A powerful choice in filters allows you to change the way the lens of the camera focuses, or highlights certain elements of the picture. One example of these, is a zoom lens. Smaller subjects, even as far down as ants and bees can be captured in slow motion or with time-lapse photography. Separate color filters in magenta or red let you choose certain light types in your shot, hand highlight certain aspects of the subject. This is great for butterflies, or to help with some harder to see choices. You can even choose between these two options with a Switchblade3 filter.
Microphones let you filter out the surrounding noise, and really focus on your subject. Whether it is a creek babbling or the smallest bird call, an external microphone is a must. Depending on what you intend to use it for, you can get additional noise dampening benefits, or you can choose more directional localization. This is a really good way to get a backdrop recording of a natural experience, stop motion photography, of just the sounds by themselves. Many creatures communicate through imperceptible sounds, and directional microphones have been a big step in tapping into these languages.
The sun continuously plays tricks on nature photographers, causing many well thought out shots to be ruined by a mere trick of the light. Lens polarizers make sure these occasional sun glares do not interrupt the shot. They also help when you are photographing shinier objects, as the glint does not affect the polarizer either.