It has been well established that humans and cows do not flourish when ingesting grains has their main source of nutrition. What kinds of animals are adapted to eating grains, and thrive on it?
PortlandAllan is partly correct in saying "birds" But not all birds are adapted to eating grass seeds (i.e., grains). Most of the grain-adapted species include chickens (and their ancestor the jungle fowl), the Columbids (pigeons and doves), sparrows and finches, and likely some others. But many birds are not specifically adapted to eating grains, such as penguins, owls and other raptors, ostriches and emus (including the kiwi), hummingbirds, etc.
There are probably a number of mammals that can tolerate some grain consumption, like certain rodents, including field mice, black rats (which in the forests of Israel eat only pine seeds), gerbils, etc.
Maybe some insects or other invertebrates are also adapted to eat grain, but I can't think of any off the top of my head.