Care is needed in assessing the rates of weight gain in birds because weight varies with time of day, and often drops (or increases less rapidly) for some hours after capture, as the bird may react to handling (e.g. Schwilch & Jenni 2001). Nevertheless, the repeated trapping of migrants in the days before they set off on migration, or at stopover sites en route, has provided information on their individual rates of weight gain (e.g. Figure 5.1), from which average and maximum rates for different populations have been calculated (Alerstam & Lindström 1990, Lindström 2003). Expressed as the daily (24-hour) gain in mass relative to lean body mass, average rates of pre-migratory weight gain (mostly fat), as measured in 58 populations, ranged from less than 1% to more than 7% (maximum 13%). Exceptionally high rates were recorded on particular days, but not sustained over a longer period. In captive passerines, weight gain was often greater on the second than on the first day of fattening, a difference attributed to growth of the digestive tract to facilitate more efficient food processing (Alerstam & Lindström 1990, Hume & Biebach 1996, Klaassen et al. 1997).
Comparing species, mean rates of weight gain decline disproportionately with increasing body size, so that large species generally accumulate reserves more slowly than small ones, and depart with relatively smaller reserves (Figure 5.5). The smallest birds have daily weight increases that, in relation to their lean weights, are five or more times greater than those of large birds. This is because the maximum limit to the daily metabolisable energy intake is proportional to basal metabolic rate (which declines with body mass), rather than to body mass itself (Lindström 1991). In field studies of various bird species, maximum rates of daily fuel deposition were rarely above 10% of lean body mass in the smallest birds studied (3-g hummingbirds) and rarely above 2% in the largest (3-kg geese). The three main groups of birds examined (passerines, shorebirds and other non-passerines) showed no significant differences in mass-specific rates of daily fuel deposition (Lindstrom 2003). Combining all species, the relationship between daily fuel deposition and body mass (M) for populations was 1.16 M~0'35 (r2 = 0.66, P < 0.001), and for individuals 2.17 M~0'34 (r2 = 0.54, P < 0.001). The slopes of these relationships were not significantly different from the maximum rate of fat deposition predicted on theoretical grounds as around M~0'27, where M is lean body mass (Lindström 1991, 2003).
These various findings all go to illustrate the enormous variation in fuelling patterns found among birds, some of which may result from nutritional constraints on fattening and others from adaptive differences between populations,
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