We have a big screened in porch on our deck and roll-down canvas shades. Our Carolina wrens nest in the rolled up ends. Makes a mess but we can sit on the deck and watch them feed their chicks.
In Tucson, the cactus wrens nest year round and always in places you would prefer they not. But, whatcha gonna do?
Well any bird is going to make a mess ... But I don't mind the wrens or a tit-mouse.
You can Google "barn swallow nest" and look at the images ... That will clear up what the nests are like (especially when multiple pairs nest).
You will also have copious amounts of bird poo everywhere ... Barn Swallows eat a lot of bugs, and poo a bunch all over the walls and everywhere else.
I mean I have house and gold finches that build nests in my hanging porch lights.
I don't mind taking them down and cleaning them when they are through for the season.
The barn swallows are simply not worth the trouble ... And as I mentioned before, the mess is only half the problem.
You won't be sitting there watching them do anything other than dive-bomb you with about 6 of their friends unless you are inside looking out a window.
Edit:
Barn swallows won't build anywhere they can see the sky from the interior of their nest.
Because of this ... When touring, you may notice how older plantation houses in the south paint the ceilings of the porches "sky blue".
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