By Keith L. Bildstein, Ph.D.
Sarkis Acopian Director of Conservation Science
Hawk Mountain Sanctuary
8 January 2016
Why are some species of birds of prey widespread and abundant whereas others are not?
It turns out that this critical ecological question, one that is fundamental to understanding biological diversity, along with many other aspects of ecology, is easy to ask, but not easy to answer.
Earlier this month two former Hawk Mountain trainees, Dr. Todd Katzner (now the head of the U.S. Bureau of Land Management’s Boise State University Raptor Research Center) and Julie Mallon (now a Ph.D. student at the University of Maryland) and I published an article in the ornithological journal The Auk that helps to explain why turkey vultures are one of the world’s most abundant and widespread avian scavengers. The field work was conducted while Julie was a Master’s degree student at West Virginia University.
The short answer to the question is that turkey vultures have evolved two so-called “key innovations” that, together, make them decidedly “above average” as avian scavengers. The long answer, which follows, is a bit more intriguing.
The turkey vulture’s first key innovation is their acute sense of olfaction. This is something that is quite handy when the food you are searching for is decaying carcasses and smells. Turkey vultures have the largest nostrils of all New World vultures, including the far more massive California condor and Andean condor, as well as the largest olfactory bulbs in their brain–the next largest being those of the king vulture, a species that does not appear to use olfaction in its search for carcasses. Two other Central and South American species of vultures, the lesser yellow-headed vulture and greater yellow-headed vulture, are both close relatives of the turkey vulture and also locate carcasses by both smell and sight.
Intriguingly, although there is no evidence that black vultures are able to locate carcasses by smell directly, the species indirectly locates carcasses by smell. Indeed, black vultures routinely seek out turkey vultures and then follow them to carcasses, capitalizing on the latter’s olfactory capacity. Old World vultures, on the other hand, show no evidence of locating carcasses via their sense of smell.
The turkey vulture’s ability to locate carcasses by smell is a key innovation because it allows individuals to search over forested as well as more open habitats for dead animals, thereby considerably expanding their feeding habitat. This key innovation explains–at least in part–why turkey vultures are the most wide-ranging of all vultures.
The turkey vulture’s second key innovation and the focus of our recent publication in The Auk provides the rest of the story. This innovation is the species dihedral wing posture, which allows it to engage in what my coauthors and I call “contorted soaring” while flying close to the ground in search of carcasses. What atmospheric scientists call “small-scale, shear-induced atmospheric turbulence” routinely occurs close to the ground. It happens anywhere the topographic features such as mountains and valleys, forest edges, and small-scale thermal mosaics, disrupt the flow of wind near the earth’s surface. This creates small pockets or eddies of swirling air that alternately push and pull on objects flying through them. Many of us have experienced this kind of turbulence during bumpy take-offs and landings at airports.
Turkey vultures are lightly-wing loaded (i.e., they have big wings for their body mass), making it easy for them to soar in large packets of warm, rising air called thermals. Other vultures, too, are able to do this. Close-to-the-ground, contorted soaring, however, is another matter entirely. It is close to the ground where the dihedral (“V” shaped) configuration of the species’ wing profile comes into play.
In addition to a relatively long, rudder-like tail that allows them to turn in tight circles while soaring in small-scale turbulence, turkey vultures have long, narrow wings that they hold above their backs in a dihedral. The latter is a key innovation that serves as an “aerodynamic self-righting (or stabilizing) mechanism.” This allows the bird to continue to soar in turbulent air even when its two wings are exposed simultaneously to different up- and down-drafts.
The unusual wing profile functions whenever one wing (either the left or the right) experiences more lift than the other. When this happens the wing experiencing greater lift tilts upward making it less parallel to the ground and reducing its aerodynamic surface and, hence, lift while the opposite occurs to the other wing. The side-to-side rocking, which at first suggests instability, actually is an energy-efficient, soaring flight that allows the birds to stay aloft. It also allows them to continue soaring without having to flap its wings to right itself, which is usually what happens when “flat-profile-wing” soaring birds encounter small-scale turbulence.
Other lightly wing-loaded soaring birds that regularly scavenge for food close to the ground, including greater and lesser yellow-head vultures, Egyptian vultures, black kites, bateleurs, and zone-tailed hawks, all have slight to strong dihedral wing configurations, as do more predatory harriers, which also forage close to the ground. Intriguingly, turkey vultures soaring at greater heights tend to do so in a less pronounced dihedral, whereas black vultures flying close to the ground frequently adopt a slight dihedral rather than their more typical flat-wing profile.
That they are able to extract useful updraft energy while soaring in small-scale turbulence near the ground means that turkey vultures can soar closer to the carcasses they are both looking and smelling for, which makes them easier to locate before turbulent winds jumble and disperse odors associated with them.
Low soaring flight has a second benefit as well. Contorted soaring allows low-flying turkey vultures to stay below the radar screen of potential competitors. This is important when a located carcass is squirrel-sized rather than deer-sized and there might not being enough food to share with others.
Ornithologists speak in awe of just how high vultures can fly while searching for prey. The world record, for example, is held by a Rüppell’s vulture that was sucked into a jet engine while soaring 37,000 feet, an altitude high enough to require special hemoglobin to capture oxygen sufficient for metabolism. But the ability to soar close to the ground also has benefits, particularly if you are searching by smell and want to keep a carcass to yourself. A keen sense of smell, coupled with a wing dihedral help makes turkey vultures what they are today, one of the most abundant and widespread of all raptors.
But of course the question then becomes “why don’t other vultures do the same?” The short answer is because they have not evolved these two key innovations. The long answer requires another blog.
You can find our paper on turkey vultures, “In-flight turbulence benefits soaring birds” on the Sanctuary’s website under “Numbered Publications” on our Science tab. Look for contribution to conservation science number 258.
Finally, if this work interests you, or any of our other research projects that better conserve raptors, please consider making a gift now. Just click here to donate now, writing “research” in the comments.