You can see a lot of difference between these Canada geese goslings. You can see the white coming in in different stages.
I had parakeet babies at one point. The hen would lay a egg every 2 days or so, for a clutch of 4 or 5. They would then hatch in that order. The size difference between the youngest & oldest chicks was remarkable.
The same happens with owls. It's to ensure that at least some of the brood survive even in years when the food supply isn't plentiful.
