Birds can fly — at least, most of them can. Flightless birds like penguins and ostriches have evolved lifestyles that dont require flight. However, theres a lot that scientists dont know about how the wings and feathers of flightless birds differ from their airborne cousins. In a new study in the journal PNAS, scientists examined hundreds of birds in museum collections and discovered a suite of feather characteristics that all flying birds have in common. These “rules” provide clues as to how the dinosaur ancestors of modern birds first evolved the ability to fly, and which dinosaurs were capable of flight.
Not all dinosaurs evolved into birds, but all living birds are dinosaurs. Birds are members of the group of dinosaurs that survived when an asteroid hit the Earth 66 million years ago. Long before the asteroid hit, some of the members of a group of dinosaurs called Penneraptorans began to evolve feathers and the ability to fly.
Members of the Penneraptoran group began to develop feathers before they were able to fly; the original purpose of feathers might have been for insulation or to attract mates. For instance, Velocirpator had feathers, but it couldnt fly.
Of course, scientists cant hop in a time machine to the Cretaceous Period to see whether Velociraptors could fly. Instead, paleontologists rely on clues in the animals fossilized skeletons, like the size and shape of arm/wing bones and wishbones, along with the shape of any preserved feathers, to determine which species were capable of true, powered flight. For instance, the long primary feathers along the tips of birds wings are asymmetrical in birds that can fly, but symmetrical in birds that cant.
The quest for clues about dinosaur flight led to a collaboration between Jingmai OConnor, a paleontologist at the Field Museum in Chicago, and Yosef Kiat, a postdoctoral researcher at the Field.
“Yosef, an ornithologist, was investigating traits like the number of different types of wing feathers in relation to the length of arm bone they attach to, and the degree of asymmetry in birds flight feathers,” said OConnor, the museums associate curator of fossil reptiles, who specializes in early birds. “Through our collaboration, Yosef is able track these traits in fossils that are 160-120 million years old, and therefore study the early evolutionary history of feathers.”
Kiat undertook a study of the feathers of every order of living birds, examining specimens from 346 different species preserved in museums around the world. As he looked at the wings and feathers from hummingbirds and hawks, penguins and pelicans, he noticed a number of consistent traits among species that can fly. For instance, in addition to asymmetrical feathers, all the flighted birds had between 9 and 11 primary feathers. In flightless birds, the number varies widely — penguins have more than 40, while emus have none. Its a deceptively simple rule thats seemingly gone unnoticed by scientists.
“Its really surprising, that with so many styles of flight we can find in modern birds, they all share this trait of having between 9 and 11 primary feathers,” says Kiat. “And I was surprised that no one seems to have found this before.”
By applying the information about the number of primary feathers to the overall bird family tree, Kiat and OConnor also found that it takes a long time for birds to evolve a different number of primary feathers. “This trait only changes after really long periods of geologic time,” says OConnor. “It takes a very long time for evolution to act on this trait and change it.”
In addition to modern birds, the researchers also examined 65 fossil specimens representing 35 different species of feathered dinosaurs and extinct birds. By applying the findings from modern birds, the researchers were able to extrapolate information about the fossils. “You can basically look at the overlap of the number of primary feathers and the shape of those feathers to determine if a fossil bird could fly, and whether its ancestors could,” says OConnor.
For instance, the researchers looked at the feathered dinosaur Caudipteryx. Caudipteryx had 9 primary feathers, but those feathers are almost symmetrical, and the proportions of its wings would have made flight impossible. The researchers said its possible that Caudipteryx had an ancestor that was capable of flight, but that trait was lost by the time Caudipteryx arrived on the scene. Since it takes a long time for the number of primary feathers to change, the flightless Caudipteryx retained its 9 primaries. Meanwhile, other feathered fossils wings seemed flight-ready — including those of the earliest known bird, Archaeopteryx, and Microraptor, a tiny, four-winged dinosaur that isnt a direct ancestor of modern birds.
Taken a step further, these data may inform the conversation among scientists about the origins of dinosaurian flight. “It was only recently that scientists realized that birds are not the only flying dinosaurs,” says OConnor. “And there have been debates about whether flight evolved in dinosaurs just once, or multiple separate times. Our results here seem to suggest that flight only evolved once in dinosaurs, but we have to really recognize that our understanding of flight in dinosaurs is just beginning, and were likely still missing some of the earliest stages of feathered wing evolution.”
“Our study, which combines paleontological data based on fossils of extinct species with information from birds that live today, provides interesting insights into feathers and plumage — one of the most interesting evolutionary novelties among vertebrates. Thus, it helps us learn about the evolution of these dinosaurs and highlights the importance of integrating knowledge from different sources for an improved understanding of evolutionary processes,” says Kiat.
“Theropod dinosaurs, including birds, are one of the most successful vertebrate lineages on our planet,” says OConnor. “One of the reasons that theyre so successful is their flight. One of the other reasons is probably their feathers, because theres such versatile structures. So any information that can help us understand how these two important features co-evolved that led to this enormous success is really important.”
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Further, these findings might contribute to scientific discussions regarding the genesis of flight in dinosaurs. According to OConnor, “scientists have only recently come to the conclusion that birds are not the only flying dinosaurs.” Furthermore, there have been discussions regarding whether dinosaurs’ ability to fly evolved once or several times. Our findings appear to indicate that dinosaur flight only occurred once, but it’s important to remember that our knowledge of dinosaur flight is still developing and that we are probably still lacking some of the earliest phases of the evolution of feathered wings. “.
For instance, the researchers looked at the feathered dinosaur Caudipteryx. Caudipteryx had nine main feathers, but its wing proportions prevented it from flying, and its feathers are nearly symmetrical. The scientists speculated that perhaps Caudipteryx’s ancestor had been able to fly, but that ability had been lost by the time Caudipteryx appeared. Because the number of primary feathers varies slowly, the flightless Caudipteryx kept its nine primaries. Other fossilized feathered creatures, such as the earliest known bird, Archaeopteryx, and the tiny, four-winged dinosaur Microraptor, which isn’t a direct ancestor of modern birds, appeared to have wings that were ready for flight.
“It’s quite astonishing that despite the wide variety of flying patterns observed in contemporary birds, they all possess the same characteristic of having nine to eleven primary feathers,” remarks Kiat. “And I was shocked that nobody seemed to have discovered this earlier.” “.
Kiat and OConnor also discovered that it takes a very long time for birds to evolve a different number of primary feathers by applying the data regarding the number of primary feathers to the entire bird family tree. According to OConnor, “this trait only changes after really long periods of geologic time.” “Evolution takes a very long time to act upon this trait and modify it.” “.
Kiat studied the feathers of all extant bird orders, looking at specimens from 346 different species that were kept in museums all over the world. Observing the feathers and wings of hawks, pelicans, penguins, and hummingbirds, he noted several common characteristics among flying animals. For example, all the flying birds had between 9 and 11 primary feathers in addition to asymmetrical feathers. The number of flightless birds varies greatly; emus have none, while penguins have over 40. It’s a remarkably straightforward rule that scientists don’t seem to have noticed.
Before being allowed to return to the wild, birds with damaged feathers occasionally require a little extra assistance. Although keratin, the same material that makes up hair, is what makes feathers, once it is fully shed, feathers cease to grow and lose their blood supply. Feathers that are damaged are irreversibly damaged until they are replaced by the body through a set timetable called molting, which can take up to a year. When a feather molts, it grows back immediately; however, once a feather breaks, it takes time for the new feather to molt. While species-specific molting patterns differ, most birds molt just a few flight feathers at a time to maintain their ability to fly. A bird may become inoperable if it sustains damage to its flight feathers or loses many of them.
After the donor feathers are ready, the impeding bird is put under anesthesia. Even though this process is painless, the bird may experience stress during it. It is crucial that the bird stay motionless throughout the process to prevent uneven feathers. Using a red-hot scalpel, the broken feathers are removed so as not to compress the shaft.
To create a bridge between the recipient and the new feather, tiny dowels are inserted inside the donor feather shaft after the tips of the feathers have been removed. To enable the dowel to cling to the recipient, part of it is left sticking out of the donor feather. The feathers are numbered so that the recipient bird will receive them in the proper sequence.
The dowels are glued into the donor feathers once they are all the right size. The donor feathers and dowels are then glued into the recipient’s feather shafts one by one. Carefully remove any excess epoxy to avoid adhering the feathers together. Before the glue hardens, the feathers are correctly layered and angled.
Following its release from anesthesia, the bird’s damaged, useless flight feathers are magically replaced with whole feathers, allowing it to take to the air once more. The impeded feathers are transient in nature and will shed in the upcoming molting cycle in the same manner as regular feathers. By then, the bird’s stay at the California Wildlife Center will only be a distant memory as it soars through the skies once more.
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