Jizz is the term used by birders to describe the immediate recognition of a bird from the briefest of glimpses. This perception of the essential characteristics of a bird, whether colour, size, shape, sound or likeness, is only something that can be demonstrated after much experience.
An analogy for Jizz might be identifying a friend across the street by the way they stand, sit, or walk. You recognize the person by their mannerisms even though you cannot see your friend's face.
It is possible that the word is derived from the US slang for "general impression of size and shape" or "giss".