I have 2 cockatiels one male (1yr) and the other is a female (~2yr) who have laid 4 eggs. Its been a month and a half and today I found a hole in one of the eggs. I understand that they take about 28 days to hatch, are the eggs OK or they are dead? I tried to candle the eggs but all I saw were dark spots inside.
After a month and a half the eggs are dead, when the eggs are overdue the parents will break them to see what's going on (some birds will eat them). I would remove the eggs and clean the nesting area before they rot.
When candling eggs the light should not disperse around the egg but focus on it to have a better view. The development of a fertile egg would look like the image below.