A thin-walled structure joined to the lungs of a bird, involved in respiration. There are usually nine air sacs, which may extend into some of a bird's hollow bones. Birds that plunge-dive into water, such as gannets, have modified air sacs beneath their skin to help cushion the impact.[1]


  1. ^ Frances, Peter; et al. (2007). Bird: The Definitive Visual Guide. Dorling Kindersley Inc. ISBN 1564582957. 

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