Birds of North America. By Popchart.
(Click on the following link to listen to an audio version of this blog …. An unexpected bird
Experts rarely agree on anything, but expert bird taxonomists are amazingly consistent in stating that there are about 10,000 living species of birds on Earth (although some couch their estimates with statements like “depending on how the word ‘species’ is defined!”). The species diversity of birds exceeds that of many other groups of vertebrates: there are, for example, just under 6,500 species of mammals, and about 6,000 species of amphibians. Avian species diversity is comparable to that of reptiles (both about 10,000 species) but is much smaller than the species diversity of fish (over 35,500 species!). All of these vertebrate diversity figures are, of course, dwarfed by the diversity of just one group of the invertebrates, the insects. There are over 1,000,000 species of insects on Earth (and, quite possibly, 20 X that number if we only had the time and energy to look into our ecosystems more carefully!).
Birds, though, are a diverse and impressive (and beautiful!) group of vertebrates.
Tyrannosaurus rex. Photo by Steveoc86. Wikimedia Commons
Birds are an evolutionary branch off of an extremely robust group of dinosaurs that dominated the Earth during the Mesozoic Era: the Therapods. The Therapods are the group that included Tyrannosaurus rex and Velociraptor and many other species). The first bird-therapod (possibly Archaeopteryx or some similar species) arose in the middle period of the Mesozoic (the Jurassic) some 150 million years ago. These bird species then began to diversify and inhabit the complex ecosystems of the Earth.
These first birds exhibited basic Therapod characteristics like hollow bones and three-toed feet and claws on all four limbs. They also had long, bony tails and teeth! Their bodies were covered with short and, possibly, quite colorful feathers. These bird-therapods underwent many remarkable anatomical and physiological evolutionary changes that eventually enabled them to accomplish powered flight. There were many bird species in ecosystems all around the Earth when the Mesozoic Era came to a crashing end 65 million years ago when an asteroid crashed into the ocean just off of what is now the Yucatan Peninsula of Mexico.
Artist rendition of Late Cretaceus asteroid impact. Picture by D. Davis. Wikimedia Commons
That asteroid is estimated to have been tens of kilometers in diameter and was moving at a velocity of 40,000 miles per hour. It dove into the shallow sea just off the coast and then burrowed several kilometers into the Earth’s crust generating an impact crater that was almost 200 km in diameter. The energy of the collision exceeded by a billion-times the energy of the atomic bomb dropped on Hiroshima at the end of WW2. It is estimated that 90,000 cubic kilometers of rocks and debris were ejected into the atmosphere at incredible velocities as a consequence of the asteroid’s impact along with 100 billion tons of sulphur and 10,000 billion tons of carbon. Massive earthquakes and tidal waves ensued along with a firestorm of super-heated air and burning debris that raced northward across the North American continent burning forests and grasslands and killing everything it encountered.
The debris and gases thrown into the atmosphere shrouded the Earth for decades blocking sunlight, generating acid rain and precipitously decreasing the temperature of the planet. It is estimated that 75 to 80% of all species on Earth went extinct as a consequence of this asteroid collision. Very notably in this large group of doomed species were all of the dinosaurs except for some of the bird-therapod species.
Why did the bird-therapods survive and the other dinosaurs did not? Possibly, a very relevant observation is that all of the surviving groups had beaks instead of teeth!
Archaeopteryx recreation. By DBCLS. Wikimedia Commons
In the ninety-some million years from their Jurassic origins up to Late Cretaceous asteroid strike, bird-theropods had evolved into a wide range of species. This Mesozoic diversification was, possibly, even more rapid and more extensive that the subsequent evolution of the bird-therapod species surviving the Late Cretaceous extinction and generated our impressive array of modern bird species. Some of these Mesozoic birds were predators, while others were insectivores. Still others ate seeds and fruit and maybe even leaves. Many species could fly, while others could only glide or walk or run. Some of these species retained the Archaeopteryx-like feature of toothed jaws, while others evolved non-toothed beaks. Beaks were especially useful for species that ate a wide variety of foods, especially foods like the tough, highly recalcitrant seeds and nuts that might have survived the asteroid inferno. Beaks may have been a physical feature of the bird-therapods that had the most generalized feeding habits and which were, then, most able to survive in the radically transformed ecosystems of the Earth!
Three distinct clades of birds survived the Late Cretaceous extinction: the Palaeognatha (ostriches, rheas, kiwis, etc.), the Galloansera (ground fowl and waterfowl) and the Neoaves (“modern birds” (95% of living bird species)). A major exploration of the DNA interconnections between and within these avian clades is part of the on-going work of a group called the “Bird 10,000 Genomes Consortium.” A preliminary analysis of their research was just published in Nature (April 1, 2024). The data from this paper generated an extensive avian family tree using genetic data from 363 species of birds (which spanned 92% of bird taxonomic families). To me, the most interesting things about this family tree is that one of the living species of birds didn’t fit in with any of the others!
Hoatzin. Photo by M. Foubister. Wikimedia Commons
The hoatzin (Opisthocornus hoazin) is the only species in its genus, family (Opisthocomidae) or order (Opisthocomformes). It is a species, then, with no close evolutionary relatives! This lack of close relations was emphasized in the family tree generated by the “Bird 10,000 Genome Consortium.” All other living bird species fit into the cladistic branches of this avian diagram. Only the hoatzins were left behind!
Fossil and DNA analysis indicates that the hoatzin arose in the late cretaceous period just before the mass extinction triggered by the asteroid collision. It is, then, a very solitary survivor of this biological calamity!
The hoatzin is a pheasant-sized bird with a long neck, a blue, featherless face, a red-orange head crest, an off-white belly and throat, a dark brown back, wings and tail with off-white highlights and edges and reddish-brown side feathers. Hoatzins live in the marshy forests that surround the lakes and slow-moving streams in the Amazon and Orinoco Basins in South America. Hoatzins are loud, relatively slow moving birds that live and nest in social groups in the trees over the streams and wetlands.
Hoatzins have a very unusual diet for a bird. They are almost exclusively vegetarian and primarily consume leaves and flowers of arums and mangroves (and 50 other plant species growing in their habitats). The hoatzin has an enlarged esophagus and crop which its uses as a ruminate chamber. These enlarged organs actually restrict the size of the hoatzin’s flight muscles and cause it to be a fairly sluggish flier. Also, when the hoatzin’s crop and esophagus are stuffed full of leaves, the bird is hardly able to move at all and must rest and digest.
Hoatzin. Photo by Kate. Wikimedia Commons
There are over 1000 species of bacteria in the crop and esophagus of the hoatzin. Many of these produce very foul smelling gases during the bird’s active digestion phases. This feature is the source of common name for the hoatzin: the stink bird. These foul smells also make the hoatzin’s meat quite unpleasant to eat. A feature which may have helped the species survive over evolutionary time!
Hoatzin nestlings also have some very unexpected features. They have claws on the ends of their wings! These claws are vestiges of the claws that their therapod ancestors possessed. A number of other bird species also exhibit non-functional wing claws, but the hoatzin nestlings are able to use their claws to climb about in their tree limb habitats and even climb back into their nest trees if they are displaced by storms or predators.
So, the hoatzin stands alone in its evolutionary space and, for the most part, in its ecological space. What an amazing bird!
I would like to thank my son-in-law, Dr. B. Lee Drake for telling me about the hoatzins and their unique lineage and habits. Apparently, hoatzins don’t live very long in captivity, so they won’t be found in any local zoos or aviaries. We will all have to head off to the Amazon and Orinoco Basins to see them in the wild! Who wants to organize the trip?