I had a rather bad experience with a swallow as a kid.
I was trout fishing next to a bridge on the creek near home.
The thought that there could be birds living under the bridge never really occurred to me at the time.
Casted my line into the water and let it run down-stream, all of a sudden I saw something move to my right, it was very fast moving, the next thing I noticed I had a pain in my right bicep, a swallow had banged into my arm and I was bleeding quite badly, all this time I was being swooped by other swallows.
I've never fished near bridges since.
Having said all that, swallows would have to be my favourite bird, for the size of them they can fly very fast.
We used to get lots of them in new dairy sheds, flying around eating sand-flies and house flies and the odd spider too, all you'd normally see is this small thing with a forked tail tearing around the shed, they were nice to watch.