I noticed these little bullet-like growths on the twigs the Bur Oak earlier this year, but I my curiosity didn't get the better of me until today. I saw them again, took a photo and vowed to figure out what they are.
I've been on a "gall" kick lately. A little voice in the back of my head has been telling me: Learn Your Galls!
I listened to the little voice, and took a stab at identifying it. It was surprisingly easy.
I googled "bur oak," "twig" and "gall." The search engine gave me 10 pages of links. I found it at the Forestry Images webpage. It's an oak rough bullet gall, made by a wasp known as Disholcaspis quercusmamm. Bullet galls are found on three oaks: the bur, overcup and swamp white oak.
The wasp lays an egg in the twig, causing the twig to grow the gall. As the plant cells grow, the galls are green or bright red, then brown or dark red.