Eastern Tiger Swallowtail, Papilio glaucus, male. Females have much more blue coloration on the hindwing. In our neck of the woods, male "tigers" are absolutely unmistakable and a very common sight. Swift and powerful of flight, they are often seen coursing high in the forest canopy, and the females often lay their eggs well aloft in Tulip Trees and ash.
As one moves northward and out of the range of a certain other butterfly - I'll showcase that one in a bit - yellow or typical forms of the female increasingly dominate. Why? Batesian mimicry. This is the term for non-toxic butterflies that have evolved coloration that resembles highly toxic butterflies. The black-form tigers resemble the Pipevine Swallowtail, Battus philenor, which is full of harmful alkaloids derived from its host plants in the Aristolochiaceae, or Pipevine Family. As predators have learned to avoid Pipevine Swallowtails, the Batesian Mimicry theory states that they will also avoid look-alikes. So, it doesn't do the tigers any good to mimic the poisonous pipevines in areas where the latter is scarce or nonexistent.
In West Virginia, where the above photo was taken, the large high-climbing Pipevine, Aristolochia macrophylla, becomes abundant and so does the swallowtail. And the incidence of Batesian Mimicry in female Eastern Tiger Swallowtails spikes sharply.
Red-spotted Purples are rather catholic in their tastes, hosting on cherries (Prunus), and a variety of species in the Salicaceae Family, which includes willows, aspens, and cottonwoods. Thus, and luckily for us, the purples are common.