Fuzzy red things on leaves?

Hello Naturalist, what the heck are these fuzzy little things growing on one of my Oak trees? I believe the tree is ~15 years old and appears to be in good health. I do not see them on other nearby trees nor do I see them on other older oaks ~50 and 100 ft. away. They seem to grow in clusters and are firmly attached to the leaf. They always seem to be at the ends of branches rather than close to the trunk. The presumably younger ones are smaller, reddish and lack the fuzzy appearance. I am not even certain if it is a developing insect or some sort of fungus. What say ye? Come over and take a look if you like.

What you’ve photographed are types of galls formed by the tree in reaction to insects (likely a tiny wasp) laying its eggs in its leaves. They are not harmful to the tree, nor to people, and are an important food source to many types of birds who eat the developing larva inside the gall.

Good question!

-Chief Naturalist John Kolar