Anyone who has carefully observed a hummingbird flitting about a nectar feeder or a patch of flowers has likely seen it happen. One moment, the graceful aerialist is darting between bright blossoms, jittery but tranquil as it laps up sugary juices. Then, suddenly, all hell breaks loose when a second bird approaches. In a flurry of chittering, gorget-flaring, and dive-bombing, the featherweights duke it out. The furious bout lasts only a few seconds but is long enough to declare a loser and send them zipping away. Such violent outbursts may be surprising given the minute size and delicate appearance of these dazzling birds, but there is good reason for their aggression—their sweet tooth, says Scott Weidensaul, an ornithologist and author who has studied and banded the winged gems. Hummingbirds are highly dependent on nectar, the sugar-loaded syrup that plants generate to entice pollinators into spreading the plant’s genes. It’s the rare food that provides enough energy to...