If you don't have enough nestboxes for your chickens, you may get broken eggs. Chickens need nestboxes in which to lay their eggs.
These should ideally be lower than their perches so that they aren't tempted to sleep in them at night (otherwise you end up with mess in the nestboxes, followed by messy eggs the next day!).
So how many nest boxes should you have? Well this does depend on the type of birds you are keeping since the more eggs they lay per year, the more hens are likely to want to use the boxes at the same time but in general, you should provide at least one nestbox to every 4 laying hens.
This short video clip shows overcrowding in nestboxes. There are 3 hens in a single nestbox all trying to get in to lay!
This is when you risk finding broken eggs in the box that can lead to vices like egg eating. Once a hen starts eating eggs and gets to like the taste of egg, she may start to break and eat eggs herself which is to be discouraged for obvious reasons!