aww. Its lovely isnt it. The last time it happened i thought maybe it was because they were stressed as they had just had a lice n easy bath and all got upset. But nothing stressful has happened to the piggies upstairs today so I dont know...All i know is it went on for ages and made me smile
Our big girl Midge 'sings' quite a lot, I've read on another g.p site it dates backs from wild instinct when the dominant pig (usually male) would stand guard and look out for trouble from predators. Just their way of saying everything is ok although when midge does her singing the other girls just freeze and stop what they are doing.