I had a rental property na few years ago, that was an old Victorian building, it had a chimney that had only been partially blocked off, so you couldn't use it and there were crows nesting in it every year.
around the same time each year, the house would be absolutely inundated by scores of flies for a few days to a week - I guess when the crows left the nest or perhaps some chicks died in the chimney or something.
it was horrendous.

the most effective way of dealing with them by far was fly paper. it's unsightly but it can clear a load of them in a few days.
another method I inadvertently found was leaving an open bottle of yoghurt drink in the vicinity, about 20 of the ****ers had gone in and got stuck in there in a few hours