Because mortality incidents that occur over sea or desert are usually impossible to detect, it is hard to tell whether they are of more than sporadic occurrence, let alone what proportion they form of the overall annual mortality. Losses that occur in particular storms may involve birds from wide areas of the breeding range, so have less impact on local breeding densities than the number of casualties might suggest. However, losses that occur in spring, when numbers are near their seasonal low, are more likely to affect subsequent breeding densities than are those that occur in autumn when numbers are near their highest. In populations in which overwinter mortality is density dependent (Newton 1998b, 2004b), losses on autumn migration could be largely or entirely offset by reduced overwinter losses, but by spring there is much less scope for such compensation before breeding begins. Most of the documented mortality events in Table 28.1 were in spring which in many northern regions tends to have more severe weather events than autumn, affecting birds on passage, or soon after arrival in breeding areas.

Of the 23 spring in-flight mortality incidents listed in Table 28.1, eight occurred during heavy rain, five during snow, two in hail, two in dense mist, four in strong winds, and two in unspecified 'bad weather'. Of eight such incidents in autumn, three occurred in heavy rain, three in snow, and two in dense mist. Incidents were categorised according to the main weather factor, as mentioned by the author, but in some a combination of factors was involved: for example, rain and mist, or rain and wind. Clearly, mist, rain and snow (or rain and snow associated with strong winds) were important causes of mass mortality among birds in flight. All 18 incidents that occurred soon after arrival in breeding areas were associated with a marked drop in temperature; ten also included snow, four included rain, and two involved the re-freezing of lake or sea-water. All eight incidents that occurred in late summer-autumn before departure were also associated with exceptional cold, including four with snow, two with rain, and two with freezing lake or sea-water. In none of these incidents were resident bird species mentioned as casualties.

In general, the survival of any bird through a difficult journey may depend not only on the weather encountered, but also on the bird's weight (and fat content) on departure, in turn influenced by the prevailing food supply, age and dominance, and levels of competition in the population (Chapter 27). In addition, some juvenile birds may die through their inexperience, making them more vulnerable than adults to various kinds of hazard, while others could die through directional or navigational errors. Vagrants are almost all first-year birds, as are individuals that in spring 'overshoot' their normal breeding range (Chapter 10).

Because the records in Table 28.1 result from incidental observations, and not from systematic study, it is impossible to estimate how often particular populations experience different types of mass-mortality events. However, in the twentieth century, Northern Lapwings Vanellus vanellus in Finland experienced at least two mass starvation events just after arrival, Common Swifts Apus apus in Finland experienced at least three such events before departure, hirundines in central Europe experienced at least two such events just before departure, and King Eiders Somateria spectabilis in the Beaufort Sea experienced at least four such events at stopover sites in spring. Cliff Swallows Hirundo pyrrhonota in Nebraska experienced at least 11 spring mortality events in 123 years (including two of extreme severity), and Lapland Longspurs Calcarius lapponicus in Minnesota experienced three such events in 25 years. These figures give some indication of the likely frequency of major mortality incidents, while smaller events in other years may pass without being noticed or documented. If such big events were to increase in frequency, they would presumably lead to changed migration timing or route (through selection), or eliminate altogether those populations occupying particular parts of the breeding range.

In addition to the major weather-induced catastrophes, other birds are presumed to die because they get drifted too far off course, or run out of fuel in places where they cannot feed. Many records exist of emaciated or dehydrated birds that have crossed seas or deserts (for records from a ship off West Africa, see Serle 1956, and from a ship off northeast Cuba, see Johnston 1968), and small islands may often attract weakened migrants from the passing stream (Spendelow 1985).

There can be no doubt, therefore, that many birds die on migration, and that in some weather-induced incidents, the numbers can be very large. In some of the examples mentioned above, the daily mortality during migration, or soon after arrival in breeding areas, was almost certainly much greater than the mean daily rate at other times of year. Estimates from the Black-throated Blue Warbler Dendroica caerulescens, Barnacle Goose Branta leucopsis, Brent Goose B. bernicla and Greater Snow Goose Chen caerulescens in the previous chapter provide striking examples, and in other species (such as Northern Lapwing Vanellus vanellus and King Eider Somateria spectabilis), the losses immediately after arrival in breeding areas in unusually cold springs exceeded the normal total annual mortality expected in these species. Following post-arrival losses in spring, local breeding populations of various species fell by 25-90%, and following heavy pre-departure losses in autumn, subsequent breeding numbers of House Martins Delichon urbica in Switzerland fell by 25-30%, and Barn Swallows in part of Denmark by more than 50% (Moller 1994b). Apart from these extreme examples, it is hard to judge the importance of such major weather events in relation to the ongoing but more diffuse losses due to food shortage, predation and other regular mortality agents. Many severe weather events are unlikely to cause mass mortalities, because birds do not normally depart on migration when conditions look bad (Chapter 4). Most losses occur when birds already aloft over water encounter storms en route. Whether the bodies of casualties reach shore, where they might be recorded, depends on many factors, such as how long they remain afloat, the direction and strength of winds and currents, proximity to land, and actions of scavengers (Bibby & Lloyd 1977).

Regarding possible effects of overwater storms on population levels, Butler (2000) pointed out that, among Nearctic-Neotropical migrants, 25 species had declined in eastern North America during the period 1966-1996, compared with only three in western North America over the same period. The eastern species migrate partly over water (western Atlantic or Gulf of Mexico), while the western ones migrate entirely over land. Moreover, among the water-crossers, declines were significantly more frequent among 13 long-distance migrants, wintering in South America, than in shorter distance migrants, wintering in Central America or on Caribbean Islands. In two of the 25 eastern species (Rose-breasted Grosbeak Pheuticus ludovicianus and Mourning Warbler Oporornis philadelphia), annual population levels (as measured by the Breeding Bird Survey) were related to the number of stormy days during the previous autumn migration (mean 39 stormy days, range 18-59 days in different years). Years with the lowest populations had followed autumns with the most storms. These findings do not necessarily imply causal relationships, but they do suggest that further investigation of weather effects on the population levels of overwater landbird migrants would be worthwhile. The autumn migration of many species between North and South America coincides with the time of hurricanes, the frequency and severity of which have increased in recent years in association with climate change.

Although such losses represent a major cost of migration, for the migratory habit to persist, they are presumably less in the long run than any losses that would be experienced if the birds stayed on their breeding areas year-round. For many species, high-latitude breeding areas are completely uninhabitable in winter. For others, the costs of reaching distant wintering areas may be offset by improved survival there, promoted by milder weather or greater food supplies (Chapter 23).

Migration among birds thus involves a trade-off: the fitness benefits of breeding and wintering in separate regions, set against the fitness costs of the journeys themselves. Mortality that occurs through adverse weather en route presumably has an important selective influence on the birds' behaviour with respect to weather, including the routes taken, while mortality that occurs soon after arrival in breeding areas, or just before departure from breeding areas, has a selective influence on the timing of migration within the annual cycle (for evidence of genetic influence on migration dates see Chapter 20; Berthold 1993, Moller 1994, Brown & Brown 2000). That mortality events associated with migration are not more frequent is testimony to the adaptive behaviour of birds in avoiding bad weather, either by not flying then, or by circumventing it.

In conclusion, although many mass-mortality events among migratory birds almost certainly go unrecorded, and most documented ones cannot be translated into population-level impacts, the sheer scale of some events must inevitably result in temporary reductions in breeding numbers over local or wider areas (as so far documented in a few studies). Almost certainly such mortality represents in some bird species a major cost of migration.

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