Depending on whose classification you're looking at, Poison Ivy (T. radicans), Poison Oak (T. diversilobum), and Poison Sumac (T. vernix) are sometimes placed in their own genus (Toxicodendron, meaning "poison tree"). However, they are close relatives of the sumacs and have often shared a genus with them (Rhus).
I've included some Toxicodendron observations in this project both because it's well worth being able to recognize (and avoid) them, and because while they aren't favorites of humans, many birds and other types or organisms put them to good use.
The easiest way to tell these three Toxicodendron species from their more innocuous cousins is their fruit. The drupes (single-seeded, fleshy fruit) of the "poison" species are white, while the drupes of Rhus are red. Though white fruit may seem like an odd color to humans, they contrast well against foliage and are very visible to birds, which are their main dispersers. So, next time you spot a white-fruited, sumac-like plant, step away and leave it for the birds.

If you're a plant needing birds to disperse your seeds, it's a good strategy to ripen your fruit when there are the maximum number of birds around. In North America, the fall migration is a perfect opportunity: thousands of hungry birds traveling long distances, eager to refuel by snacking on succulent fruit. Competition is stiff, though. Lots of plants ripen fruit in the fall, so how do you stand out from the crowd?

Some plants offer high quality fruits: juicy, sweet, and irresistible. Most of these get gobbled up fast, though there are drawbacks. Fruit not eaten immediately can go bad, and are then unlikely to be dispersed.

Most of the eastern US sumacs (ex. Rhus copallinum, R. glabra, R. typhina) use a different strategy. They, too, fruit in the fall, but make lower quality fruit: rather dry and without a lot of flesh to them, not very sweet, but with some fat content for energy (11-26% of their weight, according to Stiles (1980)). Initially, birds mostly ignore these, but the fruit don't go bad, and continue to hang on the plants through the long, cold, hungry months of winter. In fact, many of these sumacs will still have much of the last year's fruit on them well into the spring and summer.

Overwintering birds can't be as picky as fall migrants with a wealth of fruit to choose from, so sumac fruit are a hit over winter. Over 300 species of songbirds are known to eat sumac fruit, as well as turkey, quail, pheasant, and grouse (USDA, 2002).

This drawn-out fruiting strategy makes sumacs doubly desirable as garden plants: the clusters of red fruit provide flashes of color in an otherwise dull winter landscape and provide a critical food source for overwintering birds. Win-win.

Winter Robins Eating Sumac
Winter Chicadee Eating Sumac
Winter Blue Jay With Sumac

References:

Stiles, EW. 1980. Patterns of fruit presentation and seed dispersal in bird-disseminated woody plants in the eastern deciduous forest. The American Naturalist, 116(5): 670-88.

(USDA) United States Department of Agriculture. 2002. Plant Fact Sheet: Winged Sumac (Rhus copallinum). 2 pages.