From Baytree Owl and Wildlife Centre

I always find it remarkable how the smallest babies survive in a big clutch. Look at the huge difference in size between the smallest and largest baby barn owls.

Unlike other birds that lay all their eggs and then start incubating them all at once, Barn owls start incubating them as they are laid. They can keep laying eggs from 1 to 3 weeks, resulting in hatchlings with significantly different ages. This is called asynchronous hatching.

For some theories about potential advantages of asynchronous hatching vs synchronous, check out this article.