For the entire Lentocene, Antarctica was a bird-dominated continent. Mammals, exclusively geotters and bats, were widespread yet never as diversified as their feathery counterparts, which managed to fill from the smallest to the largest niche.
Other terrestrial vertebrates were completely unable to reach Antarctica, due to their great distance from other landmasses and sometimes by opposite currents.
Antarctica basically worked as a giant long-lived New Zealand, becoming a bird paradise. But these conditions weren't meant to perdure in the Cambiocene, the epoch of change. What if someone would interfere with this utopic environment? What if someone wanted to play god? And what if that god-playing someone was a white-grey, noisy and bizarre bird? Not just a simple bird, but an extremely advanced and cosmopolitan bird clade: Agricrows.

Since their appearance, agricrows were able to shape the world thanks to their intelligence and their unique culture, which include advanced agriculture and farming.
Native agricrows were completely eradicated from Antarctica during the END and were able to return only after 2 million years, during the ongoing recovery of the antarctic biota. However, these newcomers were not ordinary agricrows: they were larger, leaner...they were different. And they were not alone. 

They are commonly called Aves ex machina (Aves ex-machina) and they are incredibly intelligent agricrows with a partial eusocial behavior, living in social groups called clans. While not being a fully sapient species, the extreme organization of Aves ex machina's society works as a giant brain, like the ants of an anthill.
Every clan is commanded by the oldest members, which have full control over their subordinates. Subordinates are subdivided in several social castes, like human/ant society, which play different roles: some of them are soldiers, others are farmers, others are caregivers...and some of them are breeders. Aside from snails and some species of insects, Aves ex machina have started to breed something new: vertebrates.

The first Aves ex Machina culture that reached Antarctica was the Testu culture, the less advanced yet the most diffuse culture at its time, which bred tortoises for eggs: thanks to the slow movements and low energy requirement, these reptiles were perfect livestock for these birds.
The Testu culture was progressively eradicated by two very aggressive and advanced civilizations: the Lucer, a culture that breeds viviparous lizards, and the Rodit, which breed rodents. While being harder to rise (larger food requirements and easily fleeting), these two vertebrate groups have higher cold tolerance and could be used for food at any life stage.
While Testu culture raised their livestock through extensive grazing, Lucer and Rodit cultures developed in addition intensive farming: livestock is raised inside large half-closed hollowed structures called arenas, which can be accessed by a single entrance in the upper part; arenas are coated with big rocks and lime in order to strengthen the structure and avoid livestock escape. Livestock is fed with food waste of agriculture, meaning that all rodents and lizards raised possess a mainly herbivorous diet.
Unlike their ancestral relatives, domestic livestock has been gradually tamed by Aves ex Machina, becoming more adapted to live in tight spaces but at the same time withstand periodical translocation due to potential increased disturbance by large animals. Since islands usually lack megafauna and predators, it's not strange that Aves ex machina are great sailors, capable of flying even 500 km per day for nearly a week without resting to move from island to island, taking advantage of favorable winds. Livestock, seeds, plants and food are transported inside their mouth, which possesses an extendable gular sac like pelicans.

Thanks to their sailor capabilities, no landmasses could avoid the arrival of this agricrow, not even Antarctica. But, once somebody starts playing god, sooner or later things will get out of hand. Like it or not, Aves ex machina technology is far from being perfect, especially in the early stage of their cultures, and semi-tamed livestock could easily escape and become feral.
Like humans, Aves ex machina have destroyed the natural dispersion dynamics of plants and animals, causing a gradual biotic homogenization. 

The Cambiocene will be the epoch of change not only for Antarctica but for the world itself, because of this white-grey, noisy and bizarre bird...