The eastern deciduous forest biome, which cloaks much of the eastern half of the United States, stretching from the Gulf Coast to southern Canada, grows the lion's share of caterpillar biomass in northern North America, along with the great boreal forest of the far north. This is why the migratory pathways of so many of our Neotropical migrant songbirds goes through eastern North America, even if the species ultimately heads far to the northwest to breed. Hungry migrants are assured of finding lush food sources when they travel through a region rich in broad-leaved trees and their attendant caterpillar crops.
As for the photo above, it's a bit of a fooler. While the animal is quite caterpillar-like, it is actually a sawfly larva. Sawflies are in the order Hymenoptera (bees, wasps, ants), not the Lepidoptera (moths and butterflies). It's the larvae of the latter that we call caterpillars, and which are arguably (or not) the most important foodstuff for a great many of our bird species. Sawflies do not seem to be particularly well known, and I'm unsure of this species. It is eating black oak, Quercus velutina. Many sawfly species seem to be "chemically protected"; they sequester toxic compounds that render them unpalatable to birds. However, I have seen birds eat sawflies on several occasions.
It is the filament bearer, Nematocampa resistaria, another species that I knew of but had never seen. I quickly detoured into a parking lot, got a vial from the trunk, and caged the animal. Upon arrival home, I placed it on my serviceberry, Amelanchier arborea, and made some images. The odd protuberances on the caterpillar's dorsal surface probably act as disruptive camouflage, perhaps helping it to blend in when at rest. When disturbed, it elongates the filaments significantly, as here. I suppose it is possible that the caterpillar may use them as flails - whips by which it can repel would-be parasitoid flies or wasps.
In any event, I hope this filament bearer flourishes on my native serviceberry. They are known to eat a wide variety of woody plants, including species in the rose family, such as serviceberry. If all goes well for it, the caterpillar will morph into a rather undistinguished brown moth. If one of the local birds gets it before that, well, that's life on the food chain.