In dense urban environments, masses of corrosive pigeon shit everywhere, with feathers sticking to it. Unsanitary.

In my French Parisian city of Courbevoie, pigeon houses where eggs are shaken to sterilize them, and re-introducing magpies solved the problem - the now sparser population is fine. Magpies and crows devastate pigeon nests.

Also, the expansion of greenery led to shifting the pigeon population from mostly columba livia, which nests on rocks (and therefore on buildings) to mostly columba palumbus - which nests in vegetation and thus doesn't make a habit of shitting on buildings.