There are two species of eagle native to the Blessed Isles. Only one of these has also been observed in the Otherworld, but it is unknown why the other species has not crossed over. There is a possibility that the famously keen eye-sight of these birds can observe something about the boundaries that human eyes cannot and therefore avoid flying over.
The Otherworld-naturalised species is the White-Tailed Eagle (Haliaeetus albicilla). It can be distinguished from buzzards by size, the broad wings, the heavier head, and the white feathers on the tail. They sometimes have pale or white feathering on their heads. They can be distinguished from griffins by their lack of feline features or body parts. Despite conservation issues in our reality due to their predatory nature, the White-Tailed Eagle has remained at pre-Industrial proportions across the boundary. Their spirit form is generally described as a small, slender man much like the folkloric brownie, hob or hobgoblin but with a feathery cloak. They seldom speak to humans but can be helpful if given food, particularly dairy products. Do not offer them eggs.
The Otherworld-avoiding species is the Golden Eagle (Aquila chrysaetos), which, although the basis of much classically-influenced folklore and tradition, is the origin of a Scottish gaelic proverb that translates roughly as:
Don’t swear to fly an eagle in the Otherworld.
The basic gist of which is to say: don’t undertake a task without the proper tools.