The embryo within a birds eggs requires a narrow temperature range. This commits the parents to rigorous incubation patterns which are necessary for the embryos to develop.
Most birds delay starting incubation until the clutch is complete.
This ensures that development and hatching occur at the same time although some eggs are laid earlier than others. There are exceptions within some species. Pigeons and doves sit on the eggs from the time that the first one is laid, although the temperature required for embryonic development is not reached until the last eggs is laid. Owls and raptors begin incubation as soon as the first egg is laid, which results in the young hatching at different times and the offspring hatched first are larger than their siblings who hatch later. Most parents transfer heat directly to the eggs through a brood patch, also called an incubation patch on the breast or abdomen.