Stormbird

It was a storm of sparking feathers and shining talons crashing in amongst the soldiers as they pushed and struggled. It did not share the same struggle, it simply killed.
Each wingbeat scattered squadrons, leaving dozens as perfect targets for the creatures hunger. It set up both my comrades and my enemies withe equal enthusiasm.
Have you ever seen a hawk eat a rabbit? 
They make eating easier by pinning their prey to the ground with their talons, gripping with their beak, and pulling. The chunks are easier to swallow.
A uniform provides little protection against something such as that. 

-PFC J. S. Barnum, After the battle of the Fume Plains

Stormbirts are a uniquely terrifying prospect for travelers as there is little one can do to truly defend against them.
To repel them would require a contingent of soldiers too large to make the trip profitable, and a Daemoncaller is almost always out of the question for the same reason. In reality, it's simply a case of either running into one or not.
If you do happen across a stormbird, then your best option is to quickly-as-you-can find a spot where that creature can't follow you. A fortress, or cave with a sufficiently restrictive mouth. Attempting to hide in any place but one immune to the bird's talons and beaks will likely just make the bird frustrated as it pries you out of your chosen tomb. 
Admittedly, all of this implies that the Stormbird makes its presence in the skies above known before it is on top of you. Nor have I covered the attribute that gives the animal its name. 
While smaller than its ancestor, the Thunderbird, the Stormbirds name is as literal as it can be. Its feathers spark with tongues of lightning between pinion and flight feathers, and ready-made tempest carried on the massive bird's back. The species doesn't seem to have true control over said charged feathers, but it still increases the danger that the bird possesses. A strike of lightning is just as deadly to you and I, aimed or not. 

Thankfully, like other massive bird species, the Stormbird doesn't range terribly far from its nesting grounds unless it has to and, like the Thunderbird, it usually sticks to mountain ranges where it can nest and hunt with a good deal of privacy. 

There is really nothing that the Stormbird will not attempt to hunt. Its size, speed of flight, and lightning make it a formidable opponent for even larger species of hawks or the land-dwelling leviathans that have been appearing as of late. 

Additional Information

Average Intelligence

There is some evidence of Stormbirds, as well as other massive bird species, being able to be trained as war mounts and other such tasks, as well as their most famous trait of being able to mimic the voices of people and a bevy of other sounds. This ability is not so good as to deceive even the average human, but the conversation it is able to hold is seemingly intelligent. Akin to that of a parrot.

Scientific Name

Rex Tempestas

Origin/Ancestry

Thunderbird

Lifespan

Oldest measured Stormbird was 976 years old

Average Height

Seven meters in height.