How delightful that this is a separate word.  Perhaps the ravens consider it an office or a duty.  Have you ever had the sensation that a raven-messenger was addressing you, finally turning away when it is not understood?

• 17.035 (for the raven-messengers had been busy

What about an OED entry, I hear you ask.  Yes indeed:

† raven messenger  n. Obs. = corbie messenger n. at corbie n. 2.

a1400  (▸a1325)    Cursor Mundi (Gött.) 1892   Þat messager, men say, Þat duellis lang in his iornay, He may be cald, wid resun clere, An of the rauyns messagere [a1400 Vesp. messagers corbun; a1400 Trin. Cambr. rauenenes messangere].

so we give it the Archaic tag and follow the proffered rabbit-hole…

Oh, my.

2.   corbie messenger  n. one who returns too late, or not at all: in allusion to the raven in Gen. viii. 7.

This is a completely separate meaning from the sentient, sapient creatures who carried timely messages between Thorin and Dain.  I’m giving it the JRRT tag for a brand-new use of a previously employed word.

“corbie, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/41419. Accessed 19 September 2017.

“raven, n.1 and adj.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/158644. Accessed 19 September 2017.