Strange tree structure

Furez

Came across this strange structure/accumulation around a tree. The tree is in a fairly urban setting and is directly on the bank of a small stream. The accumulation is around 20ft high with just a few branches rising not far above it.
I looks like a collection of twigs, clay and dead ivy and there are many tunnels a bit larger than your average rabbit hole entrance into it. Seen a rook near the entrance to one today.
So it it an unconventional rookery or something else?

Ulmus

Looks like the ivy collapsed from its own weight or wind, and new ivy is growing through it. Rooks would nest in branches at the very top of a tree.

swifts need our help!

I would agree with what was said above. Jackdaws would nest within the dead twigs

Furez

Thanks it may have been a jackdaw that I had seen. It must have taken decades for that ivy to build up like that unless the daws contributed twigs too. At first I thought it was a tree that had broken high up on the trunk but it is far too wide and the tunnels seem to go in deep. Ive never seen anything like it and will keep an eye on it to id the residents.