Moult Migrations

In some waterfowl species, many individuals perform a 'moult migration' each summer, when they travel long distances from their nesting places to assemble in large numbers at traditional sites that offer food and safety. Here they pass the flightless period, replacing all their large wing feathers simultaneously within the space of a few weeks. Body moult begins before flight feather moult and continues afterwards. For habitat-related reasons, moulting sites typically hold far fewer local breeders than the summer food supply will support, leaving a big surplus for the incomers. Among geese, moult migrations are undertaken entirely by non-breeders and failed breeders, whose moulting areas nearly always lie at higher latitudes than the nesting places, from a few tens to more than 1000 km away (Figure 16.1). For example, some Canada Geese Branta canadensis from mid and west continental North America undertake a moult migration of at least 1000 km to the Thelon River area of the Northwest Territories (Sterling & Dzubin 1967), while those breeding in northwest Wisconsin scatter northward for up to 1300 km into northern Manitoba (Zicus 1981). By leaving breeding areas, such birds avoid competing with families which stay there, and by moving to higher latitudes where plant growth begins later than in the breeding areas, the birds gain from better protein-rich food, as well as from longer days. After moult, the birds migrate to their wintering areas, perhaps stopping for a time in breeding areas en route south.

Similar moult migrations occur in some duck species, but are more variable in direction than those of geese. The majority of Common Shelducks Tadorna tadorna from Europe gather each summer on the vast tidal mudflats of the Grosser Knechtsand in the German Wadden Sea, where they feed on the abundant mud-snail Hydrobia ulva. The birds converge on this site from all directions, travelling up to several hundred kilometres from their breeding areas, their numbers peaking at more than 200 000 individuals. Yearlings and young adults arrive first, followed by failed breeders, then successful adults, which leave their well-grown young behind (Patterson 1982). Smaller concentrations assemble at various other sites within the breeding range. After moult the birds drift back to their breeding areas over a period of weeks or months or move on to wintering areas, depending on whether their particular breeding areas are habitable in winter.

In various diving and dabbling ducks, the males, together with non-breeding or failed females, also perform moult migrations, leaving the successful females to hatch and tend their broods. Once their young are grown, some of the adult females perform a moult migration, being last to arrive in moulting areas, while others remain in their breeding areas and moult their flight feathers there.

Some moult migrations are not trivial. In some duck species, individuals have been found by ringing to travel distances of more than 3000 km from breeding to moulting sites and some of the biggest concentrations hold tens of thousands or hundreds of thousands of birds. In general, sea-ducks perform longer journeys than freshwater ducks. Usually birds fly quickly and directly from breeding to moulting sites, and even sea-ducks which normally stick to the coast, sometimes fly long distances overland to take the shorter routes (Salomonsen 1968). A particularly big arrival of birds on moult migration was noted at Mud Lake, Idaho, where during one night (5 August) no less than 52 000 dabbling ducks of six species suddenly appeared (Oring 1964). Examples of some of the more impressive moult migrations include the following:

King Eiders Somateria spectabilis from most of eastern arctic Canada travel to coastal areas in mid-western Greenland to moult. By that time the participants have spent as little as three weeks on their breeding areas, and they travel eastward up to 2500 km, forming a concentration of more than 100 000 birds. After moult, the birds migrate to wintering areas off southwest Greenland where they are joined by the females and young (Salomonsen 1968). Birds from the western part of the

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Figure 16.1 Moult migrations of geese. The arrows show direction and distance from origin to destination. Greylag Goose Anser anser (3), Bean Goose A. fabalis (5-9, 11), Pink-footed Goose A. brachyrhynchus (1), Greater White-fronted Goose A. albifrons (5-9, 11), Canada Goose Branta canadensis (2,1315) and Brent Goose B. bernicla (4, 9, 10, 12). From Salomonsen (1968).

Figure 16.1 Moult migrations of geese. The arrows show direction and distance from origin to destination. Greylag Goose Anser anser (3), Bean Goose A. fabalis (5-9, 11), Pink-footed Goose A. brachyrhynchus (1), Greater White-fronted Goose A. albifrons (5-9, 11), Canada Goose Branta canadensis (2,1315) and Brent Goose B. bernicla (4, 9, 10, 12). From Salomonsen (1968).

North American range migrate westward to concentrate off eastern Siberia and in Disco Bay, reaching up to 200 000 individuals.

Long-tailed Ducks Clangula clangula in eastern Siberia perform an extensive migration across the Arctic Ocean to Wrangel Island, north of the breeding range, where they gather in tens of thousands. Another large concentration occurs near Thule in northwest Greenland, again attracting birds mainly from further south (Salomonsen 1968).

Goosanders Mergus merganser from much of northwest Europe migrate from their breeding areas to North Cape in northernmost Norway, forming a concentration of about 35 000 birds (mostly males), from which they later move southward to their wintering areas. Drakes are virtually absent from breeding areas from June to October (B. Little & J. H. Marchant, in Wernham et al. 2002).

Common Scoters Melanitta nigra from the northeast migrate each summer to the west coast of Jutland, Denmark, where they reach more than 150 000 individuals. After moulting, some birds remain there, while others move on elsewhere, but not necessarily as soon as moult has finished (Salomonsen 1968).

Common Pochards Aythya ferina assemble each summer on various waters in the mid-latitudes of Europe, including the IJsselmeer in the Netherlands (50 000 individuals) and Ismaninger Reservoir in Bavaria (20 000 individuals). Collectively, they far exceed the European population, and contain many birds from further east. Initially males form nearly 100% of both populations but, as the season wears on, females gradually increase to nearly 50% by late September. Many Tufted Ducks A. fuligula and other species use the same sites, with similar seasonal changes in sex ratios (van der Wal & Zomerdijk 1979).

It takes individual ducks 3-5 weeks to regrow their flight feathers, depending on species, shelducks and geese 4-6 weeks, and swans about six weeks. However, some individuals arrive in moulting areas weeks before starting their wing moult, or leave long after finishing it, providing that food remains available. Typically, flocks arrive and leave over a several-week period, so that sites can be occupied by moulting birds for a total of 2-4 months. For example, Barrow's Goldeneyes Bucephala islandica in eastern Canada travel north from their nesting areas around 1000 km to arctic waters, and spend 3-4 months there, longer than they spend in nesting areas, and much longer than the 31-day flightless period associated with wing-moult (Robert et al. 2002).

While many species of waterfowl throughout the world gather on safe sites to moult, it is mainly species in boreal or arctic latitudes that make the longest migrations, often to sites outside the breeding range. While some birds migrate north of their breeding areas to moult, others move south of their breeding areas to moult at staging or wintering sites. For some species, therefore, the moult migration could be regarded as an extra stage of the northward spring migration or as the first stage of the southward migration, involving the same directional preferences. But for other species, the moult migration takes them in a quite different direction, sometimes at right angles to the normal migration axis. Moreover, because sites within the breeding range can attract birds from all directions, there is clearly no directional consistency between individuals. In addition, birds can periodically change their moulting sites in response to changes in water depths, or as established sites are destroyed, and new ones created, by human action. Given these facts, it seems unlikely that birds from different parts of the breeding range inherit different directional preferences for moult migration, especially considering that individual waterfowl can sometimes breed in areas hundreds of kilometres from where they were raised, or in widely separated places in different years (Chapter 17). Rather they could learn the locations of favoured sites from other individuals, this knowledge being passed on by tradition. However, not all birds in a particular region participate in a moult migration, some males remaining to moult in their breeding areas, along with many females and young.

Other wetland birds also gather in large concentrations to moult, and in some species lengthy migrations are involved. This is true of both European and American Coots (Fulica atra and F. americana) and of various grebes, all of which become flightless for a time. In western North America, most Eared (Black-necked) Grebes Podiceps nigricollis from prairie breeding areas migrate to moult on the Great Salt Lake in Utah or on Mono Lake in California, which together hold more than two million birds in late summer, supported by the masses of brine shrimps. From there, after moulting, the birds move on to winter on the sea off California (Chapter 5; Storer & Jehl 1985, Jehl 1997). In Europe, up to 186 000 Black-necked Grebes have been counted on Burdur Gola in Turkey (Hagemeir & Blair 1997), and smaller numbers elsewhere. Also, some 10 000-40 000 post-breeding adult Great-Crested Grebes Podiceps cristatus assemble to moult each year on the Dutch IJsselmeer (Piersma 1987). This latter site holds many thousands of waterbirds during the moult period, including the Common Pochards Aythya ferina mentioned above. Because of moult migration, many waterbirds occupy at least three main areas each year - for breeding, moulting and wintering; and from time to time some also make long movements within their wintering range (see later).

In all these various waterbirds, moult migration thus differs from typical migration in several respects. First, the flight direction is often different from autumn or spring migration, and may vary by up to 360° among birds converging on a single site from different parts of the breeding range. Second, only certain sectors of the population participate; goose pairs with young stay behind, as do some female dabbling and diving ducks with broods. Third, the localised and unusually high population densities found in the moulting areas are quite unique in some species, whereas in breeding and wintering areas the birds scatter over much wider areas. As in some other migrations, however, individuals follow a rigid schedule, with the timing of moult migration varying only slightly from year to year, according to the timing and success of breeding. Some species travel such long distances that they must presumably accumulate body reserves to fuel the journey, although I know of no published information on this.

The adults of many shorebird species also travel considerable distances to specific places to moult. These sites are usually at latitudes to the south of the breeding areas, so they act as staging areas on southward migration, or as wintering sites. Often among the birds from a single population, some individuals may remain after moult throughout the winter, while other individuals move on further south (as recorded, for example, in the Green Sandpiper Tringa ochropus, Bar-tailed Godwit Limosa lapponica and Sanderling Calidris alba in Britain, Wernham et al. 2002). Moulting flocks of shorebirds can consist of a few tens, hundreds or thousands of individuals, but at some sites much larger numbers occur. For example, several hundred thousand Wilson's Phalaropes Phalaropus tricolor gather at saline lakes in western North America, where they replace only the first 3-4 primaries along with the tail and nearly all the body plumage. The moult migration involves only post-breeding adults, with females arriving by mid-June and males a fortnight later (Jehl 1996). Individuals remain for up to six weeks, then double their weight before departing on an apparent non-stop flight of 4800 km to South America. Although most shorebirds do not become flightless during moult (except for the Bristle-thighed Curlew Numenius tahitiensis on Hawaii), the same factors as in waterfowl are likely to influence their choice of site: abundant food over a long period and relative security from disturbance and predation.

What could be construed as moult migrations have also been reported in divers, flamingos, cranes, terns and auks, all of which may gather to moult at places away from their nesting or wintering areas (Jehl 1990, Cherubini et al. 1996). For example, in the Little Auk Alle alle, non-breeders leave Greenland colonies in late July or August, in advance of breeding adults, and move northward to moult in rich feeding areas along the edge of the pack-ice. The main difference between this movement and that shown by many waterfowl is that the moulting locality varies from year to year, according to ice conditions (Jehl 1990).

Some passerines also perform what has been called a moult migration, but in effect they moult at stopover sites on autumn migration. Many Eurasian-Afrotropical passerine migrants break their autumn journeys in the northern tropics for several weeks and moult, later continuing their migrations to localities further south in the northern tropics or across the equator to the southern tropics (Chapter 24). In western North America, adult Western Tanagers Piranga ludovi-ciana and other passerines migrate to the southwest of the continent after breeding, moult there during the late summer rains, and then continue their journey southward to winter quarters. In contrast, juvenile Western Tanagers move to montane habitats near their natal areas to moult their body feathers before migrating (Butler et al. 2002). In northeast Europe, some Common Starlings Sturnus vulgaris migrate immediately after breeding either before their moult begins or in the early stages. Ring recoveries suggest a movement of 300-1200 km. The second phase of migration occurs in autumn after moult has finished. The two phases of movement are indistinguishable in direction, altitude of flight and temporal pattern, and both include some nocturnal flights (Kosarev 1999). Juveniles participate mainly in the first period, moulting in their migration areas, and adults mainly in the second period, after moulting in their breeding areas. The post-breeding migration of the whole population therefore occurs as two well separated waves (Figure 16.2).

These various moult migrations could be viewed either as an extra post-breeding movement in the annual cycle of the species involved, which may otherwise be resident or migratory, or in some species as a part of the autumn

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