Is it a bird or a bat?!

Opened the curtains this morning and watched the garden birds on the feeder for a few minutes, then did something else before going back to the window to see... a bird hanging upside down from the base of the suet ball feeder!

With memories of a childhood budgie being found in a similar stance as an ex-budgie,  I put some shoes on and hurried outside but the bird had gone (I searched the ground underneath but no trace).

I have never seen this behaviour before - it was clearly still alive and wasn't struggling when I saw it so presumably wasn't trapped. I have a small urban garden that attracts lots of 'ordinary' garden birds so this was likely to have been a sparrow, tit or starling - any explanations please?

Nikki