Hm, the original writeup here seems to have gone away...originally, it was noting that you never see baby pigeons, even in extremely high pigeon population areas like cities, and hypothesized that perhaps the nests are hidden from Man deliberately. My addendum: these nests aren't just hidden from mankind, but also from the other annoying city dwellers that pigeons must deal with. Squirrels will steal nesting materials. Crows will eat eggs and/or baby pigeons. Peregrine falcons will eat the parents. Rats will eat eggs or squab. And so on.