Or maybe more accurately, probably the droppings of a Mistle Thrush. This common European bird eats mistletoe berries and after processing, excretes them. The berries are covered in a very sticky goo, which sometimes necessitates the birds scraping its backside on a branch to dislodge the seeds. Having craftily worked it's way to a place out of harms way, the parasitic plant sprouts, and the cycle resumes. I bet you wish you hadn't asked now!
The clumps in these trees are mistletoe