KNOBBLY EGGS!! What's the cause?

[lonicera56]

My chickens are laying well, but recently I've noticed that some of the shells have little knobbles of extra shell on them, ranging in size from about a pin head to a grain of rice, some have only 1 or 2 bits, some are quite flecked. Does anyone know what is causing this & is it something I should be worried about?
Grateful for any advice.

 

[Anonymous]

I have the same on a few of my eggs too.

It seems to be the big egg layers that get this.

Mine seem perfectly healthy and I've put it down to the girls having to produce so many eggs, they're a bit rushed when they create the egg!