>>716245925
>>716246056
>>716247821
Rock doves, which is what your standard city pigeons are called, are usually nesting in and around cliffs, so instead of having the need to build a nest on a tree, all they really have to do is prevent the egg from rolling out of the crevice, so a couple twigs are enough.
That's also the reason why they absolutely conquered modern cities, since buildings are pretty much artificial cliff faces.