In birds followed on both outward and return journeys, it was possible to calculate how much of each year, on average, they spent on migration. The figures for passerines are based on individual ring recoveries, and for many species may be biased in favour of the faster migrating individuals, and so of limited value in this respect. However, for a few species, data on average migration times from ring recoveries or radio-tracking are available. They suggest that Barn Swallows Hirundo rustica migrating between northern Europe and southern Africa can spend around 16 weeks per year on migration (or 31% of the entire year). Among waterfowl, radio-tracked Bewick's Swans Cygnus c. bewickii spent about 33% of the year on migration, while storks and cranes gave figures up to 59% of each year in different populations, and raptors up to 42% of each year in different populations (Table 8.3). As expected, the migration period depended on the journey, with species making the longest migrations taking the longest periods. The duration of a breeding cycle (from egg-laying to independence of young) and of moult also increase with body size in birds, leaving less time to complete a long return migration within an annual cycle. Regardless of the number of feeding stops, this may constrain the length of total migratory journey that can be undertaken by large species, and hence their geographical ranges. It would be hard to imagine some large species being able to perform even longer migrations without compromising reproduction (Hedenstrom & Alerstam 1998). The problem is perhaps especially acute in some species of geese and swans which seem unable to begin migratory fattening until vegetation growth begins in spring, providing the necessary increase in food supply. Such species also make some of the slowest journeys (including feeding periods), and it is perhaps not surprising that no arctic-nesting goose or swan species migrates further than about 6000 km, although journeys twice this length are performed by many smaller birds, including soaring species. The implication is that, in some large bird species that travel by flapping flight, fuelling times may make it impossible to breed, moult and migrate over longer distances within one year. Fuelling times may thereby limit geographical ranges (Hedenstrom & Alerstam 1998).
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