She was covering her fragile youngsters very closely, in chilly, damp weather, so we were not able to see the actual hatch. We will have further updates on the eyasses soon. In the midst of this tumult, snug under Bell's feathers and warmed by her brood patches, two tiny eyasses broke out of their shells. The end of incubation is in sight, and soon we should have chicks. Incubation usually starts with the third egg and lasts about 32 - 35 days.
Embryos require heat to develop.
Incubation transfers heat to the eggs, through contact between the eggs and the adults' brood patches. During the first half of incubation, the embryo is more or less free-floating in the shell. When the adults turn the eggs, this prevents the embryo sticking to the inside of the shell. If it did, its life support system within the shell would not form completely, and the embryo would eventually die. During the second half of incubation, the embryo becomes fixed within the shell as it moves into position for hatching.