The BROOD PATCH is a featherless area on the belly. It is highly vascularized -- applied directly to eggs. Birds will turn eggs to distribute warmth. Incubation usually starts only after whole clutch is laid to insure synchronous hatching. In areas with sparse, or unpredictable resources, ASYNCHRONOUS HATCHING is favored so 1 or 2 young will survive, and the smaller chicks will starve.
Incubation time ranges from small passerines (= 10 days) to albatross (81 days), with the average being 12-14 days. There are five factors that affect incubation time. ALL 5 FACTORS MUST BE TAKEN INTO ACCOUNT.