Originally Posted by Pink Figs
It's not always easy to notice.
How many days does it take them to build a nest, do you have an idea?
And what do I do about this fledgling. He sits always in a corner in the balcony. If I shoo him away, will he be able to survive? Will his parents find him to feed him?
Pigeons are very opportunistic nesters, they will call two sticks a nest and start brooding, so there is very little advance notice before you have a family burgeoning on your balcony.
If possible, maybe try to lure your unwanted fledgling with food to some better location. Otherwise, consider it part of the landscape. This bird is less likely to be diseased if it is mostly on your balcony. Eventually, I'd expect the hormones to start jumping and it will move, just watch out for an unwanted return.