Leaving unfertilised eggs in the nest can possibly contaminate the nest and living embryos as the egg contents decay. If decaying eggs become cracked the risk is heightened. Incubating infertile eggs wastes the hen canary's energy and can disappoint the breeder. "Candling" eggs---holding them up to a candle flame--is the traditional way to tell if an egg is infertile or not. Today you can do it with an electric light. Holding eggs above a light illuminates the egg's interior, making the shell semi-transparent. Learning to recognise what you see identifies which eggs are infertile.
Wait two to five days after incubation begins to allow time for visible embryo growth to occur. After this time, take individual eggs from the nest. Hold each above intense but cool light from the flashlight or candler. Work quickly--a minute's inspection should be enough. Eggs can cool during handling for 10 minutes without harm, but hot light sources can kill embryos.
Look for a dark spot with visible blood vessels radiating outward. This is a growing embryo. Infertile eggs will illuminate with uniform brightness, showing no blood vessels. However, although most hens do not commence sitting until after laying the full clutch of eggs, others start sitting right away. When this happens, eggs laid last lag in development behind the first. Eggs that appear infertile at this early stage may be the last laid, insufficiently developed for blood vessels to show. If in doubt, leave these eggs five days longer.
Candle all eggs again after five days. Remove any that still show no signs of growth. Also remove eggs where growth has been aborted soon after incubation. Look for embryos that have not grown since first inspection. Check for telltale signs such as a fading blood vessel network or dried up streaks of blood against the shell wall. Embryos that are developing normally will now be revealed as larger shadows, eventually filling all but a small airspace and blocking out views of the surrounding blood vessels.
Candle again any eggs that remain in the nest after most have hatched. Eggs should hatch within days of each other. Also candle all eggs if no chicks have appeared by the 16th day--incubation takes 14 to 15 days. Count from the day incubation began, not the day the first egg was laid. Remove unhatched eggs immediately if candling reveals dead embryos--they could be a source of contamination.
A special flexible flashlight or candler enables you to "candle" eggs without handling them or removing them from the nest. This is especially useful for small birds like canaries where eggs are delicate. Embryo death occurs most commonly in the first three days after incubation, and the last three before hatching. Candle at both times.
Handle eggs carefully; shells are paper-thin. Wash and rinse your hands before and after candling. Residues of soap, chemicals or germs can be transferred from your hands through the thin shell, harming the chick.