The herdstalkers' hegemony

Herdstalkers (Family: Brontochionidae) are the most fierce carnivores that Antarctica has ever seen in the entire Lentocenic epoch. Despite the arrival of geotters, large niches are still monopolized by these predators. Their size is very variable, being as small as a goose or as big as a Kelenken.
The largest species, the strikestalker (Chionititan horribilis), has become a bullduck specialist: thanks to its strong legs and neck muscles, this voracious predator charges its prey, often causing fatal internal damages. The large beak has become a useful tool to flesh out the skin of large ducktails, making them unable to escape. If the targeted bullduck does not die after being brutalized by the shock, the strikestalker pursues it and continues to injure vital organs, until the herbivore finally collapses. There are no gentle manners when talking about surviving. The species distribution coincides with bullducks' range, meaning they can be found across the open-woodlands of Belgrano and the entire cold savannah of the Scrubring.

A more elusive yet giant species of herdstalker can be found in more closed environments: the martial herdstalkers (Fulminavis raptor). Unlike all herdstalkers, it's not a good runner, with very low tarsus.  These birds are in fact ambush predators of medium-sized vertebrates (like ground tyrants and rails), that are rapidly killed with strong kicks, in a similar way to secretary birds. Young anatids are sometimes killed, especially during the reproductive period. Like the strikestalker, the martial herdstalker is a solitary predator, becoming gregarious only during the reproductive season.

The most widespread species is the shadowstalker (Teravis socialis), the first pack-hunting herdstalker ever. They live in social groups of up to 12 individuals and they can be found practically everywhere in Polarica. They are formidable hunters, capable of bringing down prey several times their size.
Their main strategy to hunt large ducktails is to scare the herd, causing a stampede; in this way, panicked individuals could potentially fall and be stomped to death by other conspecifics. Youngs and elders are surgically separated from the herd on the run, becoming easy meals for these tenacious birds. When large preys are not available, shadowstalkers find no problem moving their attention towards smaller species, like rostrids and rails. Their phenotypic plasticity is so great that some coastal populations have adapted to catch fish and eat sea waste, while tundra populations consume large amounts of carrion, invertebrates and fruits. We are facing the wolf of the bird world.

Shadowstalkers live in a matriarchal society. The reproductive season starts very early, on October-September, when freezing nights become infrequent: the dominant female with the help of some juveniles creates a large nest, where she lay 3-6 eggs. While half of the pack is in charge to bring food to the nest, the other half must protect the female from predators. After one month, young and relatively immature shadowstalkers hatch from the eggs; after 2 weeks, nestlings are capable of walking and following the herd, despite becoming completely independent from their mother at 3 months. When a pack becomes too large, adults will expel juveniles, which are going to establish their pack.
Shadowstalkers have a very complex language: over 120 types of vocalizations are known, each one used for specific events.  High-frequency calls are used to interact during hunting since ducktails are not able to hear ultrasounds, while in other conditions they speak with low-frequency calls.