Old Oswestry Hillfort
Having confirmed that the name of one of the fathers of the three Guineveres in Welsh tradition is properly Gogfran, 'Jackdaw' (or chough, etc.), I'm now convinced the tradition which associated this giant with Old Oswestry hillfort in Shropshire is correct.
The jackdaw is a member of the crow family. Here is a photo of the bird:
Oswestry has an early legend of a "great bird of the crow family". The story of this bird is related in "The King in the North: The Life and Times of Oswald of Northumbria" by Max Adams:
Although most commentators on the Life of St. Oswald have preferred to see in this bird a raven, there is no reason why it couldn't have been a gogfran.