Peerie Whup

I think the Shetland name for this is a Peerie Whup (spelling?).  I don't know what the right name is.  There were a few of them following me near the Gloup in Deerness this morning.  They made a cheeping sound similar to a Shaalder.  When the flew they glided mostly with occasional single wing beats synchronised with a cheep.   Some of them landed with a curious fold of their wings, stretching them vertically showing the white understand before folding them away.