Very informative!
Vitamin D is also part of this equation, as phosphorus and calcium both need Vitamin D to absorb them.
This article could be improved by providing citations to articles to back it up or providing further reading to anyone wanting to learn more.
Here is a great article by Grubbly Farms that explains the chicken's need for all three: calcium, phosphorus, and Vitamin D.
In a perfect world, folks would feed their chickens a good all-flock (supplementing oyster shell for the layers) or layer feed that contains enough vitamins, minerals, etc. for chickens and laying hens, and then store it properly in a sealed container, and ensure never feeding spoiled, moldy, or extremely expired feed. Unfortunately, it's not a perfect world, so good if folks understood the correlation between calcium, phosphorus, and D, especially if they're having issues with their chickens as you described.
I originally wrote this for two members who were asking me questions and looking for a specific answer- I figured it there were two people, there may be more in the future. In my mind, it was easier to create an article I could reference later (and easily find again, since I’m still learning how to use the site’s search functions- in which I am enjoying the site).
I’ve done extensive research on vitamin D in birds, and a focus on poultry and passerines (chronic egg layers)- I will try to dig up my old reseach, as the actual requirement might surprise you. I see a lot of inconsistencies online as it is a topic that cannot be explained without regard to variables (even specific breed variations, etc so you cant say '13 hours' or some hogwash like that). If I were to add in the Vit D portion, it would be another couple thousand words. I'm about to get on a plane heading for surgery- so maybe by the end of the month I can add it. :)