Birds are Dinosaurs

I remember kayaking on the Charles River near Boston. The river was narrow, and overhung with trees. Vines and bushes crowded the bank while dragonflies skimmed over the water. I went around a bend, startling a Great Blue Heron into flight. It gave a honking squawk as it beat its great wings and took off looking like a prehistoric beast come to life.

That’s not surprising, since herons are not far removed from prehistoric beasts. All birds, in fact, are direct descendants of dinosaurs, specifically a group of two legged dinosaurs called theropods (which includes the monster Tyrannosaurus rex as well as the velociraptors.) It’s hard to imagine that the Ruby Throated Hummingbird is related to T rex, but more and more evidence is being uncovered to show this link.

 

 

 

 

 

 

That leads to the question: what makes something a bird? Wings, feathers, and flight come to mind, none of which are necessarily associated with dinosaurs, but there are other commonalities between today’s birds and the dinosaurs of yesteryear.

Birds have a rapid metabolism, one that requires a lot of oxygen. To solve this they have evolved a highly efficient respiratory system. In humans and other mammals air is inhaled into the lungs where it crosses the lung membrane and enters the blood stream. As the blood circulates oxygen is removed from the blood stream and the depleted blood returns to the lungs for a fresh supply of oxygen. Meanwhile, the excess carbon dioxide is exhaled. This means that each inhale necessarily mixes fresh oxygen with the stale air already in the lungs. This isn’t the most efficient system, but it works. Birds streamline the process.

Birds inhale and exhale as we do, but the air doesn’t mix in their lungs. Rather there is a one-way system within their lungs which allows the fresh air to remain separate from the stale, ready-to-be-exhaled air. This system includes a number of air sacs which are integrated into the skeleton. These sacs help move the air allowing for the unidirectional flow.

This means that birds have continuous access to fresh oxygen, even while exhaling. The air sacs also help lighten the bones as well as provide a storage system for oxygen as it moves through the lungs and blood stream. This neat trick allows geese, hawks and other birds to fly at high altitudes where oxygen is scarce as well as to maintain the high metabolic rates of hummingbirds and other flitters and fliers. As stated in the article “Bird Respiratory System”:

“The avian pulmonary system uses “flow-through ventilation,” relying on a set of nine flexible air sacs that act like bellows to move air through the almost completely rigid lungs. Air sacs do not take part in the actual oxygen exchange, but do greatly enhance its efficiency and allow for the high metabolic rates found in birds. This system also keeps the volume of air in the lung nearly constant.”^1

According to WorldAtlas.com the mild mannered Mallard Duck can fly up to 21,000 feet (6,400 meters.) Granted that couldn’t get it over Mt Everest at 29,029 feet. Only two birds can manage that achievement. The Common crane has reached an altitude of 33,000 feet (10,000 meters). Sadly, the record holding high flying bird, the Ruppell’s Griffon Vulture is critically endangered. If we don’t protect it we will lose the highest flying bird on earth, a species that has been recorded flying at 37,000 feet (11,300 meters.) That is around the maximum altitude for commercial aircraft and higher than small planes can reach.^2

 

That’s a great trick, but what does it have to do with dinosaurs? Turns out that some dinosaurs had a similar system, complete with one way oxygen exchange and air sac interspersed within their skeletons.

Other evidence for the relation between birds and dinosaurs comes from the fact that dinosaurs evolved feathers before the began to fly. That means that feathers had an evolutionary purpose other than flying. Think about ostriches, penguins, emus and other flightless birds. They still retain feathers and wings. Penguins use feathers for insulation and waterproofing while their wings have evolved into flippers for “flying” through the water. Ostriches use their wings for display both for attracting mates and for defense. They also hold out their wings for balance while running. In other words, feathers have uses beyond flight. Dinosaurs probably used feathers for similar purposes before they became airborne.

What it all comes down to is an evolutionary line can be drawn directly from the theropod dinosaurs to modern birds. The next time you think a bird looks prehistoric pat yourself on the back for your cleverness.

Be well, stay curious, learn things.
Thanks for reading

Kate,
July 2020

^1 “Bird Respiratory System.” Eastern Kentucky University | Eastern Kentucky University, http://people.eku.edu/ritchisong/birdrespiration.html. Accessed 19 July 2020.

^2 Nag, Oishimaya Sen. “Highest Flying Birds” WorldAtlas. 3 Oct. 2016, https://www.worldatlas.com/articles/highest-flying-birds.html. [7/18/2020]