Decadence
The uncertain future of rostrids
After the great radiation of the middle Lentocene, rostrids had become the second major group of megafaunal birds in Antarctica, with a large variety of species adapted to woodland and scrubland habitats.
However, their achievements weren't meant to last in time: rostrids are now nearly disappeared, with only two species remaining in total.
The cause of their disappearance is a mix of factors, from the rapid change of woodland associations to ducktail competition/disturbance, which are rapidly monopolizing Antarctica.
The last megafaunal rostrid is no larger than a cassowary and it's far related to Aurora debarkers, which went extinct after the arrival of giant bullducks to their home range. Only one population was able to survive in the mildly steep environments of the Ross Peninsula, where bullducks were not able to expand. This rostrid population rapidly speciated and is known now as the mohican dungeater (Ramphornis mohicanus), the last proof of rostrids' past success.
They've lived solitarily for millions of years in this primordial forest, mostly composed of dubious beeches and needle willows, where time has decided to stop.
Similar to their ancestors, the mohican dungeater is a facultative lignivore, supplementing its woody diet with buds, fruits, seeds and sometimes even carrion.
Herbs and leaves are rarely eaten due to strong competition with local ducktails, which means that this rostrid is often forced to eat plant litter. They have generally low fitness, living at lower densities compared to past rostrids, a hint of their slow and steady decadence.
Mohican dungeaters' social interactions are rarely physical: aside from the breeding season, each individual communicates with neighbor dungeaters by producing gurgles-like calls that can be heard from kilometers of distance.
The second and last species of rostrid that was able to persist until now is the woodclimber (Arbopes giganticus), a semi-arboreal rostrid derived from mossbrushers.
This species has a wider distribution than mohican dungeaters thanks to their unique body plan that has allowed them to reduce food competition with ducktails to the minimum.
Their third foot digit now faces backward, enabling them to efficiently grasp branches while moving on trees, with the help of wings and beak. Despite this adaptation, this rostrid rarely climbs on branches higher than 4 meters (13 feet). They also spend a lot of time feeding on the ground when ducktails are locally scarce. Their partial arboreal lifestyle has favored a frontal orientation of the orbits, enhancing binocular vision and depth perception.
Woodclimbers possess a proportionally longer intestine compared to other rostrids, subdivided into small segments, each one separated by small valves that further increase lignin and cellulose digestion.
Their digestive efficiency is comparable to a ducktail, but their marginal distribution has not allowed them to reclaim ground again: they are completely absent from lowland, where the browsing action of tower ducktails strongly disturbs this bird during nesting. Steep forests, where large towers are not able to move, are the stronghold of this species, which is now fragmented in at least five subpopulations.
The high specialization of woodclimbers (and rostrids in general) seems to be not enough to stop their pushy and better-adapted counterparts.
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