Now Pigeons, On The Other Hand...
I worked in a very old listed (and listing, actually) building. The department I managed manufactured a range of medicinal products.
As with many old buildings that can't be physically altered because of the protection order on them, the near-Victorian inner had boxed offices installed with the inevitable suspended ceilings. Problem was the outside had iron-framed windows and copious glass panels from the days when it was important to let natural light in because the alternative would have been oil or gas lamps.
Unfortunately, much of the glass must have been installed roughly when the building was still new and - as a consequence - a lot of it wasn't entirely there.
Enter: pigeons. And they did enter, through any gap they could find. When it was quiet, you could hear them cooing and moving around above the suspended ceilings.
Then, the company decided it had to access the services above the ceilings for a reason I can't remember now. It definitely wasn't to replace the ceiling, though, even though it was showing signs of its age. Something to do with the ducting and wiring.
When they removed the tiles, it turned out that hundreds of pigeons were actually nesting up there. Their poo was centimetres thick above the tiles, along with feathers and rotting corpses. Naturally the engineers who did it one weekend didn't give a damn about the desks in the offices (one of which was mine) underneath and a lot of this detritus was covering the desks and floors when I came in Monday morning.
It took weeks to stop itching after that.
Then another time we discovered maggots on the open air-cooled conveyor that cooked product ran along. It turned out a pigeon had got into the gap between the chimney stack and roof to nest, died, and the maggots that it thus gave rise to found the best way was down - into the manufacturing area.