What Can Be Done?
Egg eating is a habit they can get into, particularly if the shells are a bit on the soft side. It is important to remove all traces of the half eaten egg from the nest first of all but there are a couple of solutions which might help stop the habit as well. One is to remove the eggs as soon as they are laid so that the hen doesn't get a chance to peck at the shell but if you're unable to do this, providing plenty of soft nesting material in the nest will prevent accidental breakages when the egg is laid. Another solution is to prick a hole at either end of an egg and carefully blow out the contents and fill it with something that tastes unpleasant, hot or strong like mustard, chilli sauce - anything that tastes unpleasant to the hen then replace it in the nest. It won't take long for the habit to come to an end once they've tasted the contents! There is a slightly unusual suggestion which sounds a bit off the wall but was actually presented by an eminent poultry vet and that is to put lots of golf balls or ping pong balls in the nest to confuse the hen. Once they've pecked at a few unsuccessfully, they give up and the habit is soon broken!
If the shells are weak, this can encourage them to peck at them and eat the contents so if you provide Poultry Grit, this has crushed oyster shell as well as insoluble grit but it is important that they do have the crushed oyster shell as this provides calcium for the hens. This should be available to them all the time. Some hens can't absorb calcium into their bodies very well so adding a little cod liver oil to their layers meal will help with this and keep them in really good condition. If this doesn't help, you can buy calcium supplements which are suitable for poultry from various online sources. Many people with hens who are consistently laying poor quality eggs find this works really well for them. Some chicken keepers swear by Limestone Flour which you can buy from equestrian suppliers. A heaped teaspoon of this added to a feeder full of layers pellets can help provide extra calcium for your hens.
If the shells are regularly weak, it might also be worth giving your hens a worming treatment and to check for the presence of skin parasites.
Finally providing plenty of clean, fresh drinking water may help as the hens may be eating the eggs to provide them with liquid if they happen to be getting dehydrated. Some hens don't drink enough during the day because they struggle to use the drinker or can even be barred from using it by other more dominant hens so it's often a good idea to have an extra water source just in case, especially in hot weather when they drink twice as much as usual.
There is another possible cause for broken eggs if the hens are free ranging and the run is left open. You could have rats raiding the nests and unfortunately there isn't really a lot you can do if this is the case apart from removing all eggs which have been laid quickly. If you find that you do have a vermin problem, your local council should be able to help. The chilli filled eggs might work on rats though! Magpies raid wild birds nests and have been known to get into the Eglu run and eat eggs when hens have been free ranging!