From 2000 to 2002, the Tehachapi Mountains Birding Club conducted a census of Turkey Vultures migrating east along the Tehachapi Valley flyway corridor. Moving to their winter ranges, normally 30,000 to 40,000 Turkey Vultures pass overhead during September and October. Full census statistics are presented.
Count Background and History
Turkey Vulture Count that never wasOver the years, sightings by residents, birders and field biologists have indicated that tens of thousands of turkey vultures regularly use the relatively low elevation of the Tehachapi Grade as a passage on their migrations to seasonal ranges. Drawing on the experience and protocols of the Southern Sierra Research Center (which has been conducting bird migration counts since 1993), the TMBC began its own program in the fall of 1999, selecting the turkey vulture's winter migration path when the birds come through the area in the greatest concentration.
The count traditionally goes from Sept. 1st to October 20th; in the first year 20,042 turkey vultures were counted during that period. The following year 38,953 vultures were counted, and in 2001 31,168 were counted. The 2002 count recorded 36,435 migrating Turkey Vultures. We lost access to the Buzzard Bluff count site and are unable to find another site. Therefore the Turkey Vulture count was canceled in 2003.
Years 2000-2002 Summary
FALL MIGRATION OF TURKEY VULTURES THROUGH THE TECHACHAPI MOUNTAINS, CALIFORNIA
Each fall for ten years the Southern Sierra Research Center (SSRC) located in Weldon, Kern County California, has conducted a census of Turkey Vultures migrating through the Kern River Valley. The SSRC had in turn proded the Tehachapi Mountains Birding Club to also census the birds which were known to be migrating along another corridor which leads through the Tehachapi Valley.
Finally in the fall of 1999 such a project was intiated. Though the SSRC protocal was used as a guide, and although 19,000 TUVU’s were recorded, the project needed an overhaul. Building on the 1999 count experience, new rules and protocals were adopted. For the next three years a census was carried out. Through the effort and dedication of some 20 volunteer observers, these three counts were carried out in a very professional manner, and enjoyed by all the participants.
A fourth year was planned with observers looking forward to another year on the “Hill”. However, due to a conflict in the use of the count site, no census was carried out in 2003. At this point it is not known whether the Tehachapi Turkey Vulture Census will resume in subsequent years.
Turkey Vultures (Cathartes aura) regularly use the relatively low elevation of the Tehachapi Valley as a passage on their journey to winter ranges. For three years (years 2000, 2001, and 2002) the Tehachapi Mountains Birding Club (TMBC) undertook a census of migrating Turkey Vultures exploiting the Tehachapi Valley corridor. Beginning in late August and early September migrating TUVU’s are observed moving over the Tehachapi Valley, and continue to be seen along this corridor well into November. These three counts were conducted from September 5th through October 20th.
The count site was located within the city limits of Tehachapi north of Tehachapi Blvd., south of Hwy. 58, just east of the Tehachapi water treatment plant, west of the Tehachapi News office, above Ave. J on a treeless hill (elevation 4000 feet) at coordinates N 35 degrees 8.135 minutes W 118 degrees 27.336 minutes. From this vantage point, 200 feet above the immediate valley floor, the site provided a 360-degree view of the entire Tehachapi Valley (approximately 12 miles long from west to east and 4 miles wide north to south), and the mountain peaks and ridge lines surrounding the valley. From the junction of State Highways 223 and 58 a broad canyon leads to the Tehachapi Valley. Having gained 2500 feet in elevation, the canyon narrows to a half-mile width, then abruptly widens as it enters Tehachapi Valley in a south-easterly direction three miles northwest of the count site.
A set of instructions and protocol appears in the appendicies. The site was manned every day from 0800 to 1400 and occasionally as late as 1500 and as early as 0730. Most days there were 3 or more observers present. All Twenty observers were experienced from a 1999 trial count. Temperature, wind velocity and direction as well as per-cent of cloud cover were recorded hourly.
Turkey Vultures. A total of 107,012 Turkey Vultures were recorded for the three counts, averaging 35,671 per year. Per count day 775 TUVU’s were observed, averaging 129 vultures per hour. The peak hour was consistently 1100 to 1200, averaging 288 birds per hour. In the appendix of this report the reader will find charts and graphs summarizing this three-year study. Individual reports are available for each of the three years from the authors of this summery report.
Eighty percent of the birds come through during the last week of September and the first week of October. On the 2002 count seventy per-cent of the vultures recorded were observed during the twenty days of October. “Big Days” were defined as days when 1,000 or more birds were recorded. Thirty-six “Big Days” occurred with the biggest day being on October 2nd , 2001 when 5273 vultures were recorded.
Raptors and other species. Raptor migration along this corridor is well under way before September 1st. Further, preferring to follow the ridgelines, the count site was far from ideal for observing birds-of -prey migration. Therefore relatively few birds-of-prey were seen. However, birders being birders, the observers could not resist recording their sightings. Eight species of raptors were noted over the three years totaling 191 birds. In addition, seventy-seven bird species were noted from the count sight over the three years.
The greater Tehachapi area is also a corridor for migrating American White Pelicans (Pelecanus erythrorhynchos). Each year casual sightings from the Tehachapi area average 1200 pelicans. Kettle sizes are usually three to four hundred birds. In late September of 2003 a magnificent flight made up of three or perhaps four kettles totaling 1200 to 1500 was observed.
Weather. Three rainout days occurred in October of the 2000 count. In the fall the prevailing winds are both from the northwest and the east. That is, up-slope or down slope winds. This time of year the west wind days and the east wind days are about evenly distributed over a count period. Temperatures varied from the forties in the AM, into the eighties as the day progressed.
Overnights. Only about fifteen per-cent of the migrating TUVU’s roost overnight in the study area. When birds stop-over, the group usually numbers in the 200 range. However, from time to time, as many as 1000 vultures roost overnight in the Tehachapi Valley during their migration. Roosting occurs largely in downtown Tehachapi (City Park) and Mourning Cloak Ranch in the Golden Hills area. As the counts continued from year to year it appeared the number of “overnighters” was declining.
Strategies. Strategies used by the vultures were discussed in all the previous reports. All that needs to be noted here is that these soaring/gliding long-distance migrating birds have a strategy to work with, or to counter, all wind directions, wind velocities, and weather conditions (except heavy rain when they just do not fly) in order to complete their journey to their winter ranges.
Altitude. From several low cloud ceiling (5,000 to 6,000) days, plus reports from a GA pilot and a sailplane pilot, a judgment as to altitude of the birds as they passed over was made during the 2002 count. There were times when the birds were no more than 500 to 1,000 feet above the count site (countable without magnification), while at other times they may have been 3,000 to 4,000 above the count site (7,000 to 8,000 altitude). Such birds are visible only with 10X glass. If the vultures come through at an elevation higher than 8,000 to 10,000 feet, the observers may miss the birds, for they have no way of knowing that the birds are over the valley.
Each fall large numbers of Turkey Vultures exploit the relatively low Tehachapi Valley as a route to cross over the Tehachapi Mountains on a northwest to southeast course. The “Tehachapi” birds use a number of strategies to negotiate the variable wind currents (thermals, declivity currents and upper wind conditions, northwest to southeast winds, east to west winds) to negotiate their passage through the Tehachapi Valley (“Wind Capitol of the World” ).
Early Morning Take-offs. Lift-off in the mornings typically depends on wind. In the Tehachapi Valley the first morning hours are still. As the earth warms and the winds begin to stir, “overnighters” leave their roosts, “mill-about”, then finally, catching the forming thermals, kettle, and then are on their way usually by 0900 to 0930. However, on cold, strong, east-wind mornings the birds are up and gone early –often by 0730! This behavior has also been noted on the Weldon counts. Although this behavior is also clearly wind related, it does appear to be a departure from the norm.
Geographical Populations. Like tributaries to a large river, we think this annual flow of birds collects, then proceeds south along the east foothills of the Cascade and Sierra Nevada ranges, growing in numbers as the vultures approach their historical mountain crossings. However, it is not known from where these many TUVU populations geographically originate - California’s great Central Valley (San Joaquin and Sacramento Valleys), Oregon, Washington, or even British Columbia?
Route choice. Do the same birds come through Tehachapi each year? Likewise, do the same vultures come through Weldon each year? Route choice does not seem to be weather related. Gentic? Family group prefernce? Tradition? Further, is there a third corridor? A number of observations indicate a possible 3rd corridor. This route could be by-way of Tejon Canyon (or other canyons to the southwest of the Tehachapi Valley) leading into the west end of the Antelope Valley.
Migration Timing. What initiates the migration movement? Length of day? Weather? A combination of these two? It is generally understood that, especially with TUVU’s, the dwindling number of hours for foraging each day triggers migration. In the fall of ’02 the weather in California, as well as the northwest, was remarkably mild. It may have been that many September birds came through in October as well as on into November. Perhaps from time to time, in addition to the shortening of the days, the birds need a nudge from changing fall weather patterns.
Fidelity To Routes And Stops. The birds pass through in varying hours of the day, and overnight roosting does not occur every night. Thus there must be a number of stops along the route. When 3,000 to 4,000 birds have gone through during an 11 o'clock hour, where did they roost last night and where will they roost that night? Also, does “leap-froging” occur? Since TUVU’s are seen along the coasts and in the Sacramento Valley during winter, as the flow moves south do some birds stop while others continue on to other wintering sites? The fidelity to routes and stops seems remarkable. However, do we really know this to be the case with Turkey Vultures?
Field Studies needed. It is apparent that until these questions are addressed by scientific field studies, all we will really know is that which we now know, that being: big-bunches of Turkey Vultures cross over the Tehachapis and Weldon each fall.
Turkey Vultures migrate through the Tehachapi Valley each fall in large numbers. On average, close to 35,000 Turkey Vultures are recorded passing over the Tehachapi Valley each fall. This tracts well with the Weldon (southern Sierra) count which averages some 30,000 annually during the same period. Thus, some 65,000 Turkey Vultures exploit these two relatively low mountain passes during their migration to winter ranges.
Thanks to the Tehachapi Mountains Birding Club members whose skill and dedication made this a successful endeavor. Thanks to Judy and John Rombouts for use of the count site (Pauley Hill – AKA “Buzzard Bluff”). Special appreciation goes to the Tehachapi News for the support, publicity and for the use of a nearby necessary. We also thank Linda Crisalli for her editing in preparing this report.