To hop or not to hop? the answer is in the bird trees

Birds can use different types of gaits to move on the ground: they either
walk, hop, or run. Although velocity can easily explain a preference for
running, it remains unclear what drives a bird species to favour hopping
over walking. As many hopping birds are relatively small and arboreal, we
wanted to test the link between size, arboreality and hopping ability.
First, we carried out ancestral character state reconstructions of size
range, hopping ability and ecology on over 1000 species of birds. We found
that both hopping ability and arboreality were derived and significantly
correlated traits in avian evolution. Second, we tested the influence of
hopping ability on the morphology of the lower appendicular skeleton by
quantifying the shape differences of the pelvis and the three long bones
of the hind limbs in 47 avian species with different habitats and gait
preferences. We used geometric morphometrics on 3D landmarks, digitized on
micro-CT and surface scans of the pelvis, femur, tibiotarsus, and
tarsometatarsus. Locomotion habits significantly influence the
conformation of the pelvis, especially at the origin of hip and knee
muscle extensors. Interestingly, ecology, more than locomotion habits,
significantly changed tarsometatarsus conformation. The morphology of the
distal part of the tarsometatarsus constrains digit orientation, which
leads to a greater ability to perch, an advantageous trait in arboreality.
The results of this work suggest an arboreal origin of hopping and
illuminate the evolution of avian terrestrial locomotion.