Accidents happen of course, but I'm personally skeptical that the occasional accidents are responsible for the majority of masks "in the wild", as opposed to intentional littering, e.g., dropping the mask on the ground when someone walks out of a building.
Whatever your opinion of masks, I don't think that public health policy should be determined by whether people are littering. We should take measures to stop littering in any case. Also, I see no reason to prioritize bird health over human health (though we generally shouldn't have to choose one or the other).