Ducktails from two worlds

Mastodon bullducks are the main ecosystem engineers of Polarica: with their trampling and browsing actions, planitial forests are mostly maintained semi-open, favoring the presence of grazing animals. This does not happen in steep environments, where these titanic anatids are not able to move easily, favoring the formation of large thickets. These diffuse refuges are a perfect habitat for the coastal ducktail (Anserisilva folivora), a medium-sized pygostylids that is strictly correlated with a closed canopy.
It's a browser, stripping leaves and mosses from trees that are then ground with their false teeth. Due to habitat fragmentation, coastal ducktails have become sturdy herbivores, with strong legs that help to move from one thicket to another. They are solitary animals that can easily enter into competition with horned rotbills, that cover a similar niche: due to their digestive efficiency, coastal ducktails are usually more successful in their search for food, with lower winter mortality.
When cornered by predators, this very combative bird usually stands its ground and defends itself by pecking the opponent: thanks to its robust muscular joints of the neck, a single hit has the potential to crack a wooden board. Stay at a safe distance if you encounter one...well, if you can see one. Coastal ducktails have in fact counter-shading colors, making it difficult to find them in the dense and dark vegetation.

Life is completely different for another species of ducktail, adapted to live in the most productive habitat of Antarctica, the Scrubring. Here, woodlands don't exist, with the largest tree-shrub being not more than 7 meters high.
In open landscapes, you can't hide from predators, and the Scrubring ducktail (Desertipes gracilis) knows this very well. This bird was forged for high speed, with a thin body and a foot-tibia ratio comparable to emus and ostriches.
They are not good at changing direction while running, but they can easily reach 70 km/h (43 miles/h) and maintain this speed for several minutes, being able to outrun any terrestrial predator of Polarica. In critical situations, this bird counters predators by using kicks, which can be fatal for small predators like geotters.
Scrubring ducktails live in small family groups and are the most abundant megafaunal vertebrate of Antarctica, numbering millions. Summer densities can reach up to 18 ind./km2, meaning a biomass of 1,5 tons every km2.  By counting other large herbivores (Steppe bullducks, headbutt rotbills, etc.) of the Scrubring reach a biomass of 4 tons per km2, half the one from the famous mammoth steppe.