What's that green goo on the bottom of my shoe?

My least favorite thing that happens on campus is when I'm walking and start to feel one of my shoes slipping when it scuffs along the sidewalk... And I immediately know that i have fallen victim to the green-pooping monsters.

Canada Geese. Not to be confused with "Canadian" Geese, which in fact do not exist. They're all over UCO campus. And their poop is green and everywhere also. The bad thing about this is that it blends in with the grass. I average stepping in it at least a few times every semester.

But what I'm wondering is... Why is it green? Is it something they're drinking/eating? If so, what's in Brocncho Lake? Everyone always jokes about how nasty Broncho Lake is... Maybe there's something to these rumors.

Anyone know?