I disagree. When Gree Birds has search volume of 200 it means only the term Green Birds has searched 200 times, all related terms will have their own separate search volume.
I'd give here my estimation - for a keyword that has 1 000 000 monthly exact search volume according to the google's adword tool, my site going up and down from 8-5 places. It receives about 16-20k uniques monthly. Again with ads above natural results, etc. So let's say it should be around 3-10% depending on your keyword. One more thing I want to say... you rank 5th where? In google.com? Google' relies heavily on local search results already and espacially if your niche can give out many differant local results... and it's already understandable why you don't get traffic.
As I said before, my target country is UK. My friend which lives there and therefore represents an avarage UK visitor told me that I have the 5th position. That is why I should estimate my traffic according to 145 exact searches per day on the 5th pisition. Even If I had had 1%, I would have received at least 1 visitor a day from Google. It didn't happen, that is what Google Analytics showed me.