The birds are the first thing that almost all of us surely think of when talking about powered flight in the natural world. They are some of the most diverse and widespread animals in the world today, with around 10,000 species occupying almost every habitat. But how did birds first take to the skies?
The first birds evolved from theropod dinosaurs in the Jurassic, such as Archaeopteryx, with their closest non-avian dinosaur relatives being the Deinonychosauria, including Dromaeosauridae (e.g. Deinonychus and Velociraptor) and Troodontidae. There are two main hypotheses for the evolution of flight in birds: the arboreal (tree down) hypothesis, and the cursorial (ground-up) hypothesis. The arboreal hypothesis states that bird ancestors evolved gliding and then powered flight as they made their way from trees to the ground again. The cursorial hypothesis suggests that bird ancestors ran and jumped, perhaps to catch prey, and thus evolved flight as a way to enhance this, and to safely get back to the ground again. However, neither hypothesis can be proven or disproven with the current evidence available.
One clue on how birds may have evolved flight comes from modern-day species. Galliformes, the order of birds including turkeys and chickens, are heavy-bodied, ground-dwelling birds capable of flight. However, to reach higher points they prefer to use wing-assisted incline running (WAIR), instead of flying, where they flap their wings as they run. This improves their foot traction, allowing them to scale steep or even vertical inclines. This behaviour is also observed in young Galliformes, whose wings aren’t yet fully developed for flight. The use of WAIR in ground-dwelling birds in the modern-day implies that it may have been used by the ancestors of birds as they went from terrestrial to aerial locomotion. By flapping their forelimbs as they ran, they could enhance their foot traction and therefore overall running ability. Over many generations, these forelimbs may have evolved to become more powerful before they eventually became wings capable of powered flight. This is known as the ontogenetic-transitional wing (OTW) hypothesis.
In addition to this, birds underwent many other morphological changes when they evolved powered flight. Birds kept their ancestral small bodies, and developed an extension of the forelimb and fingers and changes in the orientation of the shoulder bones.
However, powered flight is extremely energetically taxing, so why did birds evolve flight at all? It has been speculated that one of the main reasons for the development of flight is to avoid the risk of predation, based on locomotion, behaviour, and morphology in modern birds. Other factors likely included having access to more and new foods. Birds also evolved strong and stiff but lightweight bones that helped to reduce their mass in flight so the activity requires less energy. This trait may also have been ancestral and found in other dinosaur groups.
The adaptations of the earliest birds were monumental in the development of aerial locomotion in the group. Nevertheless, these birds weren’t as efficient at flying as modern birds. Adaptations such as the keeled sternum for the attachment of powerful flight muscles would evolve later, along with more advanced wing structures, allowing birds to become the powerful and agile fliers we know today.
 Corvus albicollis, the white-necked raven. A modern bird from Eastern and Southern Africa. Available at: https://www.hbw.com/ibc/photo/white-necked-raven-corvus-albicollis/white-necked.
 An Archaeopteryx fossil found in Jurassic sediments in Germany. This genus is thought to be one of the earliest birds. Available at: https://www.livescience.com/24745-archaeopteryx.html.
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