Pigeons make a lot of noise and create debris (guano, feces, poop, etc), which clog up gutters and create a mess not only on the roof but on surrounding patios and yards.
Their nests start out as sticks and leaves but quickly become nothing but a solid mass of excrement on the roof. This mass tends to retain moisture, which rots the roof. If left alone, the roofing material underneath will eventually fail and the roof will leak.
Squirrels, while not as common, are more hazardous to the power generation system. They, like all rodents, have continuously growing teeth. They chew on whatever is around them, in order to keep their teeth worn to the proper length. If they choose to make a home under a solar installation, then what gets gnawed on is the wiring, connectors and junction boxes. We’ve seen cases where an entire junction box is missing, having been nibbled down to nothing.
In the worst cases, squirrels have been known to cause arching
which can lead to fires.