What is the smallest bird?

Context: Hummingbirds have varied specialized characteristics to enable rapid, maneuverable flight, exceptional metabolic capacity, adaptations to high altitude, sensitive visual and communication abilities, and long-distance migration in some species. Among all birds, male hummingbirds have the widest diversity of plumage color, particularly in blues, greens, and purples. Hummingbirds are the smallest mature birds, measuring 7.5–13 cm in length. The smallest is the 5 cm bee hummingbird, which weighs less than 2.0 g, and the largest is the 23 cm giant hummingbird, weighing 18–24 grams. Noted for long beaks, hummingbirds are specialized for feeding on flower nectar, but all species also consume small insects.

Answer: Hummingbird is the smallest bird.