...If the bird is seriously considering the raven's nest, it might have felt it needed to try to make a scrape nearby or, or, or ...
I'm a bit confused now, TPC. I thought that North American peregrine falcons only nested on the side of rock cliffs or in cities in man made nest boxes with gravel. Would a peregrine falcon actually consider taking over a raven's nest? I thought only Merlins would do that. 
Ravens like some of the same human-made structures that peregrines like - towers are one of them, there are lots of birds in the US that nest on all sorts of towers. When Zeus and Alice decided on the Tower Site, the only place they could lay their eggs safely was in the raven's nest. And I should mention (though I think RCF might have mentioned it sometime earlier) these nests are not small - last year's nest was about the size of the McKenzie Seeds nestbox and more flat than nest-like (bowl shaped). In this case the catwalk was like a cliff, the raven's nest the only spot with a solid surface on which to lay an egg. So that's where they nested.
At the University of Manitoba one year a pair tried to nest in a rain gutter and on the Hotel Fort Garry and at the Provincial Legislature, Abby and Garry used pigeon feces instead of pebbles to keep their eggs from rolling off the ledge. They were successful but never returned at the Leg, but spectacularly unsuccessful at the Hotel Fort Garry. In both cases they were too stress being so close to the Radisson Pair (T-Rex and Madame - both highly territorial birds).
