Cherry bird cage.

4°C, grey, NE.
What is this? There is a clump of thin branches in this cherry tree. It’s like a deformation, an odd lamp-shade shape. They have been like this for years, but most of the time, hidden by foliage. In mid-winter view, it’s too dark to see the clump.
On my morning dog-walk, I look up and speculate on it’s cause. Has some parasitic life form triggered a reaction in the growing tree. Could it be a fungus, an insect or a virus?