Variation in breeding plumage (1 Viewer)

I am very new to ringing and would appreciate any advice regarding sexing of some species. Having discussing this morning the variation which can be seen in some species with males looking very similar to females even though I would expect them to be coming into breeding plumage. I put forward the thought that this may be due to variation in the levels of testosterone. Indeed there has been some research indicating that levels of testosterone in the egg has been shown to effect the plumage of male House Sparrows and their breeding success.