It is difficult or impossible to visually sex many species, so even if the birds are a bonded pair, it does not mean they are a male and female pair. If none of the eggs are fertile over a few clutches or if there seems to be too many eggs, there is a good chance that you have two females.
A pair of birds that are too young (and this varies from species to species) will often lay infertile eggs. Other times, they might lay fertile eggs, but do not feed any chicks that may hatch. A pair of birds that are too old (this will also vary from species to species) may also lay mostly infertile eggs.
If you know for sure that your birds are a male and female pair and they are within the correct age bracket for breeding, the next step is to look at is diet. Your birds should be eating a healthy and varied diet (and again, this will vary from species to species).
Pellets often have a maintenance diet and a breeding diet, with increased nutrients required for breeding birds. Of course, pellets should not be the only food item that your birds receive. They should also be eating, fruits, vegetables, bean mix, seeds and sprouted seeds.
If you want to know before the hatching date if the eggs are fertile or not, you can candle them. To candle an egg, you hold it in front of a bright light and look at it carefully. The easiest time to see the tiny veins running through a fertile egg is 9 to 10 days after incubation begins (for most birds). Tiny finch eggs would be sooner. This works best in a darkened room with the light directed at the wide end of the egg. Be careful to candle for only a moment to avoid putting heat on the egg.
If your birds are not tame, great care must be taken before deciding to candle the eggs. If you stress the parents, the eggs could be jostled as the parents try to defend them and the development of the chicks will end before you remove them from the nestbox. The eggs would appear to be fertile, but the chick will not hatch.
Fertile eggs can also be damaged by the parents if you are continuously looking into the nestbox to watch. Some birds do not mind, but others will be stressed and will jostle and scramble the eggs before they have a chance to hatch.
Another time that chicks may die before hatching is that the eggs cooled too much after incubation began. The problem could be inexperienced parent birds or too frequent nest checking from the owner.
Other in the egg deaths occur because bacteria entered the egg through the porous shell, or too much (or not enough) humidity entered the egg.
There are a lot of things that can go wrong – but when conditions are right, the majority of eggs should be fertile and should hatch into healthy babies.