How birds got their wings: Part II
In my last post, I discussed what traits birds had in common with dinosaurs that allowed them to lay the foundation for flight. Archaeopteryx is often referred to as the turning point between reptiles and birds. It had wings, feathers, hollow bones and a unilateral breathing system. Its anatomy matches that of modern flightless birds. Further adaptations were needed to unlock the door of the bird class and finally give birds the tools that made flight possible.
Like many dinosaurs, Archaeopteryx had a small, thin sternum made of cartilage (that didn’t fossilise, so to the best of our knowledge). It was only in the early Cretaceous that the larger and ossified sternums like those in modern birds developed. Over generations, the sternum progressively began to get bigger, providing powerful wing muscles with an anchor point: the keel or “carina”. The first known bird to have had a fully developed keel is the Confuciusornis, an early Cretaceous bird. This crow-sized bird already had a lot in common with modern birds, such as a toothless beak and a short tail unlike the long bony structure Archaeopteryx had. Although many paleontologists believe Confuciusornis was able to fly, even if only for short bursts of time, it was missing many refinements modern birds have today.
One of the flight adaptations peculiar to modern birds is their shoulder development. In dinosaurs, as is the case for most vertebrates, the coracoid bone and the scapula were fused together, while in modern birds they’re separate. Finding the first species with a separated coracoid and scapula is a tricky task for scientists. Indeed, assessing the fusion of these two bones from fossils requires meticulous analysis of the bones. Osteohistologists study the history and function of the bone at the microscopic level. This enables them to compare the structure of bones from early and modern bones. Despite this technology it remains impossible to determine exactly when the separation occurred.
We do know, however, that some time in bird evolution, the coracoid bone was elongated, which provided the flight muscles with a solid attachment. The glenoid cavity, which is the junction between the humerus and the scapula in the shoulder, was also reorientated dorsolaterally to allow for a greater range of motion.
Most importantly, these adaptations lead to increased mobility for flight. The separation between the coracoid and the scapula allows for efficient upstroke movements, while the powerful muscles attached to both bones enable downstroke movement. The “furcula” or wishbone as it is commonly referred to, is the fusion between the two clavicles and although it was already present in dinosaurs, it went through some evolutionary refinements to give it the key role it has today in bird flight. Its flexibility means it can act as a spring in flight, bending and recoiling as the bird flaps its wings.
The ultimate key to flight was the airfoil created by the bird’s wings.. Their convex shape forces air to travel faster on top of the wing than underneath creating lift. Their blade-like tip reduces drag by minimizing the effect of the spirals of turbulence at the end of the wing created by the low pressure. Nature’s ingenuity goes further with different types of wings. Gliders such as albatrosses and seagulls have long thick wings which form great airfoils. This allows them to move smoothly and effortlessly through the air. While staying in the air for hours without a single flap has its perks, it also makes them extremely dependent on the wind and lack dexterity when it comes to manoeuvring. Birds with elliptical wings such as sparrowhawks are much more agile. The price to pay is that their airfoil is not as efficient, meaning they spend a lot more energy than gliders do in flight.
I have been through many of the anatomical changes early birds have gone through to reach modern “perfection”. When it comes to evolutionary adaptations, scientists have been spoiled with concrete evidence! The third part of this mini-series addresses the theories for the origin of flight, for which there is a lot less evidence, as you can imagine! However, their modern anatomy offers us exciting clues as to why they may have left the ground…
Cover image illustration: Michael Rothman
