Feeders might seem like friendly spots where chickadees, finches, woodpeckers, and nuthatches can kumbaya over a good meal, but that couldn't be further from the truth. Some of the most intense avian rivalries are hashed out among the seeds and suet, says Eliot Miller, a researcher at the Cornell Lab of Ornithology and author of a new feeder-hierarchy study. “Feeders are these arenas where birds gather and fight," he says. "There’s an increased rate of aggression to some degree.” By combing through thousands of observations from Project FeederWatch, a volunteer science count that runs from November to April, Miller uncovered a Hunger Games-like world involving 136 common North American species. As birds compete for access to feeder supplies, they reveal where they belong in the pecking order. So, by simply noting which species flew off and which ones stood their ground, Miller and his colleagues were able to build a database of victors and losers. They then used some...