As an aside, those are the reservoirs that the people of Portland are livid are going to be replaced with new, covered reservoirs. Just because they're over 100 years old and at risk for cracking, sending millions of gallons of water down a hill and into surrounding neighborhoods! We've always had open reservoirs, dammit, and just because they're a giant flooding and safety liability doesn't mean they should be replaced!
Ah, Portland. How I love you.
(in case you were wondering, yes, birds do shit in them all the time).