Thoughts on the evolution of bird flight


I had this idea while watching a bird at a local nature preserve. It was going incredibly fast along the ground. I realized, that even though it was flapping its wings, it was not actually flying; it was running along the ground while flapping its wings, and moving at a remarkably fast rate. This made me think of my high school days with modifying cars with my friends. There were two ways to increase a car’s acceleration: 1) increase the power of the engine, and 2) decrease the weight of the car. These two aspects reference the car’s “power-to-weight ratio”, and the idea is to increase that. Improving the power involved changing stuff with the engine and exhaust; decreasing weight involved swapping fiberglass body panels for the stock steel panels; however, while this lowered weight, it also increased the fragility during an accident (but we never really gave that much thought…). Another contributor to improved acceleration was reducing the rolling resistance, but other than greasing wheel bearings and increasing tire pressure, there was not a lot to be gained there. Back to the bird. I figured that by flapping its wings it was generating lift – maybe not sufficient to fly, but sufficient to reduce weight (mass was not changing but the wing lift offset the weight). By this means, the bird had effectively increased its power-to-weight ratio, and its acceleration while running was therefore increased. Furthermore, due to its speed, it had fewer steps over a given distance; thus, its “rolling resistance” was also decreased. Outrunning a predator is a key selective advantage! The earliest prototype wings are unlikely to have provided sufficient lift for flight (i.e. lift > body weight); however, any amount of lift would have increased the effective power-to-weight ratio, and improved running acceleration. As wing efficiency evolved, legs muscles could potentially degenerate, reducing weight (and flight muscle further developed, increasing lift). In this view, flight evolved from a mechanism to increase running speed (and not flight, per se). However, as sufficient lift was achieved and permitted longer and longer flights (think Wright brother’s early hops improving over time), a new avenue of predator escape emerges – flight!