I've seen the roosters peck at oyster shell occasionally but I don't worry about that. The main concern is that the feed doesn't contain 4 times the calcium that roosters and non-layers are unable to avoid if they want to eat.
I'm to the point now that I rarely feed layer anymore unless I know every bird in that flock is producing egg shells. I used to have large flocks of young layers and they were all on layer feed. But now the flocks are various ages with roosters and older birds not laying frequently or molting, as well as some not yet laying. IMO it's damaging for birds not in production to get a diet that is 4% calcium.
Layer feed is better suited for production layers where all are the same age and all are actively producing so it is a simple means of providing the hens' nutritional needs.
Research shows that birds will seek out the oyster shell when the egg enters the shell gland so they'll often pick it up in the late afternoon. Large particle oyster shell stays in the upper digestive tract longer than the crumbled particle that is in the feed. This way it makes contact with calcium absorption sites in the intestines when the bird is sleeping, not eating and the shell is forming. So oyster shell or other large particle calcium should always be offered to birds in production whether one feeds layer or not.
There are other causes of soft shells and shell less eggs than low calcium. Other causes are respiratory diseases, too little or too much vitamin D, the wrong ratio of Ca
(usually between 3 and 4% calcium, to 0.45% phosphorus), mycotoxins, stress and even excessive calcium. With new layers it can be an immature shell gland.Slipped tendons or enlarged hocks and rickets can be caused by an imbalance of calcium to phosphorus as well as a deficiency of manganese, vitamin D and other vitamins.
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/220/slipped-tendon-perosis
In an egg-laying bird, 10% of the total body calcium reserves can be required for egg production in a 24-hour period. The Ca required for eggshell production is mainly obtained from increased intestinal absorption and the reserves in medullary bone,
There's some good info here
http://edis.ifas.ufl.edu/ps029
The below study shows that male broiler breeders die at a rate nearly 4 times that of females and the cause is kidney damage from excessive calcium.
http://www.pjbs.org/ijps/fin1947.pdf
I've also seen studies with roosters given 3 different rations with 0%, 2% and 4% calcium respectively. Those with the higher calcium, in addition to kidney damage had poorer viability of sperm.
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