Birds Above All Else

There is an exchange between two characters in Tom Stoppard’s mind-bending play Arcadia (1993) that captures in a clever way the essence of science. In Act Two, Scene VII, two characters in the present day on the Coverly estate are discussing events that may or may not have occurred there in 1809. Hannah Jarvis, an author, is skeptical of the conclusions being advanced by another character, an academic, regarding what happened over a century ago. Valentine Coverly, a graduate student in mathematics, is the scion of the Coverly family that owned, and still owns, the estate.

Valentine It may all prove to be true.

Hannah It can’t prove to be true, it can only not prove to be false yet.

Valentine (pleased) Just like science.

Hannah’s statement, with its double negatives, does address a core tension in science: no conclusion, no matter how thoroughly tested, is above challenge. As Carl Sagan wrote, in The Demon-Haunted World: Science as a Candle in the Dark (1996), “Except in pure mathematics nothing is known for certain (although much is certainly false).”

But there are some propositions in science that have moved beyond Hannah’s cautionary statement into the realm of nearly universally accepted scientific facts. Evolution through natural selection is one of them. Another is the subject of paleontologist Steve Brusatte’s masterful new book, The Story of Birds: A New History from Their Dinosaur Origins to the Present (2026). In it, Brusatte writes:

Birds are dinosaurs.

This realization is one of the greatest achievements in the history of paleontology.

(I read an e-book version of this book so I won’t be providing page numbers for quotations.)

The book reaches two conclusions: birds are dinosaurs, and birds are marvelous creatures. It is a heady and graceful work, serving not just the fossil lover, but the bird lover as well. It's also rich fare for someone in search of how science has been done in the past and is done now. Basically, it's a good read.

That he is completely smitten by birds, ancient and present, is, if possible, an understatement. To offer just one example of the language he uses when extolling birds, early on he writes that birds are

some of the most unusual, the best adapted, the most fine-tuned animals that live today, and that have ever lived, in the 4.5-billion-year history of the Earth.

Though Brusatte describes his personal connections to this work, he is restrained and does not fall victim to the temptation, succumbed to in much popular science writing, of becoming the hero of the story.

Undoubtedly and deservedly so, Archaeopteryx is the hero of the paleontology side of the story that Brusatte tells. The fossils of this species, first discovered in Germany in the early 1860s, show that the animal sported feathers. It is considered the oldest bird, dating from the late Jurassic (about 150 million years ago). The picture below shows the so-called “Berlin” fossil of the species. (This image is in the public domain one of the several fossils of the species. It is published under the Creative Commons Attribution-Share Alike 3.0 Unported license.)

There are human heroes in this story, as well. Though Brusatte profiles many of the scientists who have done significant, groundbreaking work exploring the evolution, development, and family ties of birds, I am most taken by two who were involved in either first enunciating that birds are dinosaurs, or reviving that position after years of willful neglect: biologist and Darwin champion Thomas Henry Huxley (1825-1895) and paleontologist John Ostrom (1928-2005).

Huxley is a fascinating personality, faithful to his working class origins and dogged in his commitment to what he perceived as true. Brusatte highlights the bold steps Huxley took in lectures in the 1860s to lay out why he believed: "Aves (birds) has its root in the dinosaurian reptiles." The key elements to his argument were the combination of bird and dinosaur attributes that he detected in two recent fossils discoveries; these approached the issue from different directions. First, there was the earliest bird, Archaeopteryx, which featured some characteristics of reptiles, and, then, there was Compsognathus, a reptile also from the late Jurassic that sported some avian features. Huxley was brilliant in building his argument to a striking and irresistible conclusion.

Brusatte describes how, in time, this conclusion about the relationship of birds to dinosaurs fell out of favor and faded into the background. Some hundred years after Huxley had staked this claim, paleontologist John Ostrom revived it in a profound way. Ostrom had found and named Deinonychus, a coelurosaur theropod dinosaur that lived during the early to late Cretaceous (115 to 98 million years ago). Ostrom argued that the coelurosaurs, unlike their lumbering, huge brethren (think T. rex), were small, agile, and quick predators. They were also light weight, given that their bones were hollow. (Coelo, the Greek root, for their genus name means “hollow,” and, by the way, birds have hollow bones.)

Ostrom knew the coelurosaurs from his find and others which dated back to the Jurassic.  After closely analyzing the four Archaeopteryx fossils known at the time, he came to a profound conclusion. In a brief, one-page article, The Ancestry of Birds, which appeared in the March 9, 1973, issue of Nature, Ostrom listed the 21 specific morphological attributes of Archaeopteryx that he posited were shared by coelurosaurs. He introduced the list by writing:

The following coelurosaurian features of Archaeopteryx collectively are here considered as prima facie evidence of a coelurosaurian (Theropoda) ancestry of birds.

He concluded that Archaeopteryx had gained these features through "direct inheritance from a small coelurosaurian ancestor."

Of Ostrom's article, Brusatte writes:

It is probably the single most important page ever written about dinosaurs.

He summarizes what Ostrom was demonstrating as follows:

Birds are a type of coelurosaur, which is a type of theropod, which is one of many types of dinosaurs.

In support of the book’s central conclusion, Brusatte delineates in detail how the pedigree of many typical bird traits and features is actually dinosaurian in nature. These traits range from hollow bones, to feathers, to nesting, to the laying of colorful, hard-shelled eggs, to an efficient respiratory system, and to relatively small size (true of many dinosaurs), as well.

Of course, what possibly most marks an animal as a bird is its ability to fly. Archaeopteryx, the most primitive bird, was a flyer, but only a crude one. Brusatte outlines how flight probably arose and, most importantly, uses this as one of several powerful examples in the book to illustrate a central aspect of evolution. A feature that arises for one purpose may, over time, come to serve another, different one. Flight is a clear example of this. Feathers, so critical to birds' flight, arose among dinosaurs, not for flight, but possibly initially as "sensory devices," and then for insulation. Indeed, Brusatte calls feathers a "default condition" for dinosaurs (think, hair for mammals). In this case, over time, evolution repurposed the early feathers into an essential, powerful component of a constellation of features, including the forearms evolving into wings, that would be necessary for birds to take flight. Light weight from hollow bones clearly contributed.

If one were foolish enough to skip the paleontological heart of this book with its treatment of how and why birds evolved and, instead, just read the last chapter, it would still be worth the price of admission. That chapter, titled Marvelous Modern Birds: Intelligence, Cognition, Birdsong, and How Birds Are Still Evolving Today, is a précis of much that makes extant birds so universally fascinating. In this chapter, Brusatte makes good on his assertion that "birds are among the smartest, cleverest, and most cognitively advanced animals on the planet." Some of today's birds, he shows, think about the future, engage in abstract thought, fashion tools, among other impressive mental and physical feats. I was particularly taken with the oscines, the songbirds, whose songs are not innate or instinctive, but are, instead, learned by immature birds from their elders. They are "vocal learners," no mean feat, considering the complexity of many of their songs.

Finally, birds have faced dramatic threats to their survival from their origins, threats that have included such events as the mass destruction at the end of the Cretaceous, and never-ending challenges from climate change. Sadly, in recent centuries, the impact of Homo sapiens on these dinosaurs has been profound, with many species lost to hunting and wanton destruction of the environment, and billions of individuals killed by the domestic cats we seem to be unable to keep indoors (but, despite claims to the contrary, only a piddling number are lost to wind turbines). Brusatte demonstrates how birds have endured these challenges, showing that they are truly survivors. He writes:

The future is uncertain. And paleontologists like me are always more comfortable looking to the past than predicting what might lie ahead. But as the world changes rapidly around us — because of us — I am willing to make a bet. The Age of Birds will persist, even if the Age of Humans does not.