According to Professor Sally Solomon
of the University of Glasgow Veterinary School, examinations of affected eggs had revealed that pale eggs had been laid six hours prematurely, at which time the pigment had not yet been laid down. Most of the pigment is contained in the cuticle and, since thee eggs have no cuticle, eggs appear pale.
Solomon says that this is due to heat stress, which could explain why the problem tends to be seen during the warmer summer season. The problem seems to disappear once the birds go inside.