The rate at which birds can process and digest food could also constrain rates of fuel deposition. Hence, fuelling rates could be described as 'food-limited' or 'feeding-time limited' at one level, or 'metabolically limited' at a higher level (for a likely example of a metabolically limited fuelling rate see Dierschke et al. 2003). Early studies undertaken mainly outside the migration seasons suggested an upper limit to the daily metabolisable energy intake (DMEmax) of birds at around 4-5 X BMR, imposed by digestive physiology (and theoretically equivalent to a maximum rate of 2200 kJ per kg072, Kirkwood 1983). This in turn implied an upper limit on the rate of fuel deposition (FDRmax).
During migration time, however, owing to the steps birds take to improve their food-processing capacity, they can also achieve higher rates of fuel deposition. Twelve out of 22 species examined at migration times had rates of energy intake exceeding the theoretical limit (Lindstrom & Kvist 1995). Studies of captive birds in a migratory state, notably shorebirds, have shown that, given sufficient food, individuals can fatten at much higher rates than normal - up to 6 X BMR, but reaching up to 10 X BMR in shorebirds given access to food for 24 hours per day (Kvist & Lindstrom 2003). While wild birds would not normally have free access to food all day and night, these findings show what some migratory birds can achieve under near-optimum conditions.
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