I'm not sure, I've not really seen Hasdrubal
try to fly. I think many bird fledglings get quite a bit of parental attention after leaving the nest, I don't know about pigeons specifically but it's true of most species I do know about. They need to learn to get food etc somehow of course, and many if not most fledge before they can fly properly and thus still need feeding by parents who can access food. I just hope Hasdrubal has got to the stage of being able to eat without parental input: I really do not want to have to learn how to feed a scared baby pigeon manually.
And no, I didn't have any particular Hasdrubal in mind, the name just sort of turned up!
General update: Hasdrubal is still outside, looks fairly bright-eyed and not sick or weak as far as I can tell which is good. I've put a box out in case shelter is wanted overnight and have still left the food out. I tried picking it up at one point to see if putting it higher up helped attract parental attention, but it was having none of it.
I don't think there should be any serious danger: I obviously can't be absolutely certain that there are no predators in the area, but I don't think there are many. Crows are probably the biggest threat, but they rarely land down inside these deep inner city blocks. Bigger prey birds would be even less likely to come into an enclosed space like this, and it being a block probably means few routes for foxes or feral cats/polecats (of which there are very very few if any in central Vienna in the first place). So that just leaves rats as a threat, and again as far as I know we don't have any. I think rats might think twice before attacking a young pigeon in any case: they could take it, but it could probably hurt them.
Amusingly, I went out to check my plants at one point and Hasdrubal scooted, first away, then when I got to the other end of the decking back in the reverse direction... and into the door, going for a brief exploration of my flat. I had to corner it in the shoe area, and it started making lots of scared peeping noises - fortunately I had a brainwave, turned my bucket on one side, it ran into it, and I evicted it in that.
More pictures:
Mugshot with much baby fluff:
A small pigeon goes exploring:
By the door:
And in the eviction bucket: