Old chickens drink more

One of our biggest shocks when we put the Avian Aqua Miser in our tractors was how little water the birds actually drank.  We'd been spilling a gallon or two of water a day, trying to flush the poop out of our old waterers.  But when the water reservoir is enclosed and chickens drink from a nipple, the birds only consume about half a cup per day.

Temperature, obviously, affects our chickens' drinking rate, but we've discovered that age does too.  We have three tractors of chickens --- one contains three year old hens and the other two contain 16 month old hens.  The older hens drink nearly twice as much as their younger farm-mates!  I wonder whether the plentiful clean water in our homemade chicken waterers is the reason that our three year old hens are still laying at nearly the same rate as the youngsters?