A bird mesh barrier on your PV solar panels can protect the solar PV from nesting birds and other small animals. Once birds are in place, it is a lot more difficult to proceed with installing the protection, so the ideal situation is to include this in your install. Allowing birds (typically pigeons), under the panels for long periods of time can lead to an infestation which as well as potentially causing damage to you solar PV array, is also a health hazard due to excessive droppings, a noise nuisance and can lead to blocked gutters. Once you have an infestation, the cost involved to repair damage, remove nests and clean up droppings can be significant.
The law on removal of nesting birds is also worth considering. Wild bird species, their eggs and nests are protected by law in the UK, so if you find these nesting under your solar panels, you will not be able to remove them.
The RSPB states that you must not damage or destroy an active nest and you must allow the young to leave the nest before taking any action to block the entrance and avoid roofing work if you know birds are nesting there.
This bird mesh shown here prevents pigeons and other birds from getting in under the panels and making nests. It will be an additional cost to your install, but one that prevents future damage to your roof and solar panels.