As humans have taken and developed so much habitat, wild animals appropriate places of their own among our structures — where they can. Rooftop nesting sites have some advantages, in terms of protection from ground predators and humans. But they are still vulnerable to aerial predators (owls, in some case), to the elements, and to the possible slips chicks face as they start moving about. While we were at this site one day, we learned of one injured gull rescued the day before, delivered to Monterey SPCA for rehabilitation.
The hazards of early life for most wild animals are many, irrepsective of where they nest and fledge. I’m not sure of the survival ratio at this particular colony, but based on the number of ready-to-fly youngers we saw, I suspect the majority do fledge.