These are fully mature poison sumac berries. I'd never heard of sumac, let alone poison sumac, but word is that the rash from poison sumac is much worse than that of poison ivy or oak.
They do look menacing, don't they?
We don't have poison ivy in California, so I've never seen that either, but I've been told it likes to climb the trees.
Anyway, back to the poison sumac: I read that it's fairly rare, only shows up in swampy areas, although there are plenty of those around here, and its berries are much different from the regular (staghorn) sumac.
This one is the staghorn sumac, and apparently it's all over the place, though I don't think I've seen any. One is supposed to be able to tell the difference because the berries of the non-noxious plant grow in one big clump, and the leaves are jagged. This one grows in dry areas, so I'm told that you'll never see the staghorn and its evil twin growing side by side.
The leaves of both turn red in autumn and drop in winter, and then all that's left are the berries. I like the staghorn best at this stage, at least in the photos-- adds some color to the dreary landscape.