Fern Birds got their name from their fern-like feathers.
Fern Birds are tall and thin birds resembling cranes or flamingos, seemingly around six feet tall. Their primary color is green and their heads have green and white feathers resembling that of a Victoria crowned pigeon's or a crowned crane's, while their back and tail feathers are long, akin to a peacock's, which resemble a plant frond.
In concept art, soldiers were to observe the Fern Birds foraging. It can be presumed that they would have played minute roles in the film proper.
Fern Birds can use their plant-like feathers to blend in with their surroundings.