Migration mortality has proved difficult to measure as a distinct component of the overall annual mortality. This is because the chance of getting ring recoveries varies along a migration route so does not reflect the scale of mortality in different places, and with radio-marked birds, it is not usually possible to distinguish death from radio failure. However, in a study of Black-throated Blue Warblers Dendroica caerulescens, Sillett & Holmes (2002) were able to assess survival rates during the summer breeding period in New Hampshire, during the winter period in Jamaica, and over the year as a whole, respectively. They concluded that more than 85% of apparent annual mortality occurred during migration, giving a rate which was at least 15 times higher than at other times of year. The relative importance of different types of mortality to the overall migration total remained unknown.
Among Barnacle Geese Branta leucopsis travelling 3200 km between Svalbard via Bear Island to Scotland, it proved possible in 1986 to check for colour rings in the same group of birds just before and after this migration (Owen & Black 1989). About 35% of the juveniles were found to have disappeared (presumed dead) on this one journey, compared with about 5% of older birds (about half the annual total). The losses were greatest among young hatched latest in the season, which were lightest in weight at the date of departure. This amount of mortality was deemed exceptional, however, because severe weather in the breeding area forced the birds to leave earlier than usual and also stopped some from staging on Bear Island. Once juveniles reached their wintering areas, mortality dropped to a level equivalent to 10% per year, the same as adults. Evidence of similar mortality during the autumn journey of Light-bellied Brent Geese Branta bernicla hrota migrating from northern Canada to Ireland was provided by O'Briain (1987). Their migration covers at least 2500 km, part of which crosses the Greenland ice cap. In each of two years, loss of young averaged 33%, compared with 5% for adults.
As mentioned above, Greater Snow Geese Chen caerulescens atlantica migrate from breeding areas on Bylot Island in the Canadian Arctic to staging areas on the St Lawrence River in Quebec. Mortality on this autumn journey was calculated in five successive years, both from banding studies and from comparison of the brood sizes of neck-banded females before and after migration (Menu et al. 2005). The two approaches yielded similar mortality estimates, and the same pattern of year-to-year variation. The average monthly mortality of juveniles over the autumn journey was 34%, and that of adults 1%. However, after this migration, juveniles survived as well as adults (both 97% per month). The loss of juveniles over the migration period also varied greatly between years (range 29-88% over five years), and most mortality appeared to be natural (rather than due to shooting). Juvenile mortality was highest in years when: (1) temperatures at the time of fledging and migration were low (at or below freezing), (2) the mean body mass of goslings at fledging was low, and (3) the mean fledging date was late. These studies indicate how conditions on breeding areas can influence subsequent mortality on migration.
In some other species, too, most of the difference in annual mortality between juveniles and adults was attributed to greater losses among juveniles before they reached their winter quarters (Cavé 1983, Gromadzka 1989, Pienkowski & Evans 1985). In other species, mortality on the following spring journey was also heavier in juveniles than in adults. This was deduced, for example, from comparison of age ratios among museum skins of Pacific Slope Flycatchers Empidonax d. difficilis obtained at the start and end of each migration (Johnson 1973). In some migratory species, notably raptors and shorebirds, the annual survival rates among adults are so high (>90%) that working out how much of the annual mortality occurs on migration is seldom going to be easy.
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