Identification by plumage is the first and foremost technique used by the modern ornithologist. But there are so many occasions when it is just not enough. Even the best of seabird watchers need more than beak and feather detail to tell a common tern from an Arctic. When the expert birder can identify a robin at a 100 yards by the way it drops off a fence, he's always using jizz. Artists have provided a folio of illustrations of the essential character of a bird from form and behaviour, helped by Rob Hume's written interpretations.