Feather evolution following flight loss in crown group birds: relaxed selection and developmental constraints

Feathers are complex structures exhibiting structural/functional disparity
across species and plumage. Flight was lost in >30 extant lineages
from ~79.58 Ma–15 Ka. Effects of flight loss on senses, neuroanatomy, and
skeletomusculature are known. To study how flightlessness affects
feathers, we measured 11 feather metrics across the plumage of 30
flightless taxa and their phylogenetically closest volant taxa, with
broader sampling of primaries across all orders of crown birds. Our sample
includes 27 independent flight losses, representing nearly half of extant
flight­less species. Feather asymmetry measured by barb angle differences
between trailing and leading vanes decreases in flightless lineages, most
prominently in flight feathers and weakest in contour feathers. Greatest
changes in feather anatomy occur in older flightless lineages (penguins,
ratites). Comparative methods show that many microscopic feather traits
are not dramatically modified after flightlessness compared to body mass
increase and relative wing and tail fan reduction. Changes involved with
greater vane symmetry show stronger shifts, however. Relaxing selection
for flight does not rapidly modify feather flight adaptations, apart from
asymmetry. Developmental constraints and relaxed selection for novel
feather morphologies may explain some observed changes. Macroscopic
changes to flight apparati (skeletomusculature, airfoil size) are more
evident in recently flightless taxa and could more reliably detect
flightlessness in fossils, with increased feather symmetry as a potential
microscopic signal. We observed apical modification in later stages of
feather development (asymmetric displacement of barb loci), while
mor­phologies arising during early developmental stages are only altered
after millions of years of flightlessness.