Sheathbills start to diversify

With the diversification of the antarctic biota, top predators like sheathbills have started to diversify too. They are not just a large version of their ancestors anymore. Now, at least three different clades are known in the entire continent, two of them being completely flightless.

Terror birds of the ice: the herdstalkers

On the mainland (Polarica) live the most fearful sheathbills of Antarctica, comprising one genus called Teraves. They are medium-large-sized predators, with strong legs used for running and large beaks used to hit to death their prey. Wings are small and they are used only to balance when moving at high speed. Since they are known to follow herds of large anatids in search of weak individuals, these sheathbills are called herdstalkers (Megachionidae).

The largest species is the common herdstalker (Teraves raptorius), comparable in size and ecology to Andalgalornis, a species of phorusrhacid. As the name suggests, they are generalistic land predators that can be found everywhere, from forested to open habitats, despite avoiding extremely closed canopies. Their regular preys are young angucks and razorbills, while adults are sometimes taken down during the winter season, when deep snow and low energetic plant food increase the number of weak individuals. Their beak is big and sharp but does not possess a strong bite force: prey is killed by pecking them on vital areas or by tearing pieces of skin, using the beak as a knife.

In the most inhospitable areas of Polarica,'s tundra, the common herdstalker coexists with the tundra herdstalker (Teraves alpinus), a smaller but still big sheathbill. Interspecific competition is low, since the tundra herdstalker hunts smaller prey like large passerines and rails. Large anatids are consumed only by scavenging or by eating yearlings.

Insular predators and forest fliers

While herdstalkers dominate Polarica, another group of sheathbills that convergently become flightless has evolved on Ellsworth island's forests and then dispersed towards Weddel island: we are talking about the stiltpecker (Grigora sylvestris).
Unlike herdstalkers, stiltpeckers are proportionally gracile, since their different ecology: they hunt smaller prey than their mainland cousins, with a large part of their diet comprising insects, snails, and small-medium sized birds. The most bizarre adaptations of this bird are its long legs, which are used to pursue and jump on unsuspicious prey or to flee from large predatory shorebirds; despite being the largest land predators of Weddel and Ellsworth, they are surely not at the top of the trophic chain of these islands.

Above the heads of herdstalkers and stiltpeckers, a less derived lineage of sheathbills have become fully adapted to live in forests habitat: the pinebill (Arboravis transantarcitus). This sheathbill has become an obligated omnivore, eating a large number of insects, eggs, small vertebrates, leaves, fruits and seeds. Its bulky beak is capable of crushing the hard cones of antarctic conifers, like the bamboo fir.
Despite being a forest dweller, nests are rarely made on trees but are more often made on cliffs or inselbergs across their habitat. On small coastal islands, where nesting competition and predator pressure are low, nests are even made on the ground.

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