More food can be processed per unit time by increasing the size of the digestive tract, or adjusting its structure and activity to better deal with particular types of food. Changes in gut structure and action have been documented in several migratory species from passerines to geese, and usually take several days to enact (McLandress & Raveling 1983, Jordano 1987, Afik & Karasov 1995, Bairlein 1996a, McWilliams & Karasov 2005). Studies on captive Blackcaps Sylvia atricapilla revealed that the rate of energy assimilation under ad lib food was proportional to the size of the intestinal tract and liver (Karasov & Pinshow 2000). However, it is uncertain whether such gut changes anticipate increased food intake or occur in response to it. The latter seems most likely, but in any case such massive gut changes are presumably not without costs (Karasov 1996, Hume & Biebach 1996, Piersma et al. 1999). For one thing, they increase overall body mass, affecting agility and vulnerability to predation.
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