What knowledge is "jizz"?

Jizz is used by many birders to identify bird species. The definition of jizz differs between authors, but concerns aspects of the bird that are not always easy to define scientifically or describe in objective terms. Rather, impressions of the bird are favoured, including size, shape, behavior and appearance. Here we try to show that jizz is a term worth studying in detail, since its use offers insights in how to identify species, why there are differences among observers in bird surveys and why traditional knowledge about biological diversity might be worth saving when preserving species.