1. plumage 

A bird's plumage is all the feathers on its body?

e.g. Have you seen the brightly-coloured plumage of tropical birds?

2. marginal

If you describe something as marginal, you mean that is small or not very important.

e.g. If you describe something as marginal, you mean that it is small or not very important.

3. catalyst

You can describe a person or thing that causes a change or event to happen as a catalyst.

e.g. I very much hope that this case will prove to be a catalyst for change.