The avifauna of the Pocket Tundra
Despite the harsh environment of the antarctic Pocket Tundra, some birds were able to call it home. Still, most of these birds are only summer visitors, that migrate from South America (and rarely from South Africa and Oceania) to Antarctica for breeding. South American migration has two main migration routes: sturdy migrants take a direct route by crossing the Antarctic Ocean from North to South (and vice versa). Less powerful (but surely not weak) fliers take a longer but safer route, by migrating through the Scotia Arc, full of islets to rest and feed during the journey. Both routes have pros and cons, but we don't care much: they both work.
Resident bird species include mostly seabirds, like penguins, albatross, shags, and lots of species of shorebirds (Charadriiformes).
The first inland vertebrate of Antarctica
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).
Size comparison between a human hand and an antarctic tyrant.
In appearance, it would look similar to a bullfinch (Pyrrhula pyrrhula) (see image below)
A duck in the snow
Apart from the Antarctic tyrant, most other Antarctic birds are somewhat migratory. The seasonal land bird population includes around a dozen passerine species, two rail species, and notably, one species of anatid: the long-footed pintail (Anas lonigipes).
This large mallard represents the pioneering colonization of Antarctica by its kind, weighing between 1 to 1.8 kilograms (2.2 to 4 pounds) and standing as the largest Antarctic bird during this period. As the name suggests, it possesses longer and sturdier feet compared to other ducks, enabling enhanced mobility on land and facilitating swifter running speeds.
During October-November, these pintails migrate from their presumed wintering areas, likely South America and Oceania, to the northern coasts of Antarctica for breeding. They establish nests in extensive colonies comprising thousands of pairs, a collective strategy that offers effective protection against predators. Despite having webbed feet, the long-footed pintails forage for food in bogs, lakes, and rivers by diving to search for algae or small arthropods. They also feed on terrestrial food, like plants, fruits and the few terrestrial insects present on the continent.
By early March, the first flocks of pintails initiate their journey northward, returning to their wintering grounds. In less than four weeks, all pintail colonies have left Antarctica... except for one: surprisingly, a pintail colony has begun to overwinter on one of the northern Antarctic islands, King George Island. While currently a relatively small colony (comprising no more than 9,000 pairs), it appears to gradually increase in size each year. Over the next million years, this pintail colony could potentially evolve into a year-round Antarctic species.
The future prospects of this duck appear exceptionally promising here in Antarctica.
The antarctic predators
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.
