As the body reserves of a bird increase, so does its flight and migration speed and its potential flight range, but not in direct proportion. This is mainly because the extra fuel itself requires energy to synthesise, maintain and transport, so as body reserves increase, so do the flight costs per unit travel distance (Pennycuick 1989, Lindstrom & Alerstam 1992, Witter & Cuthill 1993). The costs of migratory fuel are reflected in metabolic rates which rise and fall in line with body weight. For example, in a captive Thrush Nightingale Luscinia luscinia, basal metabolic rate (BMR2) increased in almost direct proportion to body mass. Over 48 hours, BMR increased by 22.7%, in parallel with an increase in body mass of 24.3% (Lindstrom et al. 1999). Likewise, some Great Knots Calidris tenuirostris on their 5400 km non-stop flight from Australia to China in spring lost about 40% of body weight during the four-day journey. BMRs measured in birds just before and just after their flight were found to have fallen by an average of 42% in association with the reduction in body mass (Battley et al. 2001). In Barnacle Geese Branta leu-copsis travelling from Svalbard to Scotland, heart rate (reflecting metabolic rate)
2BMR = basal metabolic rate, the rate of energy consumption of a bird at rest, in a post-absorptive state.
was 29% lower just after the flight than before, again reflecting the associated decline in body weight. Resting metabolic rates decline with loss of body mass presumably because this loss involves some metabolically active tissue, such as muscle (Butler et al. 1998). Clearly, BMR varies considerably within individuals, in association with the rapid changes in their body mass, and deposition of extra fuel entails a considerable 'holding' cost.
In addition to its maintenance and transport costs, extra fuel makes a small bird less agile and more vulnerable to predation. This is another reason for a bird not to accumulate larger body reserves than necessary. Even the slight weight increase shown by small birds during the course of a normal day greatly reduces their lift-off speed and manoeuvrability, and birds accumulating migratory fat suffer much greater impediment (Witter et al. 1994, Metcalfe & Ure 1995, Lee et al. 1996, Kullberg et al. 1996, 2000, Lind et al. 1999). For example, when captive Blackcaps Sylvia atricapilla were exposed to simulated predator attacks, individuals carrying a fuel load equivalent to 60% of lean body mass (the maximum recorded in this species) were calculated to suffer reduction of 32% in angle of ascent and 17% in velocity, compared with lean Blackcaps (Kullberg et al. 1996). This degree of difference could put fat birds at substantially greater risk (Lind et al. 1999, Burns & Ydenberg 2002). These considerations should favour a migration strategy of short flights, frequent fuelling and low fuel loads wherever possible, with the alternative of long flights, infrequent fuelling and heavy fuel loads resorted to only when necessary. Avoidance of predation may also be one reason why many small birds migrate at night, when diurnal birds of prey are usually roosting (Chapter 4).
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