Noticing that songbirds, such as finches, never seem to get fat despite overeating at bird feeders, London environmental biologist Lewis Halsey wondered whether the amount of energy birds put into singing, fidgeting, or exercising could be adjusted in ways that regulate weight. In a literature review published September 18 in the journal Trends in Ecology & Evolution, he explores whether songbirds don’t need to worry about their calorie counts because they can control the way their bodies use energy.