I get this question a lot, especially in spring. The space under a park bench seat is sheltered, dark, and just wide enough for house sparrows, starlings, robins, or pigeons to tuck a nest. The trick is to make that spot unappealing before the birds get established, because once eggs are present, you usually have to wait.
First, check local wildlife laws. In many places, active nests and eggs of native birds are protected, so only discourage nesting before it starts or after the young have left.
The most effective fix is physical exclusion. Cut a piece of galvanized hardware cloth with about 1/2 inch mesh and staple or screw it tightly across the underside of the bench seat frame. This blocks the cavity completely but still lets air flow. If the bench has a recessed lip, attach a sloped metal or plastic panel underneath so there is no flat ledge for a nest to sit on. Birds prefer stable, level surfaces, and a slick slope ruins the spot.
For a quicker option, install bird spikes or porcupine wire along the inner ledge under the seat. The blunt spikes are uncomfortable but not harmful, and they stop birds from landing and building. You can also apply a non-toxic bird repellent gel to the ledge; it stays tacky and makes birds move elsewhere. Avoid anything sticky that could trap small birds or beneficial insects.
Add visual deterrents near the bench, such as reflective tape, old CDs, or a small pinwheel. These work best when they move in the breeze and catch light. Birds are nervous around unpredictable flashes, though some will get used to them, so combine them with a physical barrier.
Clean the underside regularly. If birds keep dropping twigs and grass, sweep or scrape the area out as soon as you notice fresh material. Repeated disruption in the early building stage often convinces them to choose another spot. Also try to keep food scraps away from the bench, because crumbs attract both birds and nesting activity.
If you simply want birds around but not under the seat, mount a small birdhouse on a nearby post or tree. This gives them a safer alternative and can reduce the pressure on the bench.
One final note: if a nest is already active with eggs or chicks, leave it alone until the young fledge. After that, clean the area and install a barrier so the next pair will not move in.