At this point, the nestlings are at about 75% of their adult body mass and their rate of growth slows a bit as energy goes to the fun stuff: feathers!
As adults, birds don’t grow new feathers all at once. They have a programmed molt schedule to drop old feathers and grow new ones in an orderly fashion so they avoid the naked exposure we see in these nestlings. After all, they still need to fly to avoid predators! (Sea ducks, like eiders, molt all of their flight feathers at once and they look pretty awkward flap-running away along the surface of the water!)
But growing feathers is also energetically expensive. Feathers are mostly protein (like keratin, the stuff of hair and fingernails) and make up about a quarter of a bird’s total protein. This makes molting a very demanding period of their lives, and even when they do it gradually it still takes raises their energy demand by 10% or more. And that’s when it’s a few feathers at a time…imagine all of them at once!