A noun in The Etymologies of the 1930s for a “swallow” derived from primitive ᴹ✶tuilelindō “spring-singer” (Ety/LIN², TUY).
Conceptual Development: In the Gnomish Lexicon of the 1910s, Tolkien had G. {duil >>} G. duiling or duilinc “swallow”, apparently a diminutive form based on (archaic) ᴱ✶du̯il “bird” (GL/31). Tolkien also had deleted G. duil “spring” (GL/31), and in the contemporaneous Qenya Lexicon he gave as its cognate ᴱQ. tuilindo “spring-singer, swallow” under the early root ᴱ√TUẎU that was the basis for “spring” words (QL/96). However, Tolkien marked this Qenya form with a “✱” and said it was “not related”, so perhaps Tolkien reimagined word as a derivative of ✱ᴱ√DUYU that was the basis for “bird” words in the Gnomish Lexicon, since initial d became t in Early Qenya.
ᴱN. {duilen >>} duilin “a swallow” reappeared in the Early Noldorin Grammar of the 1920s (PE13/120) and duilin “swallow” appeared in Early Noldorin Word-lists from the same period (PE13/142). N. tuilinn “swallow, (lit.) spring-singer” first appeared in The Etymologies of the 1930s, as described above.
tuilindo noun "swallow", etymologically "spring-singer" (TUY, LIN2, LT1:269, LT2:338)