Amongst the bird species that typically migrate north as the warm season ends or feed along the coast on sea wastes, one stands apart as a year-round resident in Antarctica: the Antarctic tyrant, also known as the Ex-flycatcher (Coloramphus solum). It is by far the only inland and resident vertebrate of Antarctica.

Despite its finch-like appearance, the Antarctic tyrant is, in fact, a member of the tyrant flycatcher family (Tyrannidae), as its name suggests. Its most probable ancestor is believed to be the Patagonian tyrant (Coloramphus parvirostris), a species of tyrant that once thrived in South America and successfully colonized Antarctica. Initially, their presence was seasonal due to the scarcity of their primary food source—insects. However, over generations, these birds began to adapt to a herbivorous diet, marking the first instance of herbivory among Tyrant flycatchers. This rapid evolution was largely facilitated by the geographic isolation of Antarctica, preventing finch species from easily reaching the icy continent.

The Antarctic tyrant is notably one of the largest and most robust flycatchers ever, weighing between 50 to 100 grams (2.1 to 3.5 ounces) depending on the season. As previously mentioned, it's an herbivorous species that predominantly feeds on seeds, fruits, and buds. It utilizes its short, sturdy beak to crush and cut plant matter, which is then digested in its cecum. During summer, its plant-based diet is supplemented with a small percentage of insect larvae (less than 20%). In winter, its diet comprises highly energetic plant foods like fallen Antarctic blueberries or seeds from the ice willow.

During autumn, the Antarctic tyrant enters a hyperphagia phase, consuming large amounts of food and storing fat. In just a month, it can double its weight, a crucial adaptation to survive Antarctica's harsh winter. Its moderately large size also indirectly aids in conserving body heat.

The bird's social behavior changes seasonally: in the warmer months, the Antarctic tyrant lives in breeding pairs, nesting underground by excavating holes in soft soil using their strong feet. Due to the absence of trees, nesting underground provides a safer environment to protect nestlings from predators and, notably, the fierce Antarctic winds. These underground nests typically do not exceed half a meter in length. As winter approaches, the Antarctic tyrants become more sociable, forming large flocks of 100 individuals or more, foraging for food. It's common to see them roosting together in rock shelters, providing warmth to one another.

The Antarctic tyrants rank as the most abundant vertebrate in Antarctica, surpassing the biomass of any duck and penguin species, owing to the lack of competition with other passerine species. During the summer season, their density is estimated to reach up to 70 individuals per square kilometer (27 per square mile).

Despite the scarcity of prey, two birds have established themselves as the top predators in Antarctica: the duck-eating skua and the smoky sheathbill.

The solitary duck-eating skua (Macronectes minor) stands as the most formidable predator in the Pocket Tundra. This medium-sized skua, lacking webbed feet, has assumed the role of a bird of prey, covering up to 30 kilometers (18 miles) in a day in search of food. Equipped with large and robust talons ideal for capturing prey, this skua primarily targets long-footed pintails during the summer months but also preys upon other unsuspecting birds. In winter, it undergoes a short migration to the coastal regions of the continent. During this season, seabirds become their primary prey, although weakened Antarctic tyrants may also become a target for this species.

Conversely, the smoky sheathbill (Chionis robustus) isn't a proficient predator. Despite being an adept walker, it lacks the strength and speed necessary for pursuing Antarctic birds. This species, living in small groups, adopts a generalist approach to its diet, consuming a wide range of food items—from eggs, carrion, and bird nestlings to fruits, insects, seeds, and even feces. Essentially a larger version of the snowy sheathbill, it relies on varied food sources, as all true generalist species.
Similar to the duck-eating skua, the smoky sheathbill migrates toward the coast as winter approaches, driven by insufficient food availability in inland areas. While its beak morphology resembles that of its ancestors, its neck muscles are relatively stronger, at least 10% larger than those of a snow sheathbill, enhancing its pecking force.

The feathers of the smoky sheathbill undergo seasonal molting: during summer, the coat is characterized by grey spots, which fade away in winter, rendering the bird entirely white in appearance.