From native Celtic sources comes confirmation that bird augury was widely used. An Irish version of the Historia Brittonum, by the Welsh historian Nennius, includes an ancient poem which refers to six Druids who lived at Breagh-magh and who practised 'the watching of birds.' ... The name of the wren was given in Cormac's Glossary as drui-en -- the bird of the Druids. Certainly an Irish name for the wren was drean, and a Life of St Moling confirms the etymology of the Glossary. The wren has come down to us as a bird of some significance and on St Stephen's Day (26 December) in Ireland, Scotland and the Isle of Man, and even in parts of Essex and Devon, it was hunted and killed by local boys before being carried in procession[.]This is a subject that has been of interest to me for a long time. In the old Norse poem Rígsþula, a mortal grandson of Heimdall learns the tongue:
43. Soon grew up | the sons of Jarl,
Beasts they tamed, | and bucklers rounded,
Shafts they fashioned, | and spears they shook.
44. But Kon the Young | learned runes to use,
Runes everlasting, | the runes of life;
Soon could he well | the warriors shield,
Dull the swordblade, | and still the seas.
45. Bird-chatter learned he, | flames could he lessen.,
Minds could quiet, | and sorrows calm;
. . . . . . . . . .
The might and strength | of twice four men.
46. With Rig-Jarl soon | the runes he shared,
More crafty he was, | and greater his wisdom;
The right he sought, | and soon he won it,
Rig to be called, | and runes to know.
47. Young Kon rode forth | through forest and grove,
Shafts let loose, | and birds he lured;
There spake a crow | on a bough that sat:
"Why lurest thou, Kon, | the birds to come?
48. " 'Twere better forth | on thy steed to fare,
. . . . . | and the host to slay.
49. "The halls of Dan | and Danp are noble,
Greater their wealth | than thou bast gained;
Good are they | at guiding the keel,
Trying of weapons, | and giving of wounds.
Hilda Ellis Davidson describes several more examples of Celtic and Norse mythic figures for whom learning to speak the language of birds is a part of the initiation into wisdom that allows for heroic success. (Pages 86-7.)
Understanding the speech of birds could give a hero entry into the world of ravens and valkyries, where defeat and victory were ordained, or in more everyday terms it could mean an ability to interpret calls and movements of birds and thereby receive warning of future events. Such aspects of bird lore are referred to in the Edda poems and in the ninth century Hrafnsmal [i.e., "The Tale of the Raven" -- Grim] the stanzas form a dialogue between a raven and a valkyrie. She is said to account herself wise because she understood the language of birds, and is herself described as 'the white-throated one with bright eyes,' which suggests that she herself was in bird form. Goddesses, as well as Odin himself, travel in the form of birds, and the same is true of the battle-goddesses of Ireland. One bears the name of Badb (Crow), while the Morrigan, an ominous figure who encounters Cu Chulainn in various shapes, is called Battle Crow (an badb catha). Cu Chulainn once sees her as a crow on a bramble bush and takes this as an ill omen: 'A dangerous enchanted woman you are!'...
A note in a Middle Irish manuscript in Trinity College, Dublin, lists the various cries of the raven which indicate that visitors are approaching, and attention is paid to the number of calls, the position of the bird, and the direction from which the calls come. Young warriors must have been trained in such skills... Two birds on a tree warn Sigurd against the wicked smith[.]
Thus it appears there are two levels of skill being described here. One is what you might call a 'Louis L'amour' skill, because it is the kind of awareness of nature that he often uses to endow his own heroes with special success in battle. This level is the skill of understanding the birds well enough to know what it means that they call when they do, and how they do; to know that noisy birds fall silent when something fearful or strange approaches, say, or that a flock of birds starting from a ridgeline may mean that there is a predator or a man approaching from that quarter. This is the kind of knowledge the Middle Irish note contains. This aspect represents a skill that probably was ordinarily taught to the sons of the fighting classes, as Davidson notes, and it's a skill that you can teach to your own sons if you take the trouble to learn it.
The other level is a genuinely mystical ability to speak with the birds and engage their reason in conversation. This is assumed to be a capacity available to gods and valkyrie (who very likely were goddesses themselves in the proto-myth, as the Morrigan is in the Irish myth). I find this aspect to be interesting chiefly because it assumes that the order of reason extends to crows and ravens, and wrens, at least.
That aligns with my own investigations into philosophy; if it fact it proves to be true, it ought to expand our view of how broadly consciousness is spread within the universe. We share a lot of genetic similarities with birds, but they are quite significantly different from us as well. In order for there to be a common language, even in theory, we would have to be able to work out the rules of each others' games: and success at that means that we participate in the same order of reason, even if we have different levels of access to it. We can teach birds to play some of our games, as for example in training a parrot to speak. How much does it understand? I don't know, but my father tells the story of a parrot who lived with an old woman he once went to visit. It watched him for a while, and then said: "Goodbye!" After a moment, it said again, "Goodbye!" A third time it said, "Goodbye!" After a moment more, it turned to the old woman and said, "He won't go."