Three types of limitation could influence the fuelling rates of birds. The most obvious is the rate of food intake - the amount that can be consumed per 24 hours. This rate is affected mainly by external conditions: by daylength, food availability through the day, the feeding efficiency of the bird itself, and the constraints on feeding rate imposed by competitors and predators (Chapter 27). The bird might increase its intake rate in various ways, such as feeding more rapidly or for longer than usual each day, or by selecting from potential foods the most calorific and easily digestible items. Some waterfowl and shorebirds can feed both by day and by night, and can thus achieve higher rates of food throughput than other birds, and correspondingly higher rates of fuel deposition (Zwarts et al. 1990a).
The second type of limitation is imposed by digestive efficiency which, regardless of the rate of intake, limits the amount of food that can be processed per unit time (Diamond et al. 1986, Klaassen et al. 1997). Again, birds might maximise throughput in various ways, such as ensuring that food is always present in the crop, ready for passage down the gut as soon as space becomes available, or increasing the throughput rate so as to digest food less thoroughly but in greater quantity than usual, or by modifying gut structure so as to digest food more rapidly.
The third potential constraint comes from crop capacity. Many birds, especially herbivorous species, normally fill the crop before going to roost, and digest that food during the night. If crop capacity is small relative to night length, then crop capacity could be said to limit daily intake. To my knowledge, the extent to which crop capacity can be increased at migration seasons has not been studied.
The effects of various mechanisms in raising energy intake may be enhanced by reduction in other energy-demanding activities, such as moving around. However, all these mechanisms have costs which affect the bird adversely in other ways, so they normally occur only at migration seasons or at other times when demands are high. They are apparently under endogenous control, and normally appear only at appropriate times of year, as revealed in captive birds (Berthold 1976).
The relative contributions of several of these mechanisms to migratory fattening have been studied in certain species (Bairlein 1985c, Bairlein & Simons 1995). For example, the Garden Warbler Sylvia borin at migration time eats up to 40% more per day (in terms of convertible energy), and switches from a mainly insect to a mainly fruit diet (often Elder Sambucus niger at northern latitudes and figs Ficus at Mediterranean latitudes). Increase in digestive and assimilation efficiency accounts for another 20% increase in metabolised energy during pre-migratory fattening (Bairlein 1985c, 1991a). This increased digestive efficiency involves an increase in gut weight, and in the rate of synthesis of fatty acids in the liver. In contrast, no such change in utilisation efficiency was found in seed-eaters, such as White-crowned Sparrows Zonotrichia leucophrys and Bobolinks Dolichonyx oryzivorus, the extra calories for fuel deposition being obtained entirely by longer feeding periods (King 1961, 1972, Gifford & Odum 1965). The various mechanisms to increase daily energy assimilation are discussed in more detail below.
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