In addition to empirical studies, attempts have been made to estimate theoretical migration speeds of particular species from knowledge of their likely flight speeds, and rates of fuel deposition and use (Boxes 8.1 and 8.2, Figure 8.1; Hedenstrom & Alerstam 1998, Alerstam 2003). Flight speeds can be taken from radar measurements (ideally corrected to their still-air values, Bruderer & Boldt 2001) or from theoretical estimates (Pennycuick 1969, 1975). Rates of fuel deposition can be measured from the repeat weighing of individual birds before migration or at stopover sites. Such measurements are usually taken over periods of days, so include sleeping and other non-feeding times. Rates of fuel use during flight can be measured only with difficulty, and are more easily obtained as theoretical estimates, based on body mass and other features of the bird (see Appendix 5.1). Rates of fuel (energy) gain and loss are best expressed in some common currency, such as multiples of BMR (basal metabolic rate, the rate of energy use by a resting, inactive bird). Daily rates of energy gain (above the 2 X BMR typically required for everyday life) are usually around 1 X BMR, but in good feeding conditions they can reach 2.5 X BMR (Alerstam & Lindstrom 1990). For birds that can feed by night as well as by day, such as shorebirds, rates of fuel deposition can be much higher, occasionally reaching up to 10 X BMR (Kvist & Lindstrom 2003). In these conditions, birds show extremely rapid accumulation of body reserves.
On such a basis, for a small bird with an energy deposition rate of 1 X BMR (above the 2 X BMR required for normal daily activity), and travelling by flapping flight, the predicted average migration speed is 200 km per day (Hedenstrom & Alerstam 1998). This includes both flight and fuelling periods. At the higher energy deposition rate of 2.5 X BMR, the predicted speed rises to 300-400 km per day. In larger birds, theoretical migration speeds are lower, at 70-100 km per day for an energy deposition rate of 1 X BMR, and at 150-200 km per day for an energy deposition rate of 2.5 X BMR.
Box 8.1 Calculation of theoretical migration speed. From Hedenstrom & Alerstam (1998)
The speed of migration (Vmigr) is determined by the rate at which fuel is accumulated (Pdep), the flight speed (V) between consecutive stopover sites (which depends on wind conditions and flight mode, etc.), and the flight power (Pflight), the rate at which energy is consumed), according to the following general relationship:
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