Peregrine falcons lay their eggs about 30 hours apart and start brooding just before or after the last one arrives. The eggs don’t start developing until they’re kept continuously warm for it and the goal is to get them to hatch close enough together that they all demand food equally and all get fed. Not demanding like the others conveys a possibility of being less fit and puts that chick at risk because they’ll be the last one fed of what’s left.
Our current pair is on their I think fourth year and they’ve got this thing down.
And so this morning the pips (holes in the eggs) started coming and then the chicks did, start to finish all four of them in less than twelve hours, even though their eggs appeared across about a ninety hour stretch.
Here’s Youtube video of the pips (7 min.) and (8+ min) of the newborn eyases. When the parent pulls gravel towards them while not actively brooding it’s to keep the residual warmth from their bodies close to their young.
You can see the first ones change already just in those first twelve hours.