Its territory is said to have stretched from the Tyne northwards, ultimately reaching the Firth of Forth, while its western frontier was gradually extended westward, encroaching on the Cumbric speaking kingdoms of Rheged, Gododdin and Dunbarton.
The first king of whom we have any record is Ida, who is said to have obtained the throne about 547. Aethelfrith, king of Bernicia, united Deira to his own kingdom, probably about 605, and the union continued under his successor Edwin, son of Ælla or Ella, king of Deira.
Bernicia was again separate from Deira under Eanfrith, son of Aethelfrith (633—634), after which date the kings of Bernicia were supreme in Northumbria, though for a short time under Oswiu Deira had a king of its own.
Bernicia was written about by Bede in his Historia Ecclesiastica.