What are birds?

Birds are animals belonging to the class Aves. Notable characteristics of the group are feathers, toothless beaks, the laying of hard-shelled eggs, a strong but lightweight skeleton, amongst other traits. They are widespread and come in all different shapes and sizes. The smallest living bird species is the bee hummingbird which weighs 2.6 grams, whereas the largest living bird is an ostrich which weighs a maximum of 145 kilograms, the equivalent of more than 55,700 bee hummingbirds! Of the approximately 11,154 known species of birds, 1.4% have become extinct and 22.4% are now near threatened, vulnerable, endangered, or critically endangered.

How have birds evolved?

Surprisingly, the more you learn about bird evolution, the more difficult it becomes to understand. Scientists today are still learning and debating critical questions regarding the origins of birds; however, these conversations are incredibly interesting despite not always ending in concrete answers. A major break in the understanding of bird evolution occurred when archaeologists discovered fossils of bird-like dinosaurs that appeared to have feathers. In the following years, fossil dinosaurs that had distinct wing-like structures were discovered. More and more information began to corroborate that these dinosaurs had many physical similarities with ancient and modern birds, and that they likely experimented with flight in the form of gliding or leaping.

Origin of Birds Debate

So how are birds, dinosaurs, and reptiles all related? Dinosaurs are confidently classified as reptiles. There is also an increasing population of scientists that say birds should be classified as reptiles, too. This is because many feathered dinosaurs and early bird species share a common ancestor. This ongoing scientific debate is broadly referred to as “the origin of birds” debate. For example, although many scientists classify Archaeopteryx lithographica firmly as a dinosaur (and therefore a reptile), many believe that it could be considered the first bird species. We will revisit this fossil animal, but it, and many others, are important to mention because they are examples of questions in our natural world that are still unanswered. Including: what qualifies as a bird in the first place? Let’s meet some cool extinct birds, including some that might help scientists solve these mysteries!

Archaeopteryx lithographica

One cool extinct bird is Archaeopteryx lithographica, a contender for being the earliest species of bird. The first discovery and description of this species was in 1861. Then, during the 19th and early 20th centuries, it was widely accepted by scientists as being the first species of bird. Some species (including the species below) have since been discovered that may predate Archaeopteryx and qualify as birds, but there is no consensus. Archeopteryx was smaller than a raven and like many birds had feathers, broad wings, and could fly. Discovering this species was important in understanding the evolution of flight because of its well-developed flight feathers. A study in 2011 predicts that Archaeopteryx was entirely black or at least mostly black.  Some particularly dinosaur-like features unlike birds today include sharp teeth, three fingers with claws, and a boney tail. Collectively, the known physical features of Archaeopteryx more closely resemble that of a certain group of dinosaurs than modern birds. This intermediate species between dinosaur and bird lived during the Late Jurassic period approximately between 150.8 and 148.5 million years ago.

Aurornis xui

The ancient species Aurornis xui is another cool extinct bird species that’s also arguably the first ever species of bird. Unlike Archaeopteryx, this species is definitively classified as a bird and is thought to predate Archaeopteryx. This species was first discovered in 2013 and has changed previously held predictions about the origin of birds. The scientific community now favors the theory that Aurornis xui is the origin species to all birds. Aurornis lived earlier than Archaeopteryx by about 10 million years during the Late Jurassic around 160 million years ago. The species Aurornis xui was similar in size to a pheasant and resembled modern birds in many ways. It had relatively broad wings and was near entirely covered in feathers, including its tail and legs. This physiology, alongside other traits, allowed them to fly, but their capabilities were still limited. Aurornis xui also had many different features than modern birds. For example, Aurornis had a long bony tail consisting of 30 vertebrae. The ancient bird also had claws and legs similar to Archaeopteryx, as well as several other primitive characteristics.

Dodo Bird

The dodo bird is a very well-known extinct bird that also has many cool characteristics. The dodo (Raphus cucullatus) was a flightless bird alive during the Holocene (the current geological epoch) 11.7 thousand years ago, much more recently than Archaeopteryx and Aurornis. The last confirmed sighting of a dodo bird was in 1662. It lived only on one small island east of Madagascar called Mauritius in the Indian Ocean and its closest living relative is the Nicobar pigeon. Dodos primarily inhabited wooded areas and dry coastal areas. Fossil remains indicate that the dodo bird stood approximately 1 meter tall and likely weighed between 23 and 40 pounds, possibly as much as a female bulldog! Interestingly, some studies have suggested that their weight fluctuated considerably with the changing seasons. Scientists claim they lost weight when it was hotter and gained weight when it was cooler. The dodo also had a distinctive robust beak that had a hooked tip. Its skull, not including the beak, was wider than it was long and was half the length of the beak. The dodo likely had brownish-grey plumage, yellow featherless legs, a naked head, and a tuft of feathers for a tail. Also, it had small wings and several other physiological features that were incompatible with flight.

The dodo bird has had significant cultural significance despite only a few sightings by humans. Before its extinction, it was a symbol of gluttony in European literature because of its round, squat shape. In recent times, the dodo bird has become the subject of common analogies- for example the saying “dumb as a dodo” to indicate stupidity, or “dead as a dodo” to indicate something is certainly dead or obsolete. In a more serious context, the dodo bird is also featured on the coat of arms for its native land, Mauritius.

Passenger pigeon

Another cool species of extinct bird is the passenger pigeon (Ectopistes migratorius). The passenger pigeon was officially extinct in 1914, only around 108 years ago! Compared to the millions of years some of the above species have been extinct, the passenger pigeon was still around pretty recently! The passenger pigeon got its nickname because of its migratory behaviors. They were vagrant, meaning they would migrate constantly in search of food in enormous flocks; in fact, it was the most populous bird species in North America with between 3 million and 5 million birds! Its decline, however, was due to overhunting which intensified with the colonization of North America by Europeans. The passenger pigeon had many unique characteristics, too. This bird was incredibly fast and could fly at speeds up to 62 miles per hour- faster than a car on a highway! Also, passenger pigeons were known to cooperate and fly in large groups as a strategy to reduce the threat of predators. Congregating in large numbers reduces that probability of any individual bird being killed. This is called predator satiation. The birds flew in such large and dense groups that some reports in historical literature describe them as “blackening the sky”.

Hawaiian Honeycreeper

The Hawaiian honeycreeper is a cool extinct bird, that- you guessed it- lived in Hawaii. More specifically, they were endemic to the island Honolulu. This means that was the only place in the world they could be found. According to the IUCN, Hawaiian honeycreepers are classified as extinct. They were close relatives of the still living rosefinch, but they have many adaptations that make them unique from these relatives.

Adaptive Radiation in Hawaiian Honeycreepers

There was remarkable diversity and variation of many physical traits within Hawaiian honeycreepers. This is due to a process called adaptive radiation. Adaptive radiation is when groups of the same species inhabit new ecological niches or experience environmental forces that alter resource availability, and they rapidly adapt to their new, distinct environments. Subsequently, they develop markedly different traits within the same species. This process can lead to speciation, or the divergence of one or multiple groups from an ancestral species to create new species. Arguably the most well-known example of adaptive radiation is of Darwin’s finches in the Galapagos Archipelago. The next most well-known example of adaptive radiation is likely the Hawaiian honeycreeper. In the case of Hawaiian honeycreepers, their great variation in beak size and shape exemplifies adaptive radiation. An ancestral species of honeycreeper gave rise to many different forms that could fill different ecological niches. These niches were distinct by the type of food that was available. Consequently, not all beak shapes could access the food in different areas. Many honeycreepers are nectarivorous, meaning they feed on the nectar of flowering plants. These birds evolved long, thin, curved beaks that could probe deep into a funnelform flower. Others were insectivores, meaning they fed on insects. These honeycreepers had straight, thin bills that were more suited to that niche. Others were specialist species that evolved a particular beak for specific foods like certain hard nuts or seeds. There were numerous other variations within this species which makes it a great example of adaptive radiation and of diversity within a species.

Are more birds becoming extinct?

Unfortunately, many bird species are experiencing intensifying population declines. According to the IUCN Redlist in 2019, hundreds of bird species, including the mangrove hummingbird, Somali ostrich, whooping crane, and approximately 457 others, are endangered. Many more species are classified as near threatened, vulnerable, and critically endangered. A major reason for the decline of so many avian species is human-caused habitat destruction. Logging practices deplete forests that many birds rely upon. Gas emissions from cars, factories, and chemical plants pollute the atmosphere and affect every ecosystem on earth. Chemical run-off from industrial farms and insecticides also negatively affects many bird species. Unfortunately, unless there is drastic change in human activity in the near future, many more bird species will become extinct.

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