As well as increasing energy intake during migratory fuelling, birds can also reduce expenditure, as mentioned above, by reducing locomotory activity. The switch from insects to berries, for example, may lead to reduction in the energy spent on foraging movements. Birds may also lower their metabolic rates and body temperatures when sleeping at night, as a means of conserving energy, and migrant hummingbirds usually become torpid at night, even when very fat for migration (Carpenter & Hixon 1988). Barnacle Geese Branta leucopsis show a progressive reduction in mean daily abdominal temperature (down to 4.4°C below usual), which begins just before the birds embark on their migration and continues, on average, for about three weeks (Butler et al. 2003).
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