Nest

I noticed what looks like the bottom of a nest just up above head height in an old willow I pass on one of my usual walks. It's hard to see in the photo, but above and behind the branch stump there's what looks like a cavity in the trunk. There also appears to be a possible nest entrance below the stump too. On previous walks recently I'd seen baby robins hopping about in the undergrowth nearby, so it looks as if this might have been home for them. I don't know how I missed it before, but perhaps the nest has started to collapse out of the tree now that it's (presumably) vacant.