I'd be lying if I said that I knew for sure but if I had to guess I"d say it's just genetics. I see birds with both color spursand there seems to be no pattern to it. I see young two year olds with black spurs as well as mature birds with long black hooks. I also see young and old with the lighter color spurs. So age doesn't seem to be a factor. I've killed many birds with both types from the same areas so geography or food sources in a given area doesn't seem to be a factor either.