The host and the sentry

With the reduction in size of glaciers, the Central Massif has drastically increased the surface of its alpine environments. Cold and strong winds have maintained a forest line similar to the late Lentocene, but summer temperatures can be relatively high, increasing the overall productivity of this unique ecoregion.
Large herds of hogsters can be found grazing alongside a chubby flightless bird, the trenchcrawler (Crassigena letargica), a very hardy rostrid species: it keeps its name by its habit of excavating large tunnels under the snow, which use as a safe shelter from cold and predators. Trenchcrawlers are the only lethargic rostrid, which can hibernate for even 5 months: in order to survive the cold season, the species undergo a state of hyperphagia, eating a moltitude of fruits and seeds, doubling its weight in just 60 days. Like all rostrids, the trenchcrawler has a very efficient digestive system that allows it to largely digest cellulose and even partially lignin from bark and twigs if properly chopped. This allows all the rostrids to survive even by feeding on poor-quality plants, but it brings with it a limit in their movements for some species: to cope with the weight of their long intestines, small rostrids like the trenchcrawler have a graviportal structure and therefore they are not able to run at high speed (few species can exceptionally exceed 20 km/h for short distances). The relative slowness of these rostrids makes them unable to escape from predators, from which they defend themselves with their powerful bite, capable of pulverizing the limbs of a predator.

Trenchcrawlers' dens are often exploited by another bird that is incapable of digging its burrow, due to its gracile structure: the alpine brontosparrow (Alpipes taurus). This omnivorous bird is a thermophilic species that inhabits the alpine tundra during summer, while in the other way cooler months it moves in the austral forests' lowlands.
Trenchcrawlers do not seem to be bothered by the presence of this noisy bird, since they've developed a symbiotic relationship: alpine brontosparrows are in fact excellent sentinels thanks to their tall stature, twice as their rostrid fellows. When these large passerines see a predator approaching the den site, they rapidly alert trenchcrawlers with intense whistles.
Complex cooperations like this one are rare in nature, but they give huge advantages to both species in their race for survival.