Thanks for the heads up on the oyster shell. This is also a bit off-topic, but since someone brought it up, I ran with it.
I did a simple google search and found a paper on Lead in Calcium supplements. HERE. It is written with human consumption in mind, but it mentions oyster shell supplements. It says:
"In addition, the exposure risk from lead in calcium supplements may be relatively small, even though the contribution of lead from a daily dose of those supplements (median = 2.38 µg) to the average total daily dietary intake (5-11 µg/day) may be relatively large (29). This disparity is due to the much higher supplemental concentrations of calcium, which decrease gastrointestinal lead absorption (30). Several studies of factors that influence the dietary assimilation of lead show this decrease in lead absorption (31)."
I don't know that phenomenon relates in chickens.
I did a simple google search and found a paper on Lead in Calcium supplements. HERE. It is written with human consumption in mind, but it mentions oyster shell supplements. It says:
"In addition, the exposure risk from lead in calcium supplements may be relatively small, even though the contribution of lead from a daily dose of those supplements (median = 2.38 µg) to the average total daily dietary intake (5-11 µg/day) may be relatively large (29). This disparity is due to the much higher supplemental concentrations of calcium, which decrease gastrointestinal lead absorption (30). Several studies of factors that influence the dietary assimilation of lead show this decrease in lead absorption (31)."
I don't know that phenomenon relates in chickens.