Jizz (birding); the immediately recognisable characteristics of a bird or other organism.
Sean Dooley described jizz as “the indefinable quality of a particular species, the ‘vibe’ it gives off” and notes that although it is “dismissed by many as some kind of birding alchemy, there is some physical basis to the idea of jizz.”
I asked my brother and he says the main term used is GISS, which is also mentioned in the article. It’s an acronym for general impression of size and shape. He hadn’t heard of jizz in his 30+ years of birding around the world.
So despite jizz being published first, the WWII era “GISS” is used more often.
Jizz is used quite commonly by birders in Britain. I don’t know if your brother is British or American but I’d assume it’s just a term that never caught on in America and that’s why your brothers never heard of it?
