We recently completed the gutters on a commercial site. The building is a large hall. The roof has vents at along the ridge of the roof. The openings are big enough for the birds to enter and roost in the roof.
The birds have made their home here. Their playground is the roof top. They drop grass and nesting materials as they bring it into the vent to make their nest. Their droppings have grass seed in it and this cultivates in the rich sludge that collects in the gutters over the year.
Combined, the result is shown in the above photos. The gutters are overgrown with grass, potentially blocking gutters and down pipes. It can allow water ingress into the roof cavity, resulting in water damage to the inside of the building.
Birds nesting in the roof will be noisy. The main issue would be the hazardous flammable materials they use to build their nests. They are vermin and carry bird lice. Over time the bird droppings build up in the cavity and could pose a health hazard as they can spread zoonotic diseases.
The best course of action is to remove the birds and ensure there are no eggs in the nests. The best time is when they leave the space and it is not laying season.
Block the entry points, so as to not allow them back into the roof space. They will be determined to get back in. You need to make sure you use a robust material to keep them out.
Then you will need to remove the nesting material and guano from inside the cavity. You will also need to decontaminate the area to eliminate the mites and fleas birds carry.
The cleaning process should be undertaken by a professional, as they have the correct equipment to protect themselves from the health hazards associated with this work.