Some Runaway to the Stars worldbuilding. The poles of the avian homeplanet both have permanent sea ice fields, which are difficult for life to colonize for the same reason sand dunes are. The surface constantly shifts (although much slower than a dune) as it's shaped by wind, currents, melting, and precipitation.
In the North pole, the dominant ecosystem on the ice is globe fields, which are full of conglomerate balls of moss and frost tolerant worm plants. They very slowly roll across the ice, being pushed uphill by the worms into sunlight as their thermal mass slowly melts whatever ice they sit upon.
All of the moss globes are host to a suite of small cold tolerant invertebrates, but a small fraction are also nests for globe bunnies. These small endotherms hollow out the inside of mossball and pack it with insulation. They feed voraciously on invertebrates and other mossballs, though they will also trim the "lawn" surrounding their house, to keep it at a manageable size and roundness for rolling about.
Polar avians find the globe bunnies tasty. The watercolor painting here features one of the larger coastal cities on the North pole landmass. It is essentially one large interconnected building, in classic polar avian style.