Seed preferences of wild birds may be a result of factors such as bill and seed morphology, handling time, nutritional content of the seed, and foraging behavior of the bird. To examine factors influencing seed choice of 10 species of birds that regularly visit bird feeders in the United States and Canada, we collected data on bird visits, bill size, seed size, and nutritional content of seeds for 10 seed types commonly used in bird seed blends. The presence of an outer hull, protein content, and the ratio of bill volume to seed volume were the most important variables determining seed choice. Fiber and fat content were less important. Results from this study help us understand why birds that use feeders use the seed types they do, and ultimately, may improve the composition of seed species and individual seeds found in bird food blends.