White-Tailed Eagle, Britain
The poem "The Dialogue of Arthur and the Eagle" may, according to Professor Patrick Sims-Williams, be as early as the 12th century.
Given my new idea for Dinas Powis in the Ely (Elai/Elei) Valley as the fort of Uther Pendragon (see http://mistshadows.blogspot.com/2017/11/pa-gur-yv-y-porthaur-and-ruling-center.html), I went ahead and asked Dr. Simon Rodway the following question:
Could Eliwlad could be for Elei-(g)wlad, "Elei-prince"? See forms for the Ely River below...
I only ask because the god Mabon is placed in Elei in the Pa Gur as a 'predatory bird'. He is also placed in death at Nantlle, which is exactly where Lleu is placed in the form of the death-eagle in an oak. In other words, there seems to have been some attempt to identify the two gods in Welsh tradition. I would also point out that Tremabyn and St. Mabyn are very close to the Cutmadoc place-names in Cornwall in the wooded Bodmin 'glynn', which is where the Eliwlad of the poem is placed. Eliwlad, of course, appears in spectral form as an eagle in an oak.
If we could derive Eliwlad from Eleiwlad, that would solve pretty much every problem with this name and its context. See Ely below in 1314, and Eli and Ele later for this river. These are from the Melville Richards Archive, but I'm not sure it is an exhaustive list.
ELAI Elay flu. BLAEU WILLEM JANSZ(OON) - MAP 1645
ELAI Eley 1536 CAD CATALOGUE OF ANCIENT DEEDS, I-VI, LONDON 1899-1915 vi. C 7589
ELAI Eley 1666 CFL DEEDS AT CARDIFF FREE LIBRARY, ARRANGED BY COUNTIES Glam
ELAI r. Eli 1638 CFL DEEDS AT CARDIFF FREE LIBRARY, ARRANGED BY COUNTIES Glam
ELAI r. Ely 1314 CIPM CALENDAR OF INQUISITIONS POST MORTEM, LONDON 1898FF v.332
ELAI Elye 1607 HENSOL HENSOL MSS
ELAI Ley River 1536/9 LELAND JOHN LELAND/THE ITINERARY IN WALES 18
ELAI Ele 1536/7 LELAND JOHN LELAND/THE ITINERARY IN WALES 18
ELAI Elei LL THE TEXT OF THE BOOK OF LLANDAV 204
ELAI Eley 1578 RM/BGA RICE MERRICK - A BOOK OF GLAMORGANSHIRE ANTIQUITIES 101, 112
ELAI Eley 1348 CIPM CALENDAR OF INQUISITIONS POST MORTEM, LONDON 1898FF ix.333
ELAI Eley 1229-61 CMG CARTAE ET ALIA MUNIMENTA QUAE AD DOMINIUM DE GLAMORGAN PERTINENT ii.462
ELAI Morva Lei 1514 CMG CARTAE ET ALIA MUNIMENTA QUAE AD DOMINIUM DE GLAMORGAN PERTINENT v.1782
ELAI ht. Eley 1719 CR RECORDS OF THE COUNTY BOROUGH OF CARDIFF I-IV iii.159
ELAI Elay 1760 EB/MSW EMANUAL BOWEN/MAP OF SOUTH WALES 1760
ELAI r. Eley 1586 HENSOL HENSOL MSS
Dr. Rodway replied succinctly with "Yes, it could."
The wood of Cornwall by the Cutmadoc place-names is doubtless a relocation for Coedcernyw not far east of the Ely Valley.