Hatching of Forster's Terns

Abstract

Forster's Terns (Sterna forsteri) hatch out at the large end of the egg and "peep" before hatching. The mean number of days between laying and commencement of hatching is 24.2. The breaking period varies from one to five days, longer periods involving mainly the Early Pip stage. Subsequent stages occur in less than one day. Eggs within a clutch generally hatch within one day of each other and in the order laid. In small groups of nests, hatching appears to be more closely synchronized than the onset of laying. Chicks have both upper and lower mandible egg-teeth, but lose them within five days after hatching.