Groundgeese, or Subterrapterygidae, are the family of burrowing kiwis descended from Subterrapteryx. The former genus enjoyed uncontested success at the dawn of the Arthrocene, as burrowing rats were largely wiped out during the Great Decarbonisation. Today, groundgeese occupy a more precarious position; mammalian predators and competitors have begun to evolve again, forcing the birds to adapt to a less friendly world. In particular, a new genus of carnivorous rodents poses a serious threat. In the past half-million years, over half of groundgoose species have gone extinct. Though the Subterrapterygids still have a wide distribution, ranging across all of Panapterra and beyond, only two genera are still extant.
The Mudslider (Vermavis) is a small genus once found around the globe, but now restricted to a few pockets of fertile tropical forest. The soft, waterlogged soil in these areas is difficult for other burrowers to dig through, as it is prone to collapsing and flooding. Mudsliders, however, are built to survive these dangers. Their bill is long - proportionally, even longer than their kiwi ancestors' - narrow, and downturned. This allows it to be tucked under the body when digging, keeping it out of the way as the webbed, spade-like feet reach forward and pull shovelfuls of dirt from in front of the bird's head. In the event that the tunnel caves in, the mudslider squirms, using its thin beak to cut through the earth until it pokes above the ground. With nostrils at the very end of its snout, it can then breathe as if through a snorkel, then gradually dig itself free.
The most common still-living mudslider is found in the far-western tropics of Ailuropia. This species, the Ventilating Mudslider (V. pneumaticavum), has an even more advanced method of keeping itself safe. In its environment, storms originating from the rough arctic Perithalassic occasionally bring rainfall far more intense than the jungle usually receives. Like its congenerics, it digs just a few centimeters below the surface, giving itself easy access to the air above. Uniquely, it also pokes small breathing holes every meter or so, each only about a centimeter wide and well-hidden among the leaf litter. When the floodwater arrives, the mudslider may find itself swept away as the current rushes into its underground home. By creating these ready-made breathing tubes, it ensures it can still snag a breath regularly as the rapids pull it along.
Scoopbills (Rutrostris) are social tunnelers, being more common in drier savanna and grassland habitats. This allows them to build more long-lasting subterranean chambers, as there is far less risk of destruction by natural forces. Families of up to a dozen can dig networks spanning over a hundred meters in radius and descending to a depth of three meters. The Patchland Popbird (R. vigilus) exemplifies this lifestyle, constructing intricate nests below the warm prairies on central Abeli. It uses its beak in tandem with its feet, excavating faster than mudsliders are able to. Large open spaces serve as nesting rooms, providing a deep, well-protected place for the young to grow up. Thinner tunnels connect different chambers to each other, preventing large predators from invading the lower levels. At each of the 3-5 exit points, one member of the group will constantly stand watch, alternating in shifts of about four hours. As scoopbills are slow runners, they try to stay close as they forage aboveground. When they hear the sentry's alarm call, every family member will retreat, and they will even accept unrelated individuals that strayed far from their own burrows and need a place to shelter. These accidental migrants often join the new group, taking a mate and introducing new genetic variation in the process.