Most cranes have a reduced hind toe set high on the foot, which suits walking on open ground but gives no grip. The crowned cranes of the genus Balearica kept a long, functional hallux at the same level as the other toes, so the foot can close around a branch.
The consequence is behavioural. Grey crowned cranes roost in trees at night, and both crowned crane species will perch, a sight that surprises observers who know cranes only as birds of open marsh and field. Their genus is also the oldest surviving crane lineage, and the trait is regarded as ancestral rather than a recent innovation, which makes their fossil relatives interesting for reconstructing how cranes diverged.
Nesting, however, is still on the ground, in a platform of trampled vegetation in wet grassland. That is where the species is losing out: wetland drainage for agriculture, plus capture of birds for the live trade, has driven a steep decline, and the species is now assessed as Endangered.