A major perceived cost of migration in birds is the associated mortality. This mortality has proved difficult to measure, and separate from mortality during stationary periods of the annual cycle. This chapter summarises some major recorded mortality incidents attributed to inclement weather and other factors, including: (a) in-flight losses, caused by storms and other adverse weather en route, usually over water; (b) unseasonable cold weather soon after arrival in breeding areas; and (c) unseasonable cold weather before departure from breeding areas. Cold weather often kills migrants in their breeding areas, but not the local resident species which can better withstand it. For migrants, cold and snow act to cut off the food supply, and could have a major selective effect on the seasonal timing of migration (as demonstrated in a few studies).

Records of in-flight weather-induced mortality, involving up to hundreds or thousands of birds at a time, have affected mainly small passerines, but also larger birds, including eagles and swans. Most occurred in conditions of mist, rain or snow storms, but some also involved nocturnal collisions with illuminated masts and other structures.

Records of post-arrival mortality on breeding areas have involved mainly small insectivores (especially hirundines), but also waders and waterfowl. Such incidents, associated with cold and snow, have reduced regional breeding densities from the previous year by 30-90%, depending on species and area, with up to several years required for recovery.

Records of pre-departure mortality on breeding areas have mainly affected hirundines. Two major incidents in central Europe in September 1931 and 1974 killed hundreds of thousands, or even millions, of swallows and martins. After the latter incident, Northern House Martin Delichon urbica populations in Switzerland the following year were reduced by an estimated 25-30%. Such climatic extremes that occurred in spring or late summer in particular parts of the breeding range have been recorded at approximate mean frequencies of 2-10 per century.

In addition to weather effects, many migrants die each year from collisions with tall masts and buildings. Average daily mortality in many bird species can clearly be much greater during migration periods than during stationary periods. Despite the heavy losses of birds on migration, it may be assumed that migration persists in the long term because the costs (in terms of associated mortality) are more than offset by the benefits (in terms of improved overall survival and breeding success).

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White Storks Ciconia ciconia thermal-soaring on migration

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