Unseasonable weather can also affect birds about to leave their breeding areas in autumn (Table 28.1). In central Europe in September 1931, during an early cold snap, 'immense numbers of Swallows Hirundo rustica and House Martins Delichon urbica, and some Sand Martins Riparia riparia, numbed by the cold and unable to find food, sought shelter in stables, barns and houses where, in some cases, they congregated in great masses and were so inert that they could be picked up by hand' (Alexander 1933). The 89 000 individuals that were collected and transported by air for release in Italy 'were probably a small fraction of the total lost'. Another event in central Europe in 1974 had similar effects, with hundreds of thousands of hirundines reported dead in the Alps region of Switzerland, Austria and southern Germany, and two million found alive flown by planes further south (Bruderer & Muff 1979, Reid 1981). In the next spring (1975), Northern House Martin Delichon urbica populations across Switzerland were reduced by 25-30%, but no overall effect was noted in Barn Swallows Hirundo rustica (despite some local reductions) (Ruge 1974, Bruderer & Muff 1979). However, many ringed Barn Swallows from Denmark were recovered in central Europe during this event (presumably on migration), and the following spring breeding numbers in a study area were reduced by more than one half (Moller 1994b).
Occasionally, Swifts Apus apus are unable to leave their north European breeding areas because of adverse weather. They usually leave south Finland by the end of August, but in 1986 when late summer was cold and wet, around 2000 remained into November before dying (Kolunen & Peiponen 1991). Seven were found dead in their roost sites, and others still alive were clearly starving. Scarcity of aerial insects prevented the birds from accumulating fat for migration and they remained in southern Finland until their death. Similar events occurred in 1918 and 1957, but in these years delay was caused by late breeding resulting from cold weather in spring.
Autumn mortality events have also occurred among King Eiders Somateria specta-bilis, following the early freezing of sea-water (Barry 1968). One in 1961 near Banks Island (Canada) involved an estimated 50 000 females and young. The males had already migrated and escaped the freeze. This event supposedly killed almost the entire Banks Island population of breeding females. Similarly, 21 Brent Geese Branta bernicla, hatched on Southampton Island (Canada) in the relatively late spring of 1956, were found frozen in shoreline ice the following spring. 'They were well preserved, and nothing could be found wrong with them except that their feather development was 4 to 5 days short of allowing them to fly' (Barry 1962).
Other mass-mortality events, which sometimes occur during extreme weather in summer or winter, are not directly associated with migration, and can affect resident as well as migratory species (Newton 1998b). However, it is sometimes difficult to separate migration-associated losses from other weather-induced mortality. For example, an estimated 27 000-62 000 wintering diving ducks (mainly Tufted Ducks Aythya fuligula and Pochards Aythya ferina) starved to death in Switzerland and Holland during a cold spell in March 1986, at a time when these birds would normally set off on spring migration (Suter & van Eerden 1992). This incident is not listed in Table 28.1 as migration mortality. But the authors argued that, if such weather had occurred earlier in winter, the birds would have moved further south and escaped starvation. As it was, they were programmed to move, if at all, towards their breeding areas, which prevented their usual hard-weather response.
Similarly, many large-scale mortality incidents have been recorded from time to time in seabirds (so-called wrecks), when large numbers are blown ashore, sometimes far inland (for review see Newton 1998b). It is usually unknown whether these birds were on migration at the time, or in their wintering areas. However, because some seabird species seem to remain on the move for most of the time between leaving their breeding places in one year and returning there the next, migration periods cannot always be separated from other parts of the annual cycle. The same could be said for some landbird species, which seem able to move long distances at almost any time during the course of a winter to avoid food shortages (Chapter 16).
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