Check with your local nurseries to see if they handle bird netting. It's a coarse netted plastic material, usually green, that berry and fruit growers use to prevent bird damage to their fruit. You will only need to leave it on the ferns for a short time, until the nesting season is over.
My Boston ferns are on tall pedestals in the garden, near the patio. Occasionally, there will be a bird nest in one of them, but I have a large number of shrubs, small trees and vines that they prefer, so it hasn't been an ongoing problem. The ferns aren't near a traffic area, so I never bother the birds.
One year I had a Mourning Dove build a nest on the window ledge of an upstairs bathroom. During that year, they raised three broods in that nest and I thought they were going to try for a fourth, but they gave up and went away. Interestingly, you could stand at the window and look them in the eyes and they never flinched. Somehow, they knew that I presented no danger to them.
Rb