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.
^Frances, Peter; et al. (2007). Bird: The Definitive Visual Guide. Dorling Kindersley Inc. ISBN1564582957. CS1 maint: Explicit use of et al. (link)
This article is part of Project Glossary, a All Birds project that aims to write comprehensive articles on each term related to animals.