Age of titans

Bird hips and posture do not favor gigantism. Large birds must undergo various anatomical adaptations to sustain their gigantic body, still mostly limited to no more than half a ton (with few exceptions).  With the variation of posture towards a more efficient graviportal structure, ducktails have lost their primordial size limitation, approaching the size of some large non-avian dinosaurs. Being large is indirectly useful for them in the cold environments of Antarctica, since they can better maintain their body heat and also eat proportionally less to sustain their metabolism. 

A very large species of ducktail is the steppe bullduck (Gigantoanser gracilis), found in the steppe and forest-steppe of Poalrica. It's one of the largest birds ever, reaching the size of Vorombe titan, an extinct gigantic elephant bird from Madagascar. Despite its size, it's a fairly swift herbivore, capable of escaping or charging predators at 30 km/h. They are intermediate feeders, eating both grass and low shrubs found in their range. They live in herds of hundreds of individuals, eating and trampling any type of fresh vegetation they can find.
They are the most migratory ducktails, with up to 500 km long seasonal migrations: during summer, they move to their breeding areas, in the Day-night steppe, an open ecoregion that is significantly maintained thanks to the bullduck presence. Due to its position in the center of the pole, the Day-Night steppe is affected by a single daytime and nighttime every year.
During polar winters, this steppe becomes dark and frosty, forcing steppe bullducks and other organisms to move to northerner locations, like the Scrubring, where sun or twilights are observed every day. Bullducks do not like prolonged dark due to their poor eye vision, becoming highly vulnerable to large predatory herdstalkers.