In the majority of bird species synchronous hatching is the norm. This is achieved simply: the parent does not commence incubation until the clutch of eggs, however large, is complete.

Most birds will lay an egg each day until an appropriate trigger indicates that the clutch is complete. This is why I can collect an egg a day from each of my hens - they keep laying until incubation is triggered by some stimulus. This could be the feel of a full clutch against the brood patch on a bird's belly, but there is also some endogenous control factor. For example, one of my hens may turn "broody" - she will cease laying and commence incubation on only one egg if that is all I have left her with.

In some other groups of birds - notably owls, raptors and cormorants - incubation commences when the first egg is laid, leading to sequential hatching, with the first chick gaining a significant advantage over later ones. This is very noticeable in barn owls, where five or six eggs laid at daily intervals lead to the oldest chicks being almost a week older than the youngest. This strategy ensures maximum chick survival in species with an unpredictable food source: in years of plenty all the offspring get enough food, but in years when food is less readily available the oldest, largest chicks survive and dominate the younger ones - which almost inevitably perish and may be eaten by their siblings.

This strategy sees its most elegant expression in the Cain-and-Abel syndrome, which is manifested particularly well in eagles. The first egg hatches two to three days before the second and, when food is scarce, there seems to be a degree of inevitability in the way the older chick persecutes its younger sibling to the point of death.

Norman McCanch, Canterbury, Kent, UK

The important point to note here is that development of the embryo in a bird egg is temperature dependent. It is not until the embryo is raised to a certain temperature that the single cell inside the egg shell starts to differentiate and chick growth begins. An egg left in a nest stays roughly at the ambient temperature and will not start to develop. For this reason a female can lay an egg a day and they do not develop until the female ( usually only the female but in some species also the male) starts to incubate. Incubation is not a passive process , it is more than just sitting on the eggs, it is an active transfer of heat from the adult's body to the eggs in order to raise their temperature and start the process of cell differentiation. Heat is often transferred via a "brood patch" which is a highly vascularised portion of the abdomen where blood come very close to the skin surface and this brood patch is pressed hard against the eggs shells. By not starting to brood until the clutch is complete the adult ensures that all chicks start their development at the same time and since this development is so temperature dependent all of the chicks will differentaite, grow and eventually hatch at more or less the same time if they have received similar treatment  during  the incubation period.