You must start conditioning your cockatiels the day you put up the nest boxes. A conditioned bird will not desert the nest when you candle or inspect. Check the nest box at least once a day. This will establish a routine for both you and your birds. Breeding birds must except this daily inspection. A conditioned cockatiel will allow you to actually lift her off of the eggs, but keep their privacy in mind and don't over do it.
My aviary lighting system is on an automatic timer. Lights go on at 6:30 A.M. and off at 9:30 P.M. With this in mind, I check all nest boxes with three inches of clean fresh pine shavings and usually change this when the chicks are two to three weeks old. It is again changed when the second round of eggs are laid. When you first start inspecting the box, take it down, open the lid and move the shavings around a little. When the first egg is laid pick it up do the same thing with those that follow. As the chicks hatch to the same thing. Don't be afraid to pick them up and inspect them. Any foreign odor on your hands should be removed before you handle a nest. I run my hands in the seed bin to eliminate the scent.
Some birds set tighter than others, in other words they may start incubating on the first egg. As a general rule the incubation will start on the second or third. In any case you will be able to determine a fertile egg by the fifth day. A bloodline from the embryo is evident after three days of incubation from a fertile egg.
The candling is quite simple. Hold the egg firmly but gently in front of a 100-watt bulb. By shielding the light with your hand the egg may be clearly candled.
After the embryo has started to develop you will see a maze of red lines, these will grow and develop each day. What you are actually viewing is the heart of the embryo pumping blood. By candling you can actually check the development and growth of a chick. After the third egg is laid, I candle all eggs every other day until I know the status of each egg. When I'm sure an egg is infertile I will remove it from the nest. This allows the parents to cover a large clutch of say five to six without danger of a fertile egg being left in the cold and an infertile egg being covered. A fertile egg will change color from an eggshell white to a chalk white at about twelve days. The egg will marble at sixteen to eighteen days and pip one to three days before hatching. The first sign of piping will be a small bump on the shell. This bump or pip will grow larger and develops more each day. By holding the egg close to your ear you can hear the chick working inside. Don't become over anxious and try to help the chick out, or you will lose more than you will save.
The incubation period for a cockatiel is eighteen to twenty-one days, but don't give up too early. I have had birds to hatch at seventeen to twenty-three days.
Never keep an egg or chick out of the nest box longer than is necessary. Warmth is very important at this early stage. When candling a round of eggs you can use a small light with a flexible head found in most auto supply areas, or a pin light that can be placed inside the nestbox, so there is no need to remove the eggs from the nest box. This way there is less danger of dropping an egg. The whole procedure should only take two or three minutes.
If for some reason an egg becomes chipped or cracked, assuming it isn't overly damaged, you may repair it. I use egg white or clear nail polish. Take a clean small soft brush and paint the damaged area. One coat of nail polish is usually enough but you will need several coats of egg white. I prefer the latter method.