Catching a wild animal in a closed area, if it is easily caught once you close the door, it is prohibited biblically. If the animal is not easily caught after you close the door, it is only prohibited rabbinically.

Chapter 316 Section 2

Capturing a sick deer or an old one is not prohibited biblically as it is considered caught already.

Chapter 316 Section 3

Biblically, you are only prohibited if that species is normally captured. Rabbinically, all capturing is prohibited.

Closing a small box knowing that little bugs will be caught is prohibited.