Most bird species don't start to incubate (warm) their eggs until the clutch is complete so the eggs hatch at more or less the same time.
Barn Owls begin incubation as soon as the first egg is laid and additional eggs are added every two to three days. This is termed "asynchronous" hatching.
( do I see a trivia question here ) The average hatching success rate is 4.5 and the age difference between the oldest and youngest nestlings can be as much as three weeks.
But in the end they all turn out to be the same size when they're all grown up.