A person knows when a bird egg is going to hatch because the incubation period is up; this is the period of time it takes for eggs to develop while being kept at a steady, warm temperature. Incubation periods differ between species of birds.
To know when a bird egg is going to hatch, you must know the species of bird to determine the incubation period. For example, the incubation period for dove eggs is 14 to 16 days while the incubation period for bald eagles is much longer at 35 days, and it takes up to 80 days for royal albatross eggs to hatch. Larger birds generally have longer incubation periods.
When an egg gets ready to hatch, the chick grows an egg tooth, a horny calcium deposit located on the tip of the beak. The chick uses this tooth to tear through the egg membrane and break the shell, a process called pipping. Once the chick is free of the shell, the egg tooth falls off within a few hours.
Sometimes eggs do not hatch. For instance, if an egg cracks prematurely, the blood vessels in the shell that nourish the chick become disrupted, and the chick dies. Sometimes the bird is infertile and lays an egg that can't develop into a chick.