Which bird is the smallest migratory bird?

Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. It occurs mainly in the northern hemisphere, where birds are funneled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea.

Hummingbirds are birds native to the Americas and comprise the biological family 'Trochilidae'. With about 360 species, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics. They are small birds, with most species measuring 7.5–13 cm (3–5 in) in length. The smallest extant hummingbird species is the 5 cm (2.0 in) bee hummingbird, which weighs less than 2.0 g (0.07 oz). The largest hummingbird species is the 23 cm (9.1 in) giant hummingbird, weighing 18–24 grams (0.63–0.85 oz). They are specialized for feeding on flower nectar, but all species consume flying insects or spiders.

With an average weight of 1/8 of an ounce, Hummingbirds are the smallest migrating bird. They can travel as fast as 48 kph when migrating. They fly nonstop, which can be as far as 600 miles.