|The Horaltic Pose|
|Written by Clark & Jean Moore|
It’s August, and within a month, thunderbird “want-to-be’s” will be drifting overhead. Be especially vigilant the last week of September and the first week of October – for then the Turkey Vultures will be seen in big numbers.
One of the many thrills during the TMBC Turkey Vulture Count days was observing the early morning departure of the vultures from their downtown overnight roosting sites. Depending on that morning’s wind conditions, these fly-outs would take place from 0730 to 0900, and were normally preceded by the “spread-winged posture.”
Knowing of our interest, Ramona VanRipper of the Turkey Vulture Society recently found a research piece on this subject and forwarded it to us. Robert G. Clark and Robert D. Ohmart, the researchers, proposed that the TUVU wing-spread is an energy-conserving behavior by which the bird warms itself by absorbing solar radiation.
At dawn, for the 18 days of their study, most birds would move from their overnight roost to nearby cliffs. Some, though, remained in the trees until sunrise, while a few did not move before fly-out. At sunrise, sunlight shone directly on the vultures. Thirty minutes before sunrise until ten minutes after departure was designated the “post-roost period.”
Here is the study abstract. “The behavior and spread-winged postures of Turkey Vultures (Cathartes aura) were examined at a communal roost near Superior, Arizona, from April to June, 1979. The temporal occurrence of wing-spreading during the morning pre-departure period was positively correlated with the intensity of the sun’s rays, but was independent of the ambient low overnight temperature. Wing-spreading was more common when vultures were wet than when they were dry. Turkey Vultures appear to spread their wings for at least two reasons: (1) to dry feathers, and (2) to ameliorate the thermal gradient between themselves and their environment - although the two functions are not mutually exclusive. Spreading the wings to realign feathers or to increase the mobility of ectoparasites seems unlikely.”
Tehachapi TUVU Migration
Historically, in the fall, large numbers of Turkey Vultures (Cathartes aura) are seen passing over the Tehachapi Valley, Kern County, California. Random observations by biologists in the field, as well as valley residents, indicated that perhaps 6,000 birds migrated through. Encouraged by club members, and Southern Sierra Research Center researchers Teri Gallion and Sean Rowe, who at the time were studying vulture migration along a separate corridor through the Kern River Valley, Weldon, Kern County, California, TMBC undertook to study this spectacular event over the Tehachapi Valley.
Adapting the Southern Sierra Research Center protocol, an observation count site with a 360-degree view of the valley was established. Each fall for fours years, from September 5th to October 20th, the site was staffed with TMBC volunteers. Within a half mile an overnight roosting site was clearly visible by bino and scope. Each day a count team set up shade cover and scopes, assembled chairs and tables, and laid out a multitude of recording forms. Migrating kettling, soaring and gliding vultures, as well as raptors, were counted as they crossed the count line. In addition, temperature, cloud cover, wind velocity and direction were recorded every hour on the hour. Imagine the surprise and excitement when it was found that annually, on average, 35,000 vultures pass along this route! In addition, the Weldon Count also records over 30,000 moving along a separate route. Thus, over 65,000 Turkey Vultures exploit the lower elevation southern Sierra passes to cross these mountains on their way to winter ranges. Of the 35,000 vultures passing by each fall, no more than 20% roosted overnight in the Tehachapi valley. Of those, perhaps half chose the City Park and downtown roosting site. Thus, less than 4,000 birds roosted overnight within view of the count site.
As we remarked last month, that for many years we had conjectured on the adaptive survival benefits of the Turkey Vulture’s spread-winged posture – that is, the Horaltic Pose. Although we have heard studies of this behavior are ongoing, Clark and Ohmart’s professional study was well done with their conclusions understandable and acceptable.
Following a rain, or having bathed, drying off would seem necessary for vultures. Species that in addition swim and dive (cormorants, anhingas, gulls) also spread their wings to dry off. Clark and Ohmart discount parasite control – hmmm? Also conserving energy by balancing body temperature with that of the environment makes survival fitness sense.
Observations From Buzzard Bluff
Averaging 1,000 birds a day (there were big days of several thousands) 80% of the vultures were recorded moving through from 1000 hours to 1400 hours. By late afternoon though, new arrivals would be milling-about over two roosting sites used during their migration – Mourning Cloak Ranch, three miles west of the count site, and City Park.
Since only occasionally did migrating Turkey Vultures roost overnight in central Tehachapi, perhaps ten or twelve of the thirty-five to forty-two day count window, the opportunity to observe their behavior, including the spread-winged posture, before departure, was of special interest.
Tipped off by city residents, it was usually known the previous evening that there were birds downtown. As a vulture wobbles and drifts, City Park is less than a half-mile from the count site. On these mornings the count site had to be staffed early, for depending on the weather and the bird’s individual judgment, they might depart by 0730.
All behaviors recorded by Clark and Ohmart were noted by observers. Before the sun’s rays hit the birds, many would move to different roost trees. With the sun’s arrival, preening, loafing, short flights, and the spread-winged posture would take place. Near the end of the post-roost period, milling-about signaled that departure was imminent.
This milling-about takes place around the general roost site at treetop level. Suddenly, as if the “head vulture” had commanded, groups of birds would start streaming to open grasslands slightly to the west and at the base of the Tehachapi Mountains. There, even before all the birds had left the roost site, kettling would begin.
“Wind Capitol of The World”
Some 7500-wind turbines picket the valley’s eastern arc of hills. “Wind Capitol of The World” is Tehachapi’s self-proclaimed banner. The mill blades do not move without wind – and neither do the vultures. Whether the winds are caused by deflection currents, thermals, or dry fronts from the west or down-slope out of the east - no wind, no fly.
Common thought is that this area’s prevailing winds are westerlies. In the fall though, as recorded during the count window, yes, 60 per-cent of the time wind is out of the west. However, 40 per-cent of the days are easterlies! Therefore, during the count window, and indeed the entire fall, a bird’s migrating strategy must adapt to variable wind conditions.
Occasionally early AM strong, cold, down-slope winds would pour out of the east during the count period. This causes an anomaly noted both in Weldon and Tehachapi. Roosting migrants would curb pre-flight activity, including the spread-winged posture. That is, they are off and gone by as early as 0730 hours with little post-roost period behavior!
The Horaltic Pose
Team observations, as in Clark and Ohmart’s study, noted that only a fraction of the vultures responded to the sun by posing and that posing time was relatively short. Further, posing appeared to be as related to departure time as it was to the increasing intensity of the sun.
Neither the sun’s intensity, nor air temperature, seemed to trigger pre-flight behavior. However, the developing air currents, which had been influenced by the sun’s increasing intensity, did appear to cause increased activity including the spread-winged posture.
Scattered among the contour feathers, filoplumes lack feather muscles. However, they do have sensory receptors in the skin next to the follicles. Since feathers are dead structures, there is a need to monitor feather goings-on. In addition to these sensors, there are other receptors in the skin away from the filoplumes.
As all sailplane and hot-air balloon pilots know and do, they must keep in touch with the wind and currents. When the vultures pose, are the birds awaiting and picking up sensory signals as to flying conditions? No wind, no fly, and thus no premature flapping. Waiting for proper wind conditions certainly would be an energy conserving behavior adaptation.
Well, whatever the cause, the great thunderbirds spread their wings and pose. “Now Junior, listen-up! There are no food stops, and it’s a long flight from Victoria to Mexico. So don’t flap those wings, do not leave too early – soar, soar, soar and glide, glide, glide - follow us, and always check your instruments.”