Birds require foods with high protein and fat content, requiring up to 10,000 calories a day, especially during migration, breeding season and on frigid winter days. Additionally, being efficient about what they eat is critical; birds assess potential food items for not only nutritional content and quality, but also how easy a type of food is to consume while avoiding predators. Differences in what a given bird species has adapted to eat will change what each type of bird favors, though black oil sunflower, white proso millet, nyjer (thistle) seed and sunflower chips are favorites of most backyard birds.