Can you tell me what has added moss to a swallow's nest over winter? The swallows have returned but haven't changed the nest.

Germain said:

Possibly a wren

I'd agree wren with Germain and Cris .

I have seen wrens using swallow nests in a similar setup. Though I don't seem to have posted here on the wrens using the swallow nest, the attached photos of wrens using a barn swallow nest in very close proximity to an active barn swallow nest. These photos were taken June 2023.