Laura Erickson's For the Birds

The tiniest bird in the known universe is found only on Cuba. Both adult and young males are recognized by their blue back; females have green back feathers.

Because of their tiny size, Bee Hummingbirds made it into the Guinness Book of World Records:

Smallest bird: Bee Hummingbird. The smallest bird is the bee hummingbird (Mellisuga helenae) of Cuba and the Isle of Youth. Males measure 57 mm (2.24 in) in total length, half of which is taken up by the bill and tail, and weigh 1.6 g (0.056 oz) Females are slightly larger.

And:

Vervain Hummingbird, Bee Hummingbird: Smallest bird nest. The smallest nests are built by hummingbirds. That of the vervain hummingbird (Mellisuga minima) is about half the size of a walnut shell, while the deeper but narrower one of the bee hummingbird (M. helenae) is thimble-sized.