Bird tails are, essentially, a grouping of long feathers controlled by a small muscle at the base. They are lighter and stiffer than their body feathers - as is the case with the tip and trailing edges of their wings. Collectively, these are called 'flight feathers'.
Birds use their tails for balance, to aid in steering in flight - used like a rudder. Also, without a tail, many birds would find difficulty in landing, perching and taking off gracefully. Some male birds - such as Peacocks and use their tails for display and courtship, whilst other birds (Wilson's Snipes) use their tail feather to whir and whistle as they dance.
The Woodpecker, for instance, has a row of prongs that helps her to hang against tree trunks while she beats the bark of the tree. The tail feathers act as stabilisers.
Birds displaying their tail feathers in courtship ...