What does the idiomatic expression Bird in mean?

There's no idiomatic expression called "Bird in." It's likely you're thinking of one of these:

1. "A bird in the hand is worth two in the bush": This proverb means it's better to be content with what you have rather than risk losing it by chasing something that might be better but isn't guaranteed.

2. "Early bird catches the worm": This proverb means that those who are proactive and get started early will be more successful than those who wait.

3. "To kill two birds with one stone": This means to accomplish two things with a single action.

4. "The early bird gets the worm": This is another version of the second proverb, with the same meaning.

5. "Birds of a feather flock together": This proverb suggests that people who share similar interests or qualities tend to associate with each other.

Please clarify what expression you're looking for, and I can give you a more specific answer.