Mass mortality of migrants

Migration is a season full of peril for great numbers of winged travellers. (W. W. Cooke 1915.)

Great numbers of migrating birds, chiefly warblers, had accomplished nearly 95% of their long flight and were nearing land when, caught by a norther, against which they were unable to contend, hundreds were forced onto the waters and drowned. . . . During the fall migration of 1906, when thousands of birds were crossing Lake Huron, a sudden drop in temperature, accompanied by a heavy snowfall, resulted in the death of incredible numbers. Literally thousands were forced into the water and subsequently cast up along the beaches, where in places their bodies were piled in windrows. On one section of the beach their numbers were estimated at 1,000 per mile, and at another point at five times that number. (Frederick Lincoln 1935.)

Despite its overall benefits, migration is often perceived as hazardous. During their seasonal journeys, migrating birds must travel through unfamiliar areas, and often through alien habitat, making it more difficult than usual for them to find food and avoid predators. They may run out of fuel, suffer from exhaustion, or encounter storms which could kill them. One might imagine, therefore, that the mean daily death rate would be greater during migration than at other times but, for understandable reasons, little coherent information is available on the mortality costs of migration (but see Chapter 27; O'Briain 1987, Owen & Black 1989, Sillett & Holmes 2002, Menu et al. 2005).

While the previous chapter alluded to the ongoing mortality resulting from food shortage, predation and other factors operating at stopover sites, this chapter reviews some additional major mortality incidents described in the ornithological literature. They establish the importance of such additional mortality as a frequent hazard for migratory birds, even though their effects on populations are hard to estimate. Almost all such incidents occurred during adverse weather, either during the journey itself, soon after arrival in breeding areas in spring, or just before departure from breeding areas in late summer or autumn. Typically, those incidents that occurred in breeding areas were not matched by concurrent mortality among local resident species, and could have been avoided if the migrants had arrived some days later or left some days earlier. They are therefore classed here as migration-related.

The risks of migration itself vary with the body size and other features of the birds themselves, with the length of the journey and the terrain to be crossed, and with weather at the time. In general, the risks would seem to be greater for small birds than for large ones, over long journeys than short ones, in adverse than favourable weather, and over hostile than favourable terrain. These generalisations are borne out by the events discussed in the following paragraphs (Newton 2007).

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