Which is the smallest bird in the world?

No, never heard of a bird called zoology.

I reckon the answer is Kylie Minogue.

The Bee Hummingbird or Zunzuncito (Mellisuga helenae) is a species of hummingbird that is endemic to dense forests and woodland edges on the main island of Cuba and (formerly) on the Isla de la Juventud, also part of the nation of Cuba. With a mass of approximately 1.6–2 g (0.056–0.071 oz) and a length of 5–6 cm (2.0–2.4 in), it is the smallest living bird.