Identifying a deceased baby bird (avoid if squeamish)

Our garden is blessed with a constant array of birds, thanks to a decent-sized Salix Cinerea, pond, and dense Jasmin.  The usual visitors are a flock of house sparrows, various tits, goldfinches, blackbirds, a robin, and the obligatory pigeons.

I found a baby beneath the Jasmin and Salix, but I'm not sure where it came from or what it is; I discovered a blackbird nest directly above, but it's obviously not from there:

I thought it might be a bluetit, but they only nest in tree holes and the like, don't they?