We have two mature oak trees, (probably 100+ years old) in our yard and one of them is losing large clumps of leaves, still attached to the small branches. The end of the small branches look like they have been chewed. To give you an idea of how many, I just filled two large yard waste bags with oak leaves in preparation for mowing. I noticed this last year but it was not as much. This year it is so much that the grass around the tree in a radius of 10-15 feet, is covered. The typical clump is 8-10 leaves but some of them are as many as 30 leaves. The leaves themselves look very healthy, shinny and dark green, no wilt or spots. I would very much like to know what's causing this and if there is anything we can do about it. Thank you!