It is also fancied by some birds, including the wild turkey, American robin, Northern bobwhite, Eastern bluebird, mockingbird, white-throated sparrow, Carolina wren and European starling.

The substance that causes the problem for people is an oily compound called urushiol, found in many cells of the poison ivy vine. Exposure to this allergen brings white blood cells to the site, and their overvigorous response to the foreign substance leads to an attack on skin cells as well, producing a rash, with blisters where the concentration is highest.

It is believed that urushiol is the poison ivy plant’s physical armor against the many kinds of insects that feed on it, quickly sealing the wounds with a tarry plug. question@nytimes.com