What makes shells suddenly become pale or change shape?

Egg shell appearance can be affected by life stage, nutrition, stress and disease. Anything that affects the normal functioning of the female reproductive tract in a hen can potentially cause shell changes. 

Older hens tend to lay thinner, paler shelled eggs. Younger hen tend to lay thicker, darker-shelled eggs. 

When birds have been stressed, e.g. due to severe heat, rehoming, or they are physiologically stressed, for example when they are struggling with a red mite infestation, they will often lay thinner-shelled, paler eggs. 

Various diseases of poultry, including virus and bacteria, can also affect the normal functioning of the oviduct and therefore cause abnormalities in egg shell appearance. A good summary of egg shell deformities, and potential causes can be found here.

How can you help your birds improve their shells? 

To accurately target supplementation you need to know what the cause is, but a good start, so long as your hen is feeling well, is to support them by providing extra Calcium and D3. If you think your hen is also poorly then contacting a vet would be a good idea to rule out underlying disease first. 

Older or poorly hens may find it harder to metabolise Vitamin D into its active form so providing them with the active form of Vitamin D can help them further. This is especially important if they are laying throughout winter, when sunlight levels are lower.