Barney A. Schlinger—
Setting humans aside for a moment, how many mammals did you see today? Any? If you have a dog and cat, you saw two species. A squirrel in the yard, you are up to three. Perhaps you heard a mouse or rat under your house last night. We’ll count that as another one. All told, you saw or heard four species (look in the mirror and you saw five) and, perhaps, just five individuals total. A few more if you got out of the house and saw your neighbors. Now ask yourself, how many birds did I see today? You may not have paid attention, but I’ll bet you saw many species and many individuals: gulls, pigeons, starlings, sparrows. If you know birds, you may have seen a dozen or even scores of species, and, perhaps hundreds or even thousands of individuals. And they call this the age of mammals. Well, a mammal calls this the age of mammals. In my view, from the standpoint of vertebrates, the last 65 million years has been the age of birds.
Birds are simply extraordinary ever-present organisms. Their ability to fly is, of course, without parallel. Added to flight, they run, they swim, they climb, they slide; they get about in pretty much any possible way. Their ability to populate every continent and island on earth is, again, without equal. The remotest oceanic islands have birds, sometimes millions. Their omnipresent songs are amongst the most complex and beautiful biologically created sounds on earth. They are minute and they are huge. They are, without a doubt, smart. They are utterly secretive, or, in some cases, they are some of world’s most celebrated exhibitionists.
It is in this latter area that we take special notice. They strut, they spin, they rocket off the ground, and safely careen back to earth, they dance, they show off all their vibrant colors, or special features they grow on their heads, chests, wings or tails. They perform wild acrobatics.
How do they do all of this? How did birds evolve into the most successful vertebrate group on the planet? To fully appreciate all the wonder of birds in nature, we have to look underneath their feathers and into their bones and muscles and organs and cells and, finally, into their genes. They are impeccably designed with an immeasurable number of adaptations, small and large, that make them the magical creatures they are.
Worldwide, scientists are exploring the genomes of birds of all types. From these immense sequences of DNA, genes and promoters are decoded, and new proteins identified. With laboratory testing and analysis, cell and tissue organization is observed, and their unique capabilities are identified. Each related group of birds: the ducks and geese and swans, the hawks, the owls, the gulls and terns and sandpipers, the parrots that talk, the hummingbirds that fly backwards, all the birds we know and love, all have their own unique sets of active genes, while also sharing all the essential genes that, when expressed together, make a bird. And these genes dictate the ways cells become organized and interconnected, and then perform to achieve the many avian appropriate tasks. It is well worth considering a bird beneath its feathers.
Consider a bird flying by; its flight muscles engaged. In long-distance migrations, where the act of lifting and retracting the wings occurs repetitively often many hundreds of thousands of times, their absolutely perfect muscular coordination, their ability to gain the fuel and the oxygen they require, all utilize invisible but exceptional physiological and anatomical specializations. When considered altogether, some migrations seem almost impossible to achieve, immense flights over vast oceans or deserts or mountains, aloft in the deep darkness of night, through winds and rain, but birds do so by the billions twice each year. Electricity in the muscles and nerves, quantum physics in the eyes, the flow of molecules across the gut and through the heart, the wind through the lungs and in the bones, all in exquisite coordination—times a billion.
Then there are bird’s feathers. Genes in the cells of the skin coordinate their activity to form proteins that become the intricately designed structures we call feathers. These cells may also accumulate pigments that were actively garnered from colorful fruits undergoing digestion in the bird’s gut or that were manufactured in their livers for the sole purpose of provisioning the skin cells to elaborate the feathers they make.
The songs we hear and love are the result of an extraordinary assemblage of neural circuits that hear and process sounds, that form acoustic memories, that inform motor pathways to instruct tiny muscles around the trachea to contract with exquisite timing and coordination that pull on membranes that vibrate as the birds breath sending waves of changing pressures through the air that we perceive as the wonder of birdsong.
The impressionistic colors and songs of many birds evolve, often, for purposes of attracting individuals of the opposite sex. The mating rituals of many birds can also involve unusual social interactions between and among individuals of the same sex, usually the males. They dance, they fight, they scheme. Much of all this elaborate sensory display and social behavior is deliberately activated by hormones that have also prepared and matched all the reproductive systems of males and females, the courtship, the mating, the nest-building, the brooding, the parental responses to begging, all for creation of a next generation of birds to swell the skies and fill the forests and grasslands and oceans of the planet.
Birds are amazing, and what we readily observe in nature stems from the miraculous physiological and anatomical assemblies humming along with precision, hidden a heartbeat away beneath their immaculate coat of feathers.
Barney A. Schlinger is associate dean of life sciences and professor of integrative biology & physiology and ecology & evolutionary biology at the University of California, Los Angeles. He is a past president of the Society for Behavioral Neuroendocrinology and lives in Topanga, CA.