Birds nesting in the gap between the panels and the roof is a real and recurring nuisance, especially once a system has been up for a while. Pigeons in particular like the sheltered, warm cavity under the array; the mess (droppings, nesting material) can build up, block gutters and look grim, and getting roofers to retrofit a fix afterwards can be surprisingly hard — many aren't keen to take it on. So it's a problem that's much cheaper to prevent than to cure.
The standard remedy is proofing mesh or clips fitted around the perimeter of the array, closing the gap so birds can't get underneath while still allowing airflow. The cheapest time to do this is at install — ask your installer to quote bird/pigeon proofing as part of the job, particularly if you're in an area with a known pigeon population or near other houses that have had the problem. Note that some installers will tell you it's only an issue in cities and not worth it; that's sometimes true for rural sites, but if pigeons are around, retrofitting later is the painful, expensive path.
If birds have already moved in, you'll typically need someone to clear and clean under the panels and then fit perimeter mesh to keep them out — a pest-control or specialist roofing job rather than a standard solar call-out. The practical takeaway: decide on proofing before the panels go up, factor a modest cost in if birds are a local issue, and don't assume "it won't happen to me," because it's one of the more common post-install regrets.