The Eagles were birds that served as messengers of Manwë. Among those were the Great Eagles, immense birds who were sentient and capable of speech, and often helped Men, Elves and Wizards in the quests to defeat evil. They were “devised” by Manwë Súlimo, King of the Valar, and were often called the Eagles of Manwë.
They were sent from Valinor to Middle-earth to keep an eye on the exiled Ñoldor, and on their foe the evil Vala Morgoth.
The Great Eagles were messengers of Manwë, the ruler of the sky and Lord of the Valar, being perhaps “spirits in the shape of hawks and eagles” that brought news from Middle-earth to his halls upon Taniquetil.
At a command of Manwë, for a time the Lord of the Eagles, Thorondor kept his eyries at the top of Thangorodrim, the volcano above Angband itself. While they lived there, Thorondor helped Fingon rescue Maedhros. Many years later, three of the Great Eagles came to the aid of Beren and Lúthien, bearing them away from Thangorodrim after both had drained their strength in the Quest for the Silmaril. Thorondor's folk later removed their eyries to the Crissaegrim, part of the Echoriad about Gondolin. There they were friends of Turgon, keeping spies off the mountains, bringing him news and keeping spies off the borders. Because of their guardianship, the Orcs were unable to approach either the nearby mountains, or the important ford of Brithiach to the south; their watch had been redoubled after the coming of Tuor, enabling Gondolin to remain undiscovered the longest of all Elven realms. When the city fell at last, the eagles of Thorondor protected the fugitives, driving away the orcs that ambushed them at Cirith Thoronath, the Eagles' Cleft north of Gondolin.
Thorondor wounded Morgoth in the face after Morgoth's battle with Fingolfin, and he carried Fingolfin's body to the Echoriath, where he was buried by Fingon.
The Eagles fought alongside the army of the Valar, Elves and Edain during the War of Wrath at the end of the First Age. After the appearance of winged dragons, all the great birds gathered under Thorondor to Eärendil, and destroyed the majority of the dragons.
In the Second Age, a pair of Eagles had an eyrie in the King's House in Armenelos, the capital of Númenor until the time of Tar-Ancalimon, when the Kings of Númenor became hostile to the Valar.
The Númenóreans believed that three eagles, “the Witnesses of Manwë”, were sent by Manwë to guard the summit of Meneltarma; these appeared whenever one approached the hallow and staying in the sky during the Three Prayers.
Many eagles lived upon the hills around Sorontil in the north of the island.
Eagle-shaped storm clouds, called the “Eagles of the Lords of the West”, were sent by Manwë when he tried to reason or threaten them.
- Farewell! wherever you fare, till your eyries receive you at the journey's end!
- May the wind under your wings bear you where the sun sails and the moon walks.
― Polite way to exchange good-bye with an Eagle
By the end of the Third Age, a colony under the Great Eagle lived in the northern parts of the Misty Mountains who mostly nested upon the eastward slopes not far from the High Pass leading from Rivendell, and thus in the direct vicinity of the Goblin-town beneath; they often afflicted the goblins and disrupted their plans.
These Eagles helped the Elves of Rivendell and Radagast in watching the land and in gathering news about the Orcs. As a result of feeding on the sheep of the local Woodmen of Mirkwood, their relationship was not good and the Eagles were afraid of their bows.
Those rescued Thorin's company from a band of Goblins and Wargs and carried them to the Carrock and some days later they espied the mustering of goblins all over the Mountains, to be gathered under the Great Eagle in the Battle of Five Armies near Erebor. It was only with their help that the Dwarves, Men and Elves managed to defeat the goblins. The Great Eagle became King of All Birds.
The Eagles appeared in great numbers at the Battle of the Morannon, helping the Host of the West against the Nazgûl. Several of them rescued Frodo Baggins and Samwise Gamgee from Mount Doom after the One Ring had been destroyed.
In Gnomish, one of Tolkien's early conceptions of an Elven language, a word for “eagle” is ioroth (poetic form ior). A cognate of the same meaning in Qenya is the poetic ea(r) or earen. Another Gnomish word for “an eagle” is thorn.
In the later languages, the Quenya word for eagle is soron and in Noldorin/Sindarin thoron/thorn
The Thornhoth was the name for the eagle-folk in the earliest legends.
Tolkien's painting of an eagle on a crag appears in some editions of The Hobbit. According to Christopher Tolkien, the author based this picture on a painting by Archibald Thorburn of an immature Golden Eagle, which Christopher found for him in The Birds of the British Isles by T.A. Coward. However, Tolkien's use of this model does not necessarily mean that his birds were ordinary Golden Eagles.
Other versions of the Legendarium
In the earliest version of the fall of Gondolin, the king of the eagles, Thorndor (later Thorondor), had no love for Melko (later Melkor) because he had caught many eagles and tortured them for the magic words that would enable him to fly (in order to challenge Manwë for command of the air). When the eagles refused to reveal the magic words Melko cut off their wings in order to fashion a pair for himself, “but it availed not”.
For some time Tolkien considered the Eagles as bird-shaped Maiar, as he felt it unlikely Ilúvatar would grant fëar to animals. However, the notion of a “Maia” like Thorondor having descendants contradicted later concepts and Tolkien later decided that the Great Eagles (like Roäc and the Thrush of The Hobbit) were common animals that had been “taught language by the Valar, and raised to a higher level — but without fëar.” In later texts, eagles were first envisioned by Manwë during the Music of the Ainur, and appeared shortly before the awakening of the Elves. Their origin is thus similar to that of ents.
Flying the Ring to Mount Doom
The Eagles are a dangerous 'machine'. I have used them sparingly, and that is the absolute limit of their credibility or usefulness.
― Letter 210, J.R.R. Tolkien
Many skeptical readers have wondered why the Eagles simply didn't carry Frodo and the One Ring into Mordor and drop the Ring in Mount Doom, or at least aid the Fellowship at some part of the journey, such as helping them avoiding the Redhorn Gate and Moria.
At first glance this seems incredibly easy compared to what actually happened (and it would have made a boring book).
The party of Tolkienists that accepts this as a plot hole usually respond that in any book there are usually plot holes. In a larger, far more detailed and realistic book we expect fewer (if any) plot holes, when in reality there is a far greater chance.
Considering the Eagles
Strangely, the possibility of using the Eagles has not been mentioned at all during the Council of Elrond. Although many flawed proposals are made during it (destroy the Ring, guard it, send it to the West, give it to Tom Bombadil), none of the participants thought to propose this seemingly obvious solution, especially after Gandalf described his escape with Gwaihir; even if the Eagle plan was to be countered or dismissed implausible later for some reason (like the ones above), it would be only logical to be mentioned.
On the other hand, the Council was seen deciding the fate of the Ring, not the manner; this was left to the discretion of the Fellowship. Indeed, during its existence, the Fellowship had not even decided whether they should go directly to Mordor or to seek aid from Gondor, let alone the manner to do so, before decisively been broken at Amon Hen. If Gandalf ever considered requesting the help of the Eagles after some point (eg. after passing the Misty Mountains) it's not mentioned in the narrative.