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Schmueli et al. (2000)

Passerines and Woodpigeon from ringing data, all others from radio-tracking (adults only). a: autumn, s: spring.

Flight speeds mainly from Bruderer & Boldt (2001). Bracketed speeds are from closely related species.

including the time needed for pre-migratory deposition would lower the mean flight speed to 62 km per day, the fraction of time spent in active flight to 4%, and the flight to stopover ratio to 1:24. bMainly overwater journeys.

Passerines and Woodpigeon from ringing data, all others from radio-tracking (adults only). a: autumn, s: spring.

Flight speeds mainly from Bruderer & Boldt (2001). Bracketed speeds are from closely related species.

including the time needed for pre-migratory deposition would lower the mean flight speed to 62 km per day, the fraction of time spent in active flight to 4%, and the flight to stopover ratio to 1:24. bMainly overwater journeys.

Finland in the first half of September and reach their wintering areas in France and Iberia around mid-October, the complete journey taking five weeks. The birds thus cover 3000 km at an average of 86 km per day. As they could cover 86 km in 3 hours, and the whole distance (if they could fly non-stop) in 105 hours, they must have spent up to 88% of their total five-week journey stationary, giving a ratio of flight-to-stopover of about 1:7. This agrees with the theoretical estimate given earlier for birds of this size.

Because the energy requirement per hour of flight is generally greater in large birds, and their refuelling rates are lower, they take longer to accumulate the fuel necessary for a standard journey than do small birds. In Brent Geese Branta bernicla migrating mainly over land between the Wadden Sea and the Taimyr Peninsula, the mean flight distance of 5004 km is equivalent to about three days and nights (72 hours) on the wing, assuming a mean ground-speed of 70 km per hour (Green et al. 2002). Because it took six geese on average 42 days to cover this distance, the mean rate of progress was 118 km per day (range 97-148), and only 7% of the time spent on spring migration consisted of active flying (but excluded initial fattening periods). This gave a flight-to-stopover ratio of 1:13. Long-time (foraging) stopovers made up 79% of the total migration time, and short-time stops another 14%. Including the time needed for pre-migratory fuel deposition in the Wadden Sea area would lower the mean flight speed to 62 km per day and change the flight-to-fuelling ratio to 1:25.

To take a more extreme example, consider five Bewick's Swans Cygnus colum-bianus bewickii radio-tracked on their autumn migrations between Siberia and the Baltic region (Beekman et al. 2002). These birds travelled the 2023 km journey in 34 days, on average. With a mean flight speed of 64 km per hour, an estimated 32 hours was spent on the wing, giving a flight-to-stopover ratio of 1:25. In spring, the equivalent figures from two birds were also 1:25. These estimates excluded the initial fattening period, but they broadly agree with theoretical predictions, and with the longer established observational finding that these Bewick's Swans take around two months, on average, on both their autumn and spring journeys. They also give some idea of the small proportion of the overall journey time spent by large birds in flapping flight when migrating over land, the rest of the time being spent refuelling and resting at stopover sites. We should not be misled by the fact that some swans can cross 1000 km of sea in less than two days (see later), because this makes no allowance for the period of prior fuel deposition. In practice, of course, birds often stay longer at stopovers than is needed solely to accumulate fuel, and in addition, their energy costs in flight are greatly influenced by wind conditions. On the spring journey, some birds may also be accumulating extra body reserves for breeding (Chapter 5).

Similar estimates of flight-to-stopover ratios for other species are given in Table 8.5, and generally support the notion that, among birds that travel by flapping flight, small species spend a greater proportion of the total journey time in flight than larger species. The largest species, such as geese and swans, show a flight-to-stopover ratio of 1:13 to 1:25 on overland flights. These estimates ignore long sea-crossings by geese and swans discussed later, because in estimates of their flight times, no feeding periods are included.

The situation in soaring birds is different. Their lower internal energy needs reduce the time they must spend feeding to accumulate large body reserves. In almost all the species for which relevant data are available, the flight-to-stopover ratios in adult birds varied between 1:3 and 1:7, reflecting the much reduced time spent on stopovers, but a few studies gave much longer stopover periods (Table 8.5). Considerable variation was apparent within species, with mean values of 1:4-1:7 from three studies of Peregrines, and of 1:3-1:7 from five studies of Ospreys. These stopover times are generally much lower than equivalent figures from similar-sized birds that migrate by flapping flight, and some are lower even than those from small passerines. They re-affirm the great advantage of soaring flight for large birds: it needs much less feeding time, so gives faster overall migration speeds. All the values quoted were from adult birds; and the non-breeding immatures of some species, which were under less pressure to return to breeding areas, showed much more leisured spring journeys (Table 8.5).

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