The Fellowship of the Ring was a strategy put in place when the whole gang was in Rivendell. But why didn’t they use eagles to save all that time and effort and fly the ring straight to the Mountain of Doom? While we never hear of them discussing this as an option, there are some very logical reasons as to why they chose to go on foot above any other way, including making use of a giant eagle.
The mission had to be one of great secrecy, and while using the eagles would have been significantly faster, it would have sacrificed the mission’s secrecy and greatly increased the chances of the Ring being captured. With the safety of the world at stake, Gandalf would most certainly not have wanted to take any unnecessary chances. Travelers on foot, especially as small as Hobbits, would barely be noticeable compared to massive eagles. Traveling on foot allowed for scouting ahead and moving past the enemy virtually in stealth mode. The eagles would have tipped off the enemy that something was going on and they would have been targeted by Orc archers and the Nazgul.
Getting the ring into the caldera
The Ring had to be taken through the Crack of Doom itself. It couldn’t be dropped from above into the caldera by an eagle. The Crack of Doom is located at the end of a tunnel through the mountainside. An eagle would never have fit through this tunnel, so it would have to have had a rider which would further have compromised the mission. A rider would limit the height that an eagle could fly at, as well as having a negative impact on its agility, which would be crucial in a fight against the Nazgul. Even if the eagle did make it to Mordor with its rider, it would definitely be noticed flying towards Mount Doom and would be attacked before it could land and let the rider off. We saw first hand how effective it was for the Hobbits to navigate Mordor unnoticed. After all, Frodo and Sam were actually in Mordor for a couple of weeks before they made it to Mount Doom.
The Eagles wouldn’t have done it
The eagles are well known in the books to be extremely proud creatures, thinking themselves above other species. What little help they are willing to offer is purely done as a favor to Gandalf and always comes with limitations of what they are willing to do and how far they are willing to fly. They probably would not have seen this as their fight and would not have been willing to put themselves in such danger for this cause.
The ring affects all creatures
Let’s say that the Eagles were willing to help. The ring affects all creatures and even the eagles would have been tempted by its power. Even a strong man like Boromir was no match for it. The Ring became stronger the closer it got to Morder, reaching its peak strength at Mount Doom. The willpower of the eagles would surely have been well tested.
The Council of Elrod had no idea what to expect for the long journey and in Mordor itself. Ultimately they had to plan for worst-case scenarios. Even the Fellowship’s decision was a risky endeavor.