Info

Peiponen (1991)

Common Swift Apus apus

July 1930

England

Cold and rain

>30

Watson (1930)

Within categories, incidents are listed in date order.

For other examples of heavy mortality in migrants after arrival, see Smith (1929), Brown & Brown (2000). For examples of hold-ups in migration through bad weather, see Wood (1908), Williams (1950), Berthold et al. (2002). aMuch or all of the recorded mortality occurred over water. bFemales and young, the males having already migrated.

cBirds classed as killed after arrival in breeding areas may have included some migrants with further to go, and those classed as killed in breeding areas before departure may have included some migrants already en route from higher latitudes (see especially the Swifts discussed by Kolunen & Peiponen 1991).

Within categories, incidents are listed in date order.

For other examples of heavy mortality in migrants after arrival, see Smith (1929), Brown & Brown (2000). For examples of hold-ups in migration through bad weather, see Wood (1908), Williams (1950), Berthold et al. (2002). aMuch or all of the recorded mortality occurred over water. bFemales and young, the males having already migrated.

cBirds classed as killed after arrival in breeding areas may have included some migrants with further to go, and those classed as killed in breeding areas before departure may have included some migrants already en route from higher latitudes (see especially the Swifts discussed by Kolunen & Peiponen 1991).

washed up on a beach in North Africa (Perrins et al. 1985), or the hundreds of Garden Warblers Sylvia borin and other species washed ashore in Spain following a night of heavy rainstorms (Mead 1991).

Even greater overwater losses have been recorded in the New World (Table 28.1). A massive kill, estimated at 40 000 migrants of 45 species, occurred during a tornado and storm on 8 April 1993 off Louisiana. The storm occurred when large numbers of birds were arriving at the coast after an overnight sea-crossing (Wiedenfeld & Wiedenfeld 1995). Other mortality events in the Gulf of Mexico included one of 10 000 birds (half of which were Magnolia Warblers Dendroica magnolia) washed up on Padre Island, Texas, in May 1951 (James 1956), and another of 5000 birds on Galveston Island, Texas, in May 1974 (Webster 1974, King 1976). In another incident, over the Great Lakes in May 1976, an estimated 200 000 birds were washed up on one stretch of shore on Lake Huron alone (Janssen 1976).

Lapland Longspurs Calcarius lapponicus seem to be frequent victims of spring storms. While migrating northward at night, large numbers have sometimes been grounded by wet, clinging snow. In March 1904, an estimated 750 000 were found dead on the ice of two lakes in Minnesota, each about one square mile (2.6 km2) in extent. However, carcasses were reported from a much wider area, and it was estimated that at least twice that number (1.5 million) may have been killed. Some had been attracted to town lights, and collided with buildings and other obstacles (Roberts 1907a, 1907b, Lincoln 1935a).

Waterbirds probably suffer fewer losses on overseas flights than do landbirds, being more robust and able to rest on water when navigation or flying conditions become difficult (as shown for radio-tagged Whooper Swans Cygnus cygnus on their migration between Iceland and Britain, Pennycuick et al. 1996). However, for some waterbirds on migration, problems arise from their being forced down onto dry land. In western North America, Eared (Black-necked) Grebes Podiceps nigricollis cross hundreds of kilometres of desert with few places for a waterbird to land in emergency. Snow storms have occasionally brought hundreds or thousands of birds crashing to the ground where they may die:

'During an early morning hour (about 2 am) of December 13, 1928, the residents of Caliente, Nevada, were awakened by a heavy thumping of something falling on the roofs of their houses. Those who were curious enough to step outside and investigate the unusual occurrence found scores of waterbirds in the new fallen snow. The next morning several thousand Eared Grebes were found on the ground and flat roofs of business houses throughout the city.'

Most were dead, having struck trees or buildings, but others were busy working themselves out of the snow (Cottam 1929). In January 1997, bad weather in southern Utah several times brought grebes to ground, killing an estimated total of 35 000, forming about 3% of the population that stages at the Great Salt Lake (Jehl et al. 1999).

Headwinds provide another hazard for migrants, because in effect they force the birds to fly for longer, depleting their energy reserves. This is especially important for species that cross large expanses of inhospitable habitat, such as oceans or deserts. In strong headwinds, birds exhausted over water sometimes settle on the surface, becoming soaked and unable to take off again. In addition, birds forced by headwinds to fly close to the waves become vulnerable to attacks by gulls, which have no difficulty in forcing small birds into the water, from which they can be snatched and swallowed (Hobbs 1959). However, not all land-birds that settle on water are doomed. Gatke (1895) mentioned coming across, on different occasions, a Song Thrush Turdus philomelos, Snow Bunting Plectrophenax nivalis and Brambling Fringilla montifringilla sitting on the sea surface, all of which flew when approached by a boat. Probably many birds are lost at sea while fighting headwinds, while others die or suffer reduced breeding success after arrival (for Brent Geese Branta bernicla see Ebbinge 1989).

Dead landbirds are often found washed up on beaches, and their remains show that many have been eaten at sea, presumably mainly by gulls (e.g. Alerstam 1988). Other individuals arrive on coastlines in an apparently exhausted state, as often witnessed by bird-watchers. Evidently, overwater migration inflicts continual, and occasionally heavy, losses on many landbird species.

Storm-induced deaths over water mostly involved small birds, but have included such robust species as Rook Corvus frugilegus, of which 4600 carcasses were found in one incident in southern Sweden (Alerstam 1988). At least 106 species, including passerines, cuckoos, nighthawks, rails, gulls, terns, skimmers, shorebirds and waterfowl, have been washed up following storms over the Gulf of Mexico. In addition, one large overwater incident off Israel in April 1980 involved at least 1300 birds of prey (including eagles), presumably blown over water from their usual landward route, and other incidents involving raptors are mentioned by Kerlinger (1989). One overland hailstorm killed many ducks and geese, and at least 35 Tundra Swans Cygnus columbianus (Hochbaum 1955), while another killed 100 Snow Geese Chen caerulescens (Krause 1959). It seems, then, that all sizes of birds are vulnerable to in-flight mortality from adverse weather of one type or another.

Sandstorms can be a particular hazard to birds moving through the Sahara and neighbouring deserts of the Middle East, and can be fatal to any grounded migrant. Incidents involving Northern Wheatears Oenanthe oenanthe, Barn Swallows Hirundo rustica, Quail Coturnix coturnix and White Storks Ciconia ciconia have been documented (Moreau 1928, Schuz et al. 1971). 'In the hollows between the dunes there were a few small bushes, and under each one of them the carcasses of Quail in various stages of desiccation were huddled.' (Moreau 1928).

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