If you feed the neighborhood stray cat, he'll keep coming back. He'll remember your house as an easy source of food and won't have to scavenge for as much garbage or chase down as many field mice. The same thing obviously applies to birds and bird feeders, just on a much larger scale. At that point—when hundreds of thousands, even millions of feeders are involved—what kind of effect does the feeding have on the animals' natural ecosystem? That's the question Gillian Robb and her team at Queen's University Belfast in the U.K. set out to answer through an experiment of their own and a review of the existing literature.