But what does this mean? What did professor Tolkien intend here?But in the wild lands beyond Bree there were mysterious wanderers. The Bree-folk called them Rangers, and knew nothing of their origin. They were taller and darker than the Men of Bree and were believed to have strange powers of sight and hearing, and to understand the languages of beasts and birds.
Obviously in ancient and medieval literature, with which Tolkien was more than familiar, the "Language of the Birds" meant something rather specific, although not as real and practical as I'd like.
From Wikipedia:
On the other hand, there is indeed a knowledge of signs that may be read from birds and other creatures, that a woodsman may use tactically in the wilderness to know many things, including the approach of an enemy force.In mythology, medieval literature and occultism, the language of the birds is postulated as a mystical, perfect divine language, green language, adamic language, Enochian, angelic language or a mythical or magical language used by birds to communicate with the initiated.
For instance, there's a scene in the movie Jeremiah Johnson, where Johnson and Del Gue are around the campfire, when they see a storm of birds erupt from a nearby wood. They take up arms immediately, knowing that strangers are approaching. Fortunately it is a friendly group of Flatheads.
Jon Young's What the Robin Knows: