From what I've read, it's supposed to be a behavior whose purpose is pretty much unknown. It does seem to mean something to the pigs, since typically any other pigs around will stop and listen, but just what it's meant to convey is apparently not really known. I've heard pigs chirp, but it's been very infrequent... I think I heard Linney chirp twice in her 6-year-plus lifespan, and have heard Sundae (now 5.5 years) chirp once thus far, so for my pigs it was not very common and I could never figure out what provoked it.