An eyas is a very young falcon or hawk, one that hasn't yet learned to fly. The feathers of an eyas look very different from an adult bird's, being very downy and soft.
In falconry, the ancient hobby of raising hawks for sport hunting, it's common practice to remove an eyas from its nest so that it can be hand-raised and become accustomed to humans. The human who trains and feed an eyas becomes familiar enough to the bird that it will eventually fly, hunt, and return to its trainer's gloved arm. The word, via the French niais, comes from the Latin root nidus, "nest."