I think this tree growth is called a Burl but if I’m wrong please let me know in a comment below. (Thanks Robin for the correction. This is a Gall instead of a Burl.)
My Son, Grandson and I were out exploring some woods when we discovered this tree with a large growth on it. What was cool about this tree is that the growth was being used by squirrels to sit and eat acorns. This tree would be awesome in my yard as a bird feeder.
Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites. Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified..(Wikipedia)