[cjo]

Also of interest is that birds have little oil droplets, often colored, as part of their visual set up. There are five different known types of color-filtering oil droplets (a sixth that is transparent) but it doesn't appear every bird has all of them.

I'm no bird sight expert but it's suggested here [0] that a particular bird may have 8 effective color receptors (5 cones, 3 cone-droplet pairs) for a huge number of colors humans can't see. Diagrams of the receptors themselves can be found at the wiki [1].

[0] - http://watchingtheworldwakeup.blogspot.com/2008/11/mountain-... [1] - http://en.wikipedia.org/wiki/Bird_vision#Light_perception