Birds that are producing eggs need calcium for the formation of the egg shell. The necessary amount of calcium is taken up by the feed. However, it is important that the calcium is available in the blood during the formation of the shell. If at that moment the calcium is not available, it has to be taken out of the bones from the bird.

Shell formation occurs mainly during the night. To be able to obtain the necessary calcium from the blood, this means that for shell formation, the optimum feeding time is in the afternoon. In this way, most of the calcium will be digested and circulating in the blood during shell fomation.

If the feed is given long before calcium is needed for shell formation, for instance directly when the light goes on, the calcium will be released from the bones. This is together with the phosporus that is bound in the bones and the calcium. When calcium becomes available again, this calcium will, together with the necessary phosporus, be built in the bones again.

In warm, tropical conditions, feeding in the middle of the day might cause problems. Due to the digestion of the feed, heat is produced by the birds that might cause problems with heat stress, as this feed is digested in the warmest period of the day. For this reason, feed is often given early in the morning.

When feeding is done early in the morning, the following rules have to be respected:
- feed a very coarse source of calcium, to delay the digestion of the calcium and delay the release in the blood.
- feed extra calcium, for instance in the form of lime stone or oyster shells, in the afternoon, to allow the birds to take up calcium from the blood during egg shell formation
- ensure that there is sufficient phosphorus in the feed, as extra phosporus will be released together with the calcium from the bones, and this phosphorus is needed the next day to build in sufficient calcium in the bones again.