Complex sentence: 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.
Split into simpler sentences: In the Cormorants, the gular skin is often coloured, contrasting with the otherwise plain black or black-and-white appearance of the bird. This presumably serves some function in social signalling, since the colours become more pronounced in breeding adults.