The armed rails of Polarica

Shortly after the mid-Lentocene, two separate groups of rails developed two unique structures to defend themselves from predators: 

• hedgebacks (clade: Spinoavialidae) who transformed some of their feathers into dangerous quills that can be used for both offensive and defensive purposes; 

• armadrails (clade: Scutiornitidae) which modified their plumage in an unassailable keratinous armor;

The two groups can be still found now, in the late Lentocene, but they were not able to diversify much since their appearance, because of the intense competition with ducktails and ground tyrants.
Armadrails possess a limited distribution, being found only in the warmest areas of Polarica: the most interesting species is the sloth armadrail (Armadillornis subtemperatus), a swan-sized bird that inhabits the Amery Bay.
They are the only known bird to have lost their endothermy, in favor of mesothermy: Their metabolism is lower than any known bird but higher than a reptile, maintaining an optimal body temperature between 30°C and 35°C (86-95°F).
While this adaptation is extremely useful to reduce food intake, sloth armadrails are forced to hibernate during the cold season, in order to not lose their limited body heat. They do not possess a complex stomach, so these armadrails must eat three times more than a ducktail to acquire the same energy from food. They can double their weight in autumn by feeding on highly energetic food, like fruits, seeds and eggs.

Storing fat for the winter is not a problem for another armed rail, the crown hedgeback (Hystricopteryx tridactylus) which is active all year, eating invertebrates during the warm season and carrion during the cold one.
Their long snout is perfect to feed on wormflies and earthworms, which are caught by using their sticky tongue. They possess a wider distribution compared to armadrails, being found across any forested habitat of Polarica and in some coastal islands of the continent.
Aside from their spiky back, male crown hedgebacks possess a long quilled tail, which has evolved as a sexual display but can be also helpful when facing large predators; it's not rare to see geotters stabbed to death by this small yet aggressive bird. You don't mess with an hedgeback, never.
Hedgebacks are tridactyl with prehensile feet, enabling them to roost on low branches, where they raise their young. They surely can't be defined as an arboreal species, but they typically nest on trees because of sheardogs, which can easily destroy an entire clutch in a few minutes.  
The first month of life is challenging for this species: hatchlings are born immature and they must remain in the nest for even two weeks before being able to follow their parents, developing their typical spiky feathers only after 30 days. However, the few young that will be able to survive after this period will become practically invulnerable to any predator.