Native birds and new invaders
Passerines, as always, are globally (and "antarctically") the most widespread and biodiverse group of birds. Hundreds of species are known to live in Polarica, even if more than half are only summer breeders. Recently, a new conspicuous invasion of birds has interested the icy continent, with significant consequences on the native species.
Perching birds of the undergrowth
Different species of birds have slowly become more terrestrial in recent millions of years, not due to the absence of predators (that are in contrast very diffuse) but due to the lack of any competition. The Weddell groundfinch (Caudopasser settentrinalis), endemic to Weddell island, is one of the various perching birds that have adapted to live on the ground, foraging for seed and insects in the undergrowth. They've reduced wings, preferring to run than fly from predators. They still possess a very long tail, used as a sexual display during the breeding season. They usually live in small groups of 5-10 individuals, that nest together inside thickets and sometimes in abandoned ground tyrants' dens. They are very prolific, capable of quadruplicating their population during the warm season. To survive winter, groundfinches create larger groups and often cooperate in searching for seeds and other plant matters under the snow.
Even Berkner island has its endemic ground passerine, an emberizid: the Berkner bunting (Geoemberiza arborea). They are bad fliers too, with strong feet for a semiarboreal lifestyle: in fact, this large bunting is capable of searching for food even on the top of the austral forest canopy. During the good season, some populations are known to move seasonally in the tundra habitat of the only two "mountains" of the island, the Thyssenhöhe and the Reinwarthhöhe domes. Unlike the ayayema, Berkner buntings are very easy to spot, especially thanks to their melodic calls during the long antarctic twilights that characterized this island.
Clash of old and new clades
In the course of the last millions of years, other bird groups were able to colonize Antarctica.
The barbed neckpigeon (Collicolumbi spermophagus) is the first-ever columbid of Antarctica, migrated from South America not many centuries ago. From few northern populations, this very adaptable species was able to rapidly expand across the entire austral forest of Antarctica. Birds can be extraordinary fast at colonizing new territories, thanks to flight. Barbed pigeons are mostly granivorous birds with unusually long necks with filiform feathers, mostly used for sexual display. Aside from that, their body plan is still pretty close to their most probable ancestor, the urban pigeon.
Another important addition to antarctic biodiversity is the sowering raven (Picorvus variabilis), a nearly cosmopolitan corvid with many ecotypes, all adapted to different types of habitat.
They are relatively clever birds that have adopted a very specialized food gathering, a sort of rough agriculture. This greedy and mostly herbivorous bird takes with him the most delicious species of fruity plants (mostly shrubs and herbs), sow them together and then waits for their growth. After the seeds sprout, plants are not fed with water but only weeded. Despite this action can create the local creation of monocultures, many migrant sowering ravens were the source of the arrival of many new plants in Antarctica, increasing the overall biodiversity of the continent.
Some native species have suffered from the last invasions, like the barkpeckers, which were greatly impacted by the arrival of woodpeckers (Picidae), that are way more adapted to debarking trees in search of food. The last known member, the seed barkpecker (Dendripasser acutis) was able to survive only by shifting their diet towards nuts and snails, which are crashed by smashing them with rocks.
On the other hand, various species were luckier: the tickpit (Micropteris commensalis) for example is so specialized that no bird could potentially compete with him. They are very small birds that live most of their lives on the back of large terrestrial anatids, usually bullducks, eating their parasites. Nests are made in ground holes or tree cavities depending on the place, but always near ducktails herds. During winter, tickpits are known to rapidly slow their metabolism and become mesothermic, to reduce energy dispersion: body heat is maintained by living inside bullducks plumage as a commensal. To physically tolerate the host movements, tickpits have evolved relatively strong bones and also very strong feet to better grasp bullduck's feathers. A very unique symbiosis has slowly started between these two birds.