Mar 19 2009
Question: After the mother peregrine has laid an egg, why doesn’t she sit on it all the time?
Answer: Peregrines usually lay 3-5 eggs per clutch but they don’t begin incubation (to raise the eggs’ temperature) until they’ve laid their next-to-last egg.
The mother bird shelters the eggs to make sure they don’t get wet or freeze and if it’s very hot she shades them. But she prevents early development of the first few eggs by not incubating them until her clutch is nearly complete. The result is that all the eggs hatch on the same day. The last egg laid sometimes hatches a day or two later because it was laid after incubation began.
For more information, see my blog at: http://www.wqed.org/birdblog/2008/03/16/why-isnt-she-sitting-on-the-eggs/
(photo from National Aviary falconcam at Gulf Tower, Pittsburgh, 18 March 2009)