Wild bird feeding is one of the most common forms of human-wildlife interactions in the Western world. Originally a practice providing nutritional assistance to over-wintering birds, especially in more northern latitudes, birds throughout the cities of the world are now provided with considerable amounts and a variety of foods year-round. Despite the global nature of the practice, remarkably little is known about the outcomes and implications of what may be seen as a supplementary feeding experiment on a massive scale. Although many claims are made about the benefits of feeding, there are growing concerns about the spread of disease, poor nutrition, risk of dependency and many other important issues. Constructive debate among increasingly vigorous proponents and opponents is currently constrained by a lack of reliable information. Here we argue that bird feeding provides an important, if challenging, opportunity for fundamental research in urban ecology.