Shale

The Shale are birds that guard the Sky Border in Foo. They are first heard of when a boulder that was knocked off the hard border for attempting like so many others to expand above the Sky Border had gained the ability to talk after it accidentally crushed someone who happened to be standing in the wrong place at the wrong time. It mentioned how the Shale would group together to form barriers whenever anything tried to go past the Sky broder and knock the would-be trespasser down.