Rachel, I think Hydestone is okay with feeding layer during the final month before laying. Excess calcium seems to have the most danger for younger birds with developing organs. As far as having oyster shells out, they apparently are fairly good at self-regulation when they are given a choice between eating extra calcium, or not.
I'm no sage but if you want facts and figures on excess calcium: You may want to read what
the veterinarians at DeKalb Poultry have to say.
Urolithiasis is a disease of chickens resulting from kidney tissue damage at the cellular level, the nephron. This damage eventually may lead to uremia, visceral gout, and death after enough time has elapsed and more nephrons are lost. . . once kidney insult occurs, the mortality from urolithiasis usually continues for life. . . Kidney cells are not regenerated once they are lost.
The vets recount specific management mistakes that have led to kidney damage in pullets:
1 - feeding left over lay ration from previous flock (doubled expected mortality throughout life of flock)
2 - feeding low levels of phosphorus (increased mortality rate)
3 - feeding layer ration too early (under 15 weeks of age)
4 - using a large particle calcium source for pullets
Steve