Rostrids
Geese of the forest

Rostrids (Family: Rostridae) comprise half the species diversity of ducktails; still, its members are pretty derived.
They are all folivorous, with few species of grass (monocots) present in their diet.
Despite their size variation (from just 20 kg/44 lbs to over 100 kg/220lbs), all species have in common: 

• the lack of the fourth digit of the foot

• the absence of a chambered gizzard

• strong jaw and neck muscles

• non-atrophied wings

• an exclusively forest-scrubland distribution 

Since they do not have a ruminant-like digestive system, rostrids have developed an enlarged cecum similar to horses and lagomorphs, where they digest cellulose. It's not optimal, but it works. 
Rostrids' bacterial flora is extremely biodiverse, hosting several bacteria/fungi with lignase enzymes, able to digest lignin. It is believed that up to 40% of the rostrid diet during winter is composed of lignin-rich materials (barks, roots and branches).
Due to the difficulty of digesting lignin, rostrids must eat their first feces (coprophagia) like rabbits, increasing nutrient assimilation and contrasting vitamin deficiency (e.g., B12 deficit). Because of this, rostrids are often called dungeaters. (other names are rotbills, debarkers and mossbrushers)

The largest rostrid species is the short-necked dungeater (Xilophagaves brevicollis), a very compact bird that is restrained to the relictual coastal woodlands of Polarica: the only large metapopulation is the one that lives in the D.D., with up to 300.000 individuals estimated. Despite its relictual range, their descendants will probably expand when this short Ice Age stops. At least, if they can survive.
Dungeaters, like other rostrid species, are usually solitary herbivores that eat leaves, mosses and branches all year round. Despite living at low densities, their significant debarking action helps create meadows, favoring the presence of herbs and willow species.
During the mating season, males challenge rivals in a ritual dance, flapping their blue and green wings, a common behavior of all rostrids.

In terms of abundance, the dwarf mossbrusher (Ramphornis minutus) is the most numerous rostrid and also the smaller. Its rounded beak works as a vice, stripping pieces of mosses and rotting wood to eat. Nests are never made on the ground, but at 1-2 meters of height over large and dense bushes or basal branches: they are mediocre climbers, often awkwardly climbing small trees to escape ground predators rather than running. They are still ground birds, with poor adaptations for an arboreal lifestyle.

In terms of distribution, the most widespread rostrid is instead the running rotbill (Brachiorampho rapidus), the only species of its group that can be found in semi-open environments: running rotbills are way more common in shrublands, where their long legs with elongated tarsus help him at fleeing from predators. During autumn, this species is known to eat a large number of fruits to store fat for the winter.