Weather And Inflight Mortality

Migrants often encounter bad weather en route, whether rain, mist or adverse winds. Over land, birds can usually seek shelter from storms, but this option is not available over water. Heavy rain can saturate the plumage, increase wing-loading (already high in many migrants through fuel deposition), and cause the loss of body heat. These stresses, often coupled with disorientation, sometimes force migrants down where they may be killed by collision, drowning or chilling (Frazar 1881, Saunders 1907, Cottam 1929, Williams 1950, Woodford 1963, Kennedy 1970). Flying birds are also sometimes killed by hail, or by electrocution in lightning storms (Hochbaum 1955, Roth 1976, Glasrud 1976). Clearly, it is important for birds to avoid flight at times of adverse weather, and most of the recorded mortality incidents listed in Table 28.1 refer to landbirds that encountered storms over water or other terrain where they could not take shelter.

Migrant landbirds caught by mist or storms over water must often be lost without trace, being consumed by gulls and other predators, or washed on to remote shorelines. Nevertheless, reports of mass deaths of migrants caught in bad weather (Table 28.1), and of birds arriving exhausted and emaciated, are fairly frequent (Morse 1980, Dick & Pienkowski 1979, Evans & Pienkowski 1984, Pienkowski & Evans 1985, Spendelow 1985). Examples include the large numbers of Quail Coturnix coturnix found drowned during a spell of sea fog off North Africa (Moreau 1927), the large numbers of dead Swifts Apus apus and martins

Table 28.1 Some large-scale mortality incidents associated with migration

Species

Date

Location

Conditions

Numbers

Source

A. Mortality during spring migration

Various species (>23 species) April 1881

Off Louisiana coasta

Gale

'Many thousands'

Frazar (1881)

Lapland Longspurs Calcarius lapponicus

March 1904

Minnesota-Iowa

Snowstorm

1.5 million

Roberts (1907a, 1907b)

Mainly Lapland Longspurs Calcarius lapponicus

February 1922

Nebraska

Snowstorm

'Thousands'

Reed (1922), Swenk (1922)

Magnolia Warblers Dendroica magnolia and others (39 species)

May 1951

Off Texas coasta

Rainstorm

>10 000

James (1956)

Ducks, geese and swans

April 1954

Wisconsin

Hailstorm

'Many'

Hochbaum (1955)

Various (>14 species)

May 1954

Minnesota

Snowstorm

>175

Frenzel & Marshall (1954)

Hirundines (4 species)

May 1956

Saskatchewan

Snow and cold

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