One of the most striking features of birds is their ability to fuel high-intensity endurance exercise with fatty acids that are stored in adipose tissue and delivered continuously to the working muscles by the circulatory system. This facility makes birds exceptional among vertebrate animals (McWilliams et al. 2004). Yet studies have increasingly revealed other amazing ways in which migratory birds have overcome physiological constraints in energy storage and migratory flight. For fuelling, these include various ways of temporarily increasing calorie intake rates, including the extension of feeding times (sometimes into night-time), nighttime migration to leave the day for feeding, the switching of metabolic emphasis from carbohydrate to fat, change of diet, and increase in the size and effectiveness of the digestive tract. For flight, they include reduction in the size of digestive organs immediately before or during the flight, and catabolism of protein to supply metabolites needed for the utilisation of fat and other purposes. These are all measures which the bird can take for short periods only, as they also have costs, and conflict with other activities or with requirements at other stages of the annual cycle.
Growth in the gut and liver facilitates rapid fat deposition, but in the days before departure by long-distance migrants these organs may shrink, while the pectoral muscles and heart continue to grow, providing the increased metabolic power necessary for sustained flight or for heat production at high altitudes (Piersma et al. 1999). Improving intake and assimilation, or cutting expenditure, are all ways in which birds can increase the efficiency of migration (Piersma 2002), enabling some species to perform extreme journeys otherwise impossible, or to carry body reserves from one region in order to facilitate reproduction in another far away. What astonishing creatures birds are!
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