I am losing eggs just lately, not hatching ones, just in the nests.  I have a feeling its magpies or crows, as they have nests in the trees, and I can hear the young squawking.  Today for example, I am at least 10 eggs down, which is a lot, but I am sure the hens are laying.  Rats too will take them, but I would guess at the birds.  They are clever, and I have watched them listen for the hens to declare they have laid and then swoop down.