Why do pigeons have such strange nests, what is wrong with these birds? (7 photos)
Modern city pigeons are the descendants of those postal pigeons, which people once pulled out of their usual warm native natural habitat (southern Europe, Southwest Asia and northern Africa), and forced them to carry letters all over the world. When the need for winged postmen disappeared, thousands of birds remained in various cities, switching to a sedentary lifestyle. 
They had to arrange their own life in unusual conditions and an unsuitable climate, get food (by beating it out of passers-by) and raise chicks. And they coped as best they could.
Like any standard bird, a pigeon makes a nest. Only it looks like a nest only remotely. 
Pigeon nests, if you can even call them that, look so careless that there are many jokes about it on the Internet. 
Let's not find fault and say that this is a nest
What's wrong with pigeons, why don't they know how to build normal nests? No, they wouldn't have time to unlearn it so quickly
In general, pigeons don’t really need this at the moment. They build a nest on the flat, hard surface of city balconies and attics of buildings. The nest won't fall from there, so there's nothing to worry about. But this is not the main reason. 
When the pigeons form a pair, the male begins to collect material for construction in order to show the female the seriousness of his intentions, and then something breaks, and instead of the usual nest, a small chaotic pile of twigs, sticks and everything else he could find appears. The female is absolutely not demanding about the nest. It is enough for her to see that the male has designated a place for future laying. 
What's the matter
In nature, pigeons nested on rocks for thousands of years before moving to cities. They built nests on small ledges. They found a more or less level area, marked exactly the same loose something there (just to mark the place), and calmly raised the chicks. 
Why do they even put a couple of twigs in place of the nest? You can do without them. Well, first of all, a visual reference is important to them. Secondly, very ancient residual instincts still speak in them. Many thousands of years before switching to nesting in rocks, the ancestors of modern pigeons built full-fledged nests in trees. 
With the move to the rocks, the need for normal nests disappeared, and even the paws of modern pigeons are no longer comfortable for sitting on branches, but with the advent of warmth, that distant memory of their ancestors awakens in the heads of the sizars, forcing them to do something at least vaguely reminiscent of a standard bird house.
