For my last blog during my tenure at the Florida Keys Wild Bird Rehabilitation Center, I decided to focus on adaptations birds evolved for flight. Even though birds aren’t the only animals that can fly (bats and many insects can fly), part of the reason we are fascinated by them is because of this ability to take to the air. Using their own faculties and without technology, birds can fly almost anywhere in the world. Most of us have imagined what it would be like to be a bird, soaring through the sky, free to go anywhere at any time. But how are birds able to fly and we humans cannot? Over the past 150 million years, birds have evolved many adaptations to not only make flight possible, but to optimize their aerial abilities. Some of these adaptations, like wings, are more obvious, while others are hidden beneath their body’s surface.
Gravity pulls all matter, including animals, towards the earth. Birds overcome this force through unique adaptations. These adaptations are focused on mechanisms that provide enough power to efficiently lift and stay aloft in the air, while also keeping bodily weight to a minimum. It takes more energy to lift and keep an object off the ground if it’s heavier, which makes reducing bodily weight advantageous.
Wings and feathers are the most apparent adaptations for flight. By flapping wings that have a large surface area, birds can lift themselves off the ground and fly through the air. Large, well-developed pectoral muscles power wing flapping and are attached to a uniquely keeled sternum (i). While all flying animals have wings, only birds (Class Aves) have feathers. Downy feathers keep birds insulated, but stiff contour feathers allow them to fly. Contour feathers are very light and form the aerodynamic shape of a bird’s body and wings. If you were to look under a microscope, you’d notice that all the contour feathers are connected to each other via tiny hooks on the barbules of each feather (i). These connections enhance the collective strength of the feathers, making flight possible.
Unlike the skeletal system of humans and other ground-dwelling vertebrates, most birds have hollow bones, which can reduce their overall weight. To put this in perspective, the skeleton of a frigatebird with a seven-foot wingspan weighs less than the feathers covering its body (ii)! Despite being hollow, bird bones are incredibly strong and resistant to stress from bending, the most common stress during flight (iii). Other weight reduction adaptations include the elimination and fusion of bones in the pelvis, fingers, and legs, all of which are separated in most vertebrates (ii). For example, birds only have three of the five finger bones that humans have present in their wings (i).
Other features of birds that effectively reduce weight include (i):
• Beaks without teeth.
• Absence of a bladder.
• Very small reproductive organs including the testes, ovaries, and oviducts. Females only have one ovary. However, during the breeding season, reproductive organs enlarge.
• Unlike the earliest known ancestral bird, Archaeopteryx, extant birds have very short tails.
Flying is a very active process that requires a high metabolism and a lot of energy, more than walking and running (ii). Consequently, gas exchange and the delivery of oxygen to flight muscles needs to be efficient. Indeed, avian respiratory systems are proportionately larger and more efficient than humans (ii). While the human respiratory system comprises one-twentieth bodily volume, the avian respiratory system comprises one-fifth (ii). Birds utilize some of their hollow bones as extensions of their body’s air sacs, collectively increasing respiratory capacity. Efficiency is increased by a one way, two-stage flow respiratory system and “crosscurrent circulation” gas exchange (ii). While a human has to take a breath once every four and a half heartbeats, birds only need to take one breath every six to ten heartbeats (ii). To quickly deliver oxygen and nutrients during flight, birds have a robust four-chambered heart with two pumps, just like mammals. Circulatory systems of reptiles, amphibians, and fish are less advanced.
During flight, birds often need to make quick decisions with their movement. Compared with their most closely related living relatives, non-avian reptiles, a bird’s brain is proportionately much larger (ii). The center for processing images received from the eyes is especially large, and the nerves that run between the eyes, brain, and flight muscles are fast and efficient (ii). These advanced features allow birds to quickly receive, process, and respond to environmental input. This is essential for catching prey, avoiding predators, or weaving in between trees in a forest during flight.
Birds are remarkable animals that have evolved adaptations in many organ systems to not only make flight possible, but efficient. These adaptations have proven very successful, helping give rise to the roughly 10,000 beautiful bird species currently sharing the world with us. We may not be able to fly without our modern technology, but we can always appreciate our aerial neighbors and their ability to fly places we’ve only imagined.
Steven Warchocki, Education Intern, 2016
(i) Campbell, N.A, et al. (2008). Biology (8th ed). San Francisco: Pearson Benjamin Cummings.