Another source of loss among migrants is the unseasonable cold which may occur in some years soon after migrants have arrived in their breeding areas. Typically, such losses are restricted to migrants, local resident species being better able to withstand the cold. In each documented example, as mentioned above, the migrants could have avoided the cold spell if they had arrived in breeding areas some days later than they did. Such losses could often have included passage migrants en route to other breeding areas, as well as migrants breeding locally.
Insectivorous migrants seemed especially vulnerable at such times, because cold and snow greatly reduced their food supplies, but large-scale losses also occurred among newly arrived waterfowl and waders. Ground-feeding birds may starve when fresh snow and ice makes their food unavailable (King 1974, Roseberry 1962, Vepsalainen 1968, Bull & Dawson 1969), and aquatic feeders are susceptible to delayed melting or re-freezing of water surfaces (Smith 1964, Barry 1968, Fredrickson 1969).
One such catastrophe affected newly arrived Cliff Swallows Petrochelidon pyrrhonota in Wisconsin, when a sudden drop in temperature caused insects to become dormant. The swallows did not fly during this period, but clung to their old nests, with one observer collecting 'a milk pail full' of birds that had died (Buss 1942). Another unusual period of cold caused heavy mortality of Cliff Swallows across the north-central Great Plains in 1996, reducing a study population by about 53% (Brown & Brown 1998, 2000). When short of food in cold weather, swallows and swifts often seek shelter in buildings, huddle together for warmth, and may suffer from hypothermia and starvation (Koskimies 1950, Smith 1968, Keskpaik 1972, Lyuleeva 1973, Brown & Brown 2000). Other migratory insectivores also die in such conditions, but less conspicuously (Table 28.1; Ligon 1968, Eckhardt 1977, Tramer & Tramer 1977, Whitmore et al. 1977, Zumeta & Holmes 1978, Mead 1979a).
In spring 1964, an estimated 100 000 King Eiders Somateria spectabilis - about one-tenth of the whole west Canadian population - died at migration staging areas in the Beaufort Sea. This mortality occurred when newly opened leads among sea-ice re-froze, preventing the birds from feeding (Barry 1968). In a more recent incident, birds found dead had lost about half of their body weight, and many survivors had insufficient body reserves for breeding, so that the effects of the freeze extended beyond the immediate mortality (Fournier & Hines 1994). Such mass starvation events in spring were documented in this species several times during the twentieth century (Barry 1968, Fournier & Hines 1994), and occasionally in autumn (see later). Similar events, involving the re-freezing of open water, have also killed hundreds of American Coots Fulica americana and other newly arrived waterfowl (Fredrickson 1969).
In the spring of 1966, frost and snow on 11-17 April caused massive mortality of newly arrived Lapwings Vanellus vanellus across southern Sweden and Finland, with reductions in local breeding populations the same year of 30-90% depending on region (Vepsalainen 1968, Marcstrom & Mascher 1979). In such conditions, some birds retreated southward (on 'reverse migration'), others abandoned their territories and re-assembled in flocks, moved from mountains to valleys or from inland to sea coasts, and some females about to lay resorbed their eggs. A similar event occurred in the same region in 1927 (Kalela 1955), and a smaller event affected Eurasian Oystercatchers Haematopus ostralegus in Scotland in 1979 (Watson 1980).
Migrants that had died during unusual cold snaps in breeding areas had simply starved to death: carcasses were light in weight and practically devoid of fat reserves (e.g. Ligon 1968, Vepsalainen 1968, Fredrickson 1969, Morrison 1975, Whitmore et al. 1977, Marcstrom & Mascher 1979, Watson 1980). At such times, birds were often reported in unusual places, such as roadsides or human
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