I would like to point out that the 'bleaching' of the yellow is easy to gauge on yellow skinned hens, but not so easy on white skinned hens.
The bleaching is caused by the hen using her natural yellow pigmentation to help 'color' the yolks. The more eggs she has produced, the more of her natural pigmentation she has consumed.
An even better tutorial can be found here:
http://www.poultry.msstate.edu/extension/pdf/culling.pdf
The people I worked with in South America would typically cull about 9,000 hens per year (1/3 of their flock). I once accompanied the owner on a 2,000 bird culling. As we walked through the hen houses, he would just point to a bird, and the helpers would put her in a holding pen, ready for sale/slaughter. After culling 2,000 hens, his egg production for the entire next week was down about 200 eggs. He certainly had an "eye" for it. His only comment was "I
should have done it a few weeks ago."
