|Hello Man Yee!
Thank you for your kind words! I'm not 100% sure about dark streak but I think it is for two purposes. Firstly - camouflage - black eye is too well visible on bright feathers. Secondly - I think it is to protect eye during fights or attack of predator - predator don't know where to hit to damage real eye when all streak including eye is equally dark.
I just founded third answer from very different field:
Q: Why do American footballers paint black marks across their cheeks?
A: Cuts the glare of the sun or bright lights.
Walkabout With Kaszek (28)