The Quenya word for “eagle”, appearing in a number of compounds, derived from primitive ✶thorono and the root ᴹ√THOR(ON) “come swooping down” (PE22/159; Let/427; Ety/THOR; PE21/33). It had couple variants such as sorno (Let/427) and sorne (Ety/THOR), but consistently appeared as soron- in compounds. Its stem form isn’t entirely clear: its most common plural form was sorni (Ety/THOR; SD/290), which is the expected result from the Quenya syncope, but in one place it had the plural soroni and the presence of the variant sorne muddies the waters.
Conceptual Development: This word appeared as ᴱQ. sorne or sor (sorn-) “eagle” in the Qenya Lexicon of the 1910s derived from the early root ᴱ√ŠORO [ÞORO] (QL/86). The form sorn- “eagle” also appeared in the Poetic and Mythological Words of Eldarissa where Tolkien indicated the primitive form was sorni- (PME/86). The form ᴱQ. soron appeared in Early Noldorin Word-lists of the 1920s as a cognate of ᴱN. thorn “eagle” (PE13/154), and Tolkien mostly stuck with that form thereafter, though he occasionally used variants like sorne and sorno as noted above.
In the Declension of Nouns from the early 1930s, Tolkien listed a large number of declined forms for ᴹQ. soron “eagle”, and those declensions used soron- (or sorun-) as their base. In The Etymologies of the 1930s, however, Tolkien gave plural sorni and genitive sornen indicating a stem form sorn-, but that document also gave sorne as an alternate form of soron “eagle”. Nevertheless, I think from the 1930s forward, it is more likely that Quenya syncope would have come into play in the declension of this word, so it would have plural sorni, dative sornen, ablative sornello, etc. The only noun case where the primitive stem would be preserved would be possessive soronwa.
soron (or sornë) (þ) noun "eagle", before an ending sorn- as in pl. sorni, "gen.sg....sornen"; in LotR-style Quenya this would be the dative singular instead (THOR/THORON). SD:290 has the pl.soroni "eagles", changed to sorni as in the Etymologies. Early "Qenya" has the forms sor, sornë (LT1:266)