At VulPro, we switch the precious Cape Vulture eggs in the nests with dummy eggs. We incubate the real eggs artificially in an incubator. This not only safeguards the eggs from accidental damage or loss in the breeding enclosure due to inexperience etc, but allows us to know in advance if the egg is fertile and developing normally or not. In the case of an infertile egg, knowing this within the first week of being laid, gives us the option of pulling the dummy egg from the parents to allow them to double clutch.
More at: http://www.vulpro.com