Interesting close-up a few minutes ago. This is the first time I've seen greenery up there! Is that fallen tree branches from all this wind and rain? (although don't know where they would come from in downtown Winnipeg!) Or do the chicks actually have a wee little forest beginning to grow up there?! 
(LOL) That would certainly give them some shade on the hot sunny days! 
Sand/dirt debris accumulates on this roof, that's why the peregrines nest on it, the debris gives them a substrate for their nest scrape. The other thing the substrate does is collect seeds so we always have some vegetation appearing at this time of the year. It gets killed off/cut back with the winter months and usually in the late summer when there just isn't enough rainfall (might not be a problem this year) to keep them hydrated in such thin "soil". If you are seeing "branches" think bird limbs not tree limbs
Its the debris from past meals mostly - bones, some wings. Doesn't mean something else couldn't have landed there, but it is 14 stories up, and that's a bit high for flying debris to travel through the downtown canyons. If I see something "that doesn't belong here", I'll certainly point it out to y'all however!
