What is the meaning of in a bird inioms?

Bird in the hand - something certain rather than something uncertain

Birds of a feather flock together - People of the same sort are likely to become friends.

Early bird catches the worm - If you get up early in the morning, you will have more opportunities.

Killed two birds with one stone - Accomplished two things with one action

A little bird told me - A secret source

Let the cat out of the bag - To share a secret

When pigs fly - Never, not going to happen