By holding the eggs close to a high powered light in a dark room, it is possible to actually watch an embryo grow and develop. Much care must be given to avoid damaging the shell or causing the internal membrane to rupture. Many breeders use this method to determine if eggs are fertile. There are many styles of egg candlers on the market.
Portable hand held candlers are great for checking eggs while they are still in the nestboxes.
Larger, more powerful egg candlers are usually reserved for nurseries where eggs are artificially incubated. The subjects being examined are Blue and Gold Macaw, Green Winged Macaw, Umbrella Cockatoo and Goffin's Cockatoo eggs.