This is the nest of a black-capped chickadee. It sits in one of the nest boxes in our yard. The nest is made of moss, covered with a thin blanket of what looks like deer hair. As of Thursday it held four eggs. I expect two or three more to be added.
The box base is four inches square. The nest is three inches deep. That’s 48 square inches of moss, all delivered by the chickadee(s), piece by piece.
This is a serious nest, an impressive amount of work.