmons 1977, Bluhm 1988). In association with this, many species of ducks form pairs while in winter quarters.

The above generalisations apply to small or medium-sized birds in which the moult occurs as a distinct event in the annual cycle (Table 11.1). In some larger birds, breeding and moult may take so long that they cannot both be fitted within the annual cycle without overlapping (Stresemann & Stresemann 1966). In most raptors, for example, moult begins during incubation (earlier in females than males) and overlaps with most of the breeding cycle, although it may be arrested during chick-rearing. Small or medium-sized raptor species which migrate short distances can normally finish their moult before the post-breeding migration, but long-distance migrants typically arrest moult during migration, and continue after reaching winter quarters. In some of the largest flying birds, such as vultures, condors and albatrosses, each moult cycle lasts more than a year, but again may be arrested during difficult periods, such as chick-rearing. Otherwise such birds appear to moult more or less continuously, and may have two or more moult waves in the flight feathers at once (so-called serial moult, Box 11.1).

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