The passerine radiation
Passerines are still the most biodiverse group of vertebrates of Antarctica, comprising over 100 species, both resident and migratory. They occupy small niches, even if some isolated clades can reach a relatively large size. Thanks to the arrival of the first antarctic forests, passerines of any family have radiated in a multitude of species, often with a specialized dietary habit.
Passerines of any kind
The most biodiverse group of birds in Antarctica is the finch family (Fringillidae). When the first South American finches started to colonize Antarctica, they rapidly underwent a process of niche partitioning. Every species was adapted to different types of seeds or fruits, reducing interspecific competition. Seedsplitters (Durostra sp.) are a clear example of this evolutionary process: all species inside this genus are granivores, but they are all specialized to crack and eat different types of fruits. Larger species are efficient at extracting seeds from conifers, oak and beech, while smaller species typically eat fleshy fruits of flowering plants.
Finch radiation has also produced a bizarre species that convergently evolved some adaptive traits analogous to woodpeckers: the barkpecker (Passeripicus oreivatis). This insectivorous finch searches its prey (bark beetles and other insect larvae) on dead trees by moving away rotten barks with its long and robust beak: prey are then picked up by using its sticky tongue.
Another ecologically interesting species is the wavepipit (Populornis malus-felix), a miniaturized pipit that does not exceed 12 grams (0,4 ounces). Their wings are short and round compared to their ancestors, meaning a reduction of flight speed in favor of better maneuverability, a typical adaptation of forest-dwelling birds. Curiously, the wavepipit does not necessarily live in woodlands, but it is linked to the presence of large anatids. Wavepipits' typical diet is based on anatid ectoparasites. Their abundance is extremely fluctuating depending on the abundance of their prey; a large population boom occurs periodically, followed by a significant die-out in the next years.
A new aberrant family of tyrants... and a conservative pipit
After 18 million years, the Antarctic tyrant lineage is still present and well widespread on Antarctica, with some clades that have become extremely terrestrial and pretty large for a regular passerine. The most successful group is the Territyrannidae family, commonly called ground tyrants. They are all characterized by robust legs that are used to dig their den or search for roots and insects underground. Like their most recent ancestor (the false ptarmigan), their diet is primarily composed of plant matter and only secondarily by invertebrates (from 30% to 5%). Thanks to their (mediocre) flying abilities, ground tyrants can be found everywhere in Antarctica, from Weddell island's forests to the most inaccessible tundra of Polarica. Some species have adapted to make their den under the ice!
Forest-dwellers like the antarctic sparrowquail (Criocoturnix ronnii) have elongated bodies and gallinaceous beaks, while tundra-dwellers like the burrowing tyrant (Rambornix cuniculus) are more compact for optimizing their metabolic rate; due to their more herbivorous diet, burrowing tyrants also possess large and rounded beaks.
While these tyrant clade experienced large radiation, other passerine lineages have remained more conservative, like the descendants of the diving pipit: the kingdipper (Neolimnanthus brodensis) is anatomically identical to its ancestor, excluding its larger size and its partially webbed feet. Its diet is still composed of aquatic arthropods, but it also contains small fish and, rarely, aquatic birds' eggs.
