In my world, there exists giant eagles kind of similar to the ones in LOTR. These eagles are large enough to ride and are domesticated.
As a result, people have been using this giant eagles in warfare. First, they have been used as scouts. Later on they were used as transports. One day, somebody got the idea of collecting a huge number of rocks and dropping them from the eagles. This was similar to Lazy Dog Bombs used in WW2. The eagles fly 1 mile up in the sky at a speed of 120 kilometers per hour.
Pertinent Information:
These giant eagles can carry up to 500 kilograms of weight, can fly for up to 6 hours at a full load, are as durable as a warhorse, and are as rare as elephants.
The enemies are standard late medieval Western Europeans fighting in the open plains. Typical enemy forces comprise lightly armored pikemen/archers and heavily armored knights. No other species.
Question:
Would it be feasible for a country with late medieval-era technology to make a lot of kinetic projectiles, such as flechettes, for use as an effective weapon against contemporary militaries?