LotR -- Book 3 - Chapter 02 - The Riders of Rohan

[Nilpaurion Felagund]

Over here its shepherds rather than sailors who are either delighted or warned, but it is (or was) a common saying. (davem)

I was about to post the shepherd version of the quote (really! I just forgot), and was thinking if, in fact, it had anything to do with Legolas' words.

Just one word kept coming back to me: shepherds. The Ents, the Shepherds of the Trees, may be the ones being warned (or delighted)--after all, it was the same day when Merry and Pippin blundered into Treebeard. A delight and warning to the old Ent the two have been!

Perhaps Legolas just saw it and interpreted it as if it was for their own, which it wasn't:

As before Legolas was first afoot, if indeed he had ever slept. "Awake! Awake!" he cried. "It is a red dawn. Strange things await us by the eaves of the forest. Good or evil, I do not know; but we are called. Awake!"

LR III 2 - emphasis mine

Treebeard may have read it himself, and that could explain the fact why he was there on the eaves--well, relatively near to it--of his forest.

But that's already two chapters in advance.