There are about 40 species of cormorants found throughout the world and we have three of them here in California -- the double crested, Brandt's, and pelagic. The name "cormorant" is to derived from Latin -- Corvis marinos -- literally ravens of the sea. That's a pretty good name because most of them are black and are found in the ocean, though several species do flourish in fresh water as well.
I remember as a child reading "The Story of Ping." In this book there was a section about a Chinese fisherman using cormorants to fish. In many parts of Asia, especially Japan and China, men still capture cormorants as chicks, bond with them, and raise them to adulthood. The fishermen constrict the bird's neck with a brass ring.
The cormorants are excellent fishers and capture fish that are too large for them to swallow. They then bring these large fish back to the fisherman in exchange for small fish that will fit down their necks. I've been fascinated with cormorants ever since reading this book.
The uro-pygial, or preen gland, is the only skin gland that birds have. It is located at the base of the tail. You often see birds reaching with their beaks in that general area as they preen. The oil, in addition to providing the precursor to vitamin D, also waterproofs and protects the feathers. Oil is naturally buoyant and some cormorants, like our double crested, have a poorly developed gland thereby allowing the bird to sink and swim more efficiently. However the price they pay is periodically having to dry their feathers. This is a classic picture of cormorants in a "salutation to the sun" yoga pose.
The double crested is the commonest cormorant and thrives inland in rivers and lakes, as well as oceans and bays. They are found fromAlaska to Central America and all throughout the United States. Some of the best nesting sites here in the San Francisco Bay Area are the bridges, including the San Rafael/Richmond Bridge where they nest just outside the eastbound fast lane! But keep your eyes on the road please! No bird watching.