My grandmother had numerous chickens.  And she knew which egg had been laid by each individual hen.  Not all eggs are exactly the same shape, shade or hue.  

Knowing who laid what was important because if a certain color egg didn't show up in the coop, it was getting laid somewhere else--and Gran knew that hen was hiding eggs to sit on and hatch.

Biddies are clever.  They sneak off to the loft, or field, and scratch a nest together out of hay or straw and sit on their eggs.  They knew they couldn't do that in the hen house.  Gran wouldn't let them. 

Because she knew the individual eggs and which hen laid them, she knew which eggs were missing and which hen to watch.  I remember helping her track a wayward hen down, and finding a huge clutch of eggs out away from the barn.  Gran was, of course, much smarter than the hens. 

I got to throw the eggs out of the nest and help her replace them with white glass dummy eggs.  The hen didn't know the difference--and each day we could collect a fresh egg.  Gran wanted eggs, not baby chicks.  Eggs could be sold.  Chicks had to be fed--and when she wanted to raise a new brood, she had a plan to do that.  When she decided to do it--not when the hen decided to do it.  Gran was the ruler of the hen house.