1 Fire hazard:

Bird nests in ventilation shafts can start fires. When birds nest inside buildings vents, especially clothes dryer vents, the blocked lint can ignite.

2 Leakage:

Birds can cause leakage by clogging roof-gutters due to the nests, bones and etc.

3 Health issues:

Birds are able to carry more than 60 transmittable bird-borne diseases including Psittacosis, Salmonellosis, Fungal diseases and Avian Influenza which can be fatal for humans.

4 Material degradation:

Since bird droppings are very acidic they can eat away at construction materials such as rooftop surface or air-conditioners.

5 Noise, poor sanitation and decreased aesthetics:

Last but not the least, birds make lots of noise and mess on your rooftop. This attracts new birds to nest and roost there. Birds can also attack people when they feel danger.