I get very upset when I hear of people who knock down swallow and martin nests because they leave a mess
We had some nests in really inconvenient places last year, and the droppings were going all over sheep medicines, lawnmowers etc.
A really easy solution was to use a sheet of thin plywood (or thick cardboard) about 1ft/1ft, suspended beneath the nest. The birds weren't bothered, yet it caught all the
, and kept it off our stuff.
Of course another way around the annoyance is just to change your perspective. You see guano. I see dead flies!!