Subcutaneous fatA layer of fat just underneath the skin to provide energy during migration, also called 'pre-migratory fat'. Its development causes a very noticeable increase in weight in a bird preparing for migration and it enables a migrant to travel long distances without having to stop for food, an invaluable aid in sea or desert crossings.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver