Eager beaver
The term eager beaver refers to a person who is hardworking and enthusiastic, sometimes considered overzealous.

Bee in one’s bonnet
Someone who has a bee in their bonnet has an idea which constantly occupies their thoughts.

Birds of a feather flock together
If two people are birds of a feather, they are very similar in many ways, so they naturally spend time together.

A dark horse
Someone who is more clever or skilful than anyone expects

A lone wolf
Someone who is not very social with other people

A guinea pig
Someone who is part of an experiment or trial

The travel bug
A very strong desire to travel

To have butterflies in your stomach
To be nervous

An early bird
A person who gets up early in the morning, or who starts work earlier than others.

A home bird
Somebody who prefers to spend his social and free time at home.

A busy bee
A busy, active person who moves quickly from task to task.

A lone bird/ wolf
Someone who prefers his won company or who has little social contact with others.

An odd bird/ fish
An eccentric person whose behavior or way of life is regarded as strange.

A rare bird
Somebody or something of a kind that one seldom sees.

A dog in the manger
A person who selfishly prevent others from using, enjoying or profiting from something even though he/ she cannot use or enjoy it himself.

A cold fish
Somebody who is not often moved by emotions, who is regarded as being hard and unfeeling.

A dark horse
Someone whose past is a mystery; a person who keeps their interests and ideas secret, especially someone who has a surprising ability or skill.

A lame duck
A person or enterprise (often a business) that is not a success and that has to be helped.

A sitting duck
An easy target.