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In ornithology, gular skin refers to an area of featherless skin found in some species of bird, which joins the lower mandible of the bill to the bird's neck. It is prominent, for example, in members of the order Pelecaniformes. In the cormorants the gular skin is often coloured, contrasting with the otherwise plain black or black and white appearance of the bird, and it presumably serves some function in social signalling since the colours become more pronounced in breeding adults. In frigatebirds, the gular skin or sac is used dramatically. During courtship display, the male forces air into the sac, causing it to inflate over a period of 20 minutes into a startling huge red balloon.
