Is it likely a bird would swoop in or are my fears unfounded do you think?
Free-ranging piggies or piggies in a run with no lid would be a very tempting target for a bird of prey. We get kestrels and sparrow hawks around here, so I am glad that my runs have roofs. Crows and magpies would also find piggies tempting. You might think that your piggies would be too big for a kestrel or sparrow hawk but they will both try for prey quite a lot bigger than themselves.