I considered that it may be a chicken breaking the egg for food, as this can be a common problem, however, finding the culprit would be a challenge. How are you supposed to tell which chicken stole the egg? I asked a friend this question, and wisely he answered " she will be the one with egg dripping down her beak" He also advised collecting the eggs as often as possible, however, I pointed out that surely the chickens would have the "inside" information regarding egg production as they are the ones laying the egg.
We seemed to have out witted the lizards as there have been no breakages for a while...until we caught the real thief. Our cats follow us everywhere including through the orchards, so when Diablo our smallest one decided to follow us one day and jump onto the nesting boxes this was normal. However, Diablo proceeded to jump in the nesting box, and gently tap one of the eggs against the side of the box until it smashed.
Next, he sat quite happily licking his golden prize, until there was no more egg, and only the smallest amount of yolk on his nose and whiskers! Well we found the animal with the egg dripping down their face, who would have thought it would have been a cat. However, in this mad house, nothing surprises me any more!