Have you ever watched birds flit to your feeders and wondered how they achieved the magnificent ability of flight? The evolution of flight has been an intriguing puzzle ever since evolutionary biologists realized that modern birds evolved from dinosaurs. An article published recently in PLoS ONE adds a new hypothesis to the recipe for avian flight.
The new hypothesis adds to the three existing main theories about the evolution of flight in Archosaurs (the taxonomic group that includes crocodilians, dinosaurs, and birds). One of the most puzzling parts about the evolution of flight is that all of the feathered dinosaurs that have so far been unearthed appear to be a link between scaly, ground-dwelling dinosaurs and our modern birds. These dinos had feathered limbs that were clearly advantageous in some way, but the size and shape of their feathers and the musculature of their bodies would not have enabled them to actually fly. So the question is: what was the step (or steps!) in between ground-dwelling reptiles and airborne birds that pushed evolution toward flight?
The oldest of the theories that takes dinosaurs from the ground to the air is the cursorial or “running” model. This model describes flight as evolving from the behavior of predatory terrestrial dinosaurs. In this model, an Archeopteryx-like dinosaur would jump while running in pursuit of prey and flap its feathered wings. The theory behind this is that lift and glide would be achieved through the short jumps while the dinosaur would also expend less energy and reach higher speeds. Eventually, flight would have evolved from these short gliding jumps, from the ground up.
The second major theory about the evolution of avian flight — the arboreal theory — suggests that birds evolved from tree-dwelling dinosaurs. This theory states that Archeopteryx and its relatives lived in the trees and glided from tree to tree and down to the ground, using its feathered forelimbs for balance and lift. This second model stands up better to the tests of feasibility.
A third theory came about in the early 2000s from University of Montana ornithologist, Ken Dial, who looked at behavior in modern birds and came up with the “wing assisted incline running” (WAIR) model. Ken looked at Chukars, a species of partridge that appear to actually run vertically up tree trunks. These birds create downforce by flapping their wings, which allows their feet to maintain greater traction on vertical or steeply inclined surfaces. WAIR hypothesizes that early birds started out with WAIR behavior to navigate inclines to escape predators. This may have been a stepping stone toward full-fledged flight, as feathered dinosaurs moved up into trees.
For more information on these previously existing theories and more, go here.
The new hypothesis about the evolution of flight, like the Chukar research, also comes from observations of extant birds — in this case raptors. The article, published by researchers from the Museum of the Rockies, Montana State University-Bozeman, and Brown University focuses on Deinonychus and other dromaeosaurids (such as the famous Velociraptors) — predatory dinosaurs that had a specialized enlarged “killing claw” that allowed them to take down prey much larger than themselves, just like many existing raptor species do. The researchers hypothesize that once dromaeosaurids took down their large prey, they stood atop their prey, grasping it with the killing claw and using “stability flapping” to stay on top of the animal while it was still alive. Once their claws are locked onto the prey, they are no longer useful in terms of stability, so flapping the feathered forelimbs would be essential to keep the dinosaur balanced on top while it devoured its meal. Imagine standing on top of a round, wobbly rock – you would have to “flap” your arms to stay balanced. Imagine how much more balance you could achieve if you had feathers to create a bit of lift and draft to keep you stable.
Watch modern raptors use stability flapping:
Employing stability flapping does not mean that these dinosaurs had the actual capacity to fly, but this “flapping first” hypothesis shows that flapping could have been an intermediate step in the evolution of flight. Dinosaurs like Archeopteryx and Deinonychus have long puzzled researchers because they did not seem to possess the musculature that would create enough lift to actually fly, but they would have had the ability to use stability flapping while on top of prey. Their short, broad, feathered forelimbs would not have been well suited for soaring or speed, but would have provided great maneuverability, agility, and lift — much like in modern Accipiters. The musculature, shape, and size of the forelimbs of these precursors to birds would have been a perfect fit for stability flapping, and this could be an essential missing step in the evolution of flight.
Since PLoS ONE is an open-access journal, you can check out the article for yourself and read about the “flapping first” hypothesis as well as some other fascinating new theories about predatory behavior in Deinonychus, Velociraptors, and other similar dinosaurs here.
One of the most common tricky bird identification challenges that FeederWatchers face is separating Sharp-shinned Hawks and Cooper’s Hawks. Both hawks are becoming increasingly more common at FeederWatch sites and, to the untrained eye, can appear very similar. This is why a recent interaction caught on camera by Cornell Lab of Ornithology staff member Jay McGowan piqued our interest.
It’s not often that one gets to see these two easily-confused species together! During the interaction in these photos, the Sharp-shinned Hawk mobbed the Cooper’s Hawk, much like you would often see a crow mob a raptor.
Seeing these two species juxtaposed next to each other allows us to really see some the features that distinguish them. Notice, first of all, the size-difference between the two. The Sharp-shinned Hawk is noticeably smaller in size than the Cooper’s. Next, notice the small head of the Sharp-shinned Hawk (almost comically small), which barely projects beyond the leading edge of its wings, compared to the large brutish head of the Cooper’s Hawk, which sticks out noticeably. Also take note of the spindly, pencil-thin legs of the Sharp-shinned Hawk next to the thick, stocky legs of the Cooper’s Hawk.
For those of you wondering why the Cooper’s Hawk is upside-down in the second photo: raptors that are being mobbed often try to scare away their harassers by flashing their outstretched talons. The easiest way to do this in mid-air is to perform a quick barrel roll to show off the weaponry. In this situation, the Sharp-shinned didn’t appear to be very intimidated!
Based on the above ID tips and our Tricky ID page, can you tell which of the hawks (from this same pair) is in this photo?
Guest post by Kayla Garcia, Cornell Class of 2013
Animals are confronted with many new challenges as the globe becomes increasingly urbanized. For birds, urbanization may affect the food supply, the predator community, and the ability to find cover that offers protection from predators. Young birds that recently left the nest, in particular, suffer from high rates of mortality. Predation is a primary cause of fledgling mortality, and predator numbers tend to be greater in urbanized areas than in rural areas. Common predators of urban songbirds include raptors, domestic cats, raccoons, and unexpectedly for many, chipmunks.
In a recent study, researchers examined post-fledging survival across a rural-to-urban gradient at 26 sites in the Columbus, Ohio metropolitan area. The two bird species, Northern Cardinal (Cardinalis cardinalis) and Acadian Flycatcher (Empidonax virescens), were chosen because of their different nesting strategies: cardinals fledge at a younger, less mature age and often breed in urban environments, while flycatchers stay in the nest longer and are less likely to nest in urban areas. Radio transmitters were attached to fledgling birds and their survival and movements were tracked after fledging.
Somewhat surprisingly, the researchers found that overall survivorship was not strongly influenced by urbanization in either species. Further, cardinals were more likely to survive in more urbanized areas during the first three days post-fledging. Cardinals were also more likely to choose areas with more understory plant cover and fledglings were more likely to survive in areas with a dense understory. Because fledgling survival increases as the amount of understory vegetation increases, planting dense shrubs in suburban and urban yards will likely benefit the birds, even in highly urbanized landscapes.
For tips on how to create a more bird-friendly yard, visit the Cornell Lab of Ornithology’s new YardMap project.
Source: Ausprey, I. J., and A. D. Rodewald. 2011. Postfledging survivorship and habitat selection across a rural-to-urban landscape gradient. The Auk 128:293-302.
Kayla Garcia is a junior studying Natural Resources and Entomology at Cornell University. Her love for nature began with sneaking out of the house to watch alligators while growing up in central Florida, though now she mostly sticks to birds, smaller herps, and arthropods. In the future, she hopes to share her love and enthusiasm for the natural world in part through educational outreach and public service. When not holed up in one of Cornell’s many excellent libraries, Kayla enjoys making tortillas, exploring Ithaca, and puzzling over the poor hunting skills of her pet vinegaroon (a kind of scorpion), Waylon.
Normally found in the mountains of the west, the Gray-crowned Rosy-Finch is a very rare find in eastern North America. FeederWatcher Nancy Loomis of Booneville, New York, was thrilled to host only the second confirmed Gray-crowned Rosy-Finch ever seen in the state. Understandably, Nancy could not identify the bird when it first appeared at her feeders. She snapped a photo and posted it on the Cornell Lab of Ornithology facebook page for identification. Over the next few days, Nancy hosted many eager bird watchers who would normally have to travel thousands of miles for a chance at seeing this species.
Thanks, Nancy, for keeping watch and for opening your home and sharing this rare find with other birders!
Have you ever had a crossbill at your feeder? How about both species of crossbills? At once? FeederWatcher Kathy Garber of Hudson, Ohio was lucky enough to have just that! Kathy had an immature male White-winged Crossbill and a female Red Crossbill hanging out in her yard and visiting her feeders for a few weeks earlier this month and was able to snap a few photos of the unlikely pair.
White-winged Crossbills are rare in Ohio. The conditions for Red Crossbills have to be just right – with a cone crop that suits the birds’ appetite. So, to have both birds at one feeder is quite unusual!
We sure wish Kathy would share her crossbills with the rest of us!
FeederWatcher Patricia Jones-Mestas was lucky enough to catch this moment of mutualistic behavior recently in Parker, Colorado. Black-billed Magpies are known to pick ticks off of the backs of deer, moose, and other large mammals. This behavior is mutually beneficial for both species — the Magpie gets a meal, and the deer gets rid of ticks. Unfortunately there’s no option for “deer” as a FeederWatch feeder type!
Spring is almost here and that means beautiful cherry blossoms will be coming out soon. You can watch the cherry trees near you bloom and help out a fellow citizen science project by recording cherry blossom data for Project Budburst’s Cherry Blossom Blitz. Participating in Cherry Blossom Blitz is easy and free and the short count period runs from March 20th to April 30th. The data collected will be used to study phenology – the study of the timing of biological events, such as flowers blooming or animal hibernation. To learn more about Cherry Blossom Blitz and how to participate, check out their website.