Overall, this study affirms something I've long been saying: That
flight in birds is a burden if they don't need it.
Flight places a
low ceiling on body mass, which is a disadvantage when processing
large amounts of poor-quality plant fodder.
herbivores dominate the base of Neornithes - both Palaeognathae and
Galloanserae. One hypothesis is that terrestrial-based volant birds
(including those in continental ecosystems) became better adapted to
evade or defend against terrestrial predators - and over time spent
more time on the ground, and eventually lost the ability to fly.