flight behavior of black and turkey vultures: implications for reducing bird–aircraft collisions

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FLIGHT BEHAVIOR OF BLACK AND TURKEY VULTURES:IMPLICATIONS FOR REDUCING BIRD–AIRCRAFT COLLISIONS

TRAVIS L. DEVAULT,1 Department of Forestry and Natural Resources, 195 Marsteller Street, Purdue University, West Lafayette,IN 47907, USA

BRADLEY D. REINHART, Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USAI. LEHR BRISBIN, JR., Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USAOLIN E. RHODES, JR., Department of Forestry and Natural Resources, 195 Marsteller Street, Purdue University, West Lafayette,

IN 47907, USA

Abstract: Mid-air collisions with black vultures (Coragyps atratus) and turkey vultures (Cathartes aura) regularly causesubstantial damage to military and civilian aircraft. Information concerning the flight behavior of black and turkeyvultures potentially could improve predictive models designed to reduce bird strikes by aircraft. We examined theflight behavior of black and turkey vultures at the Savannah River Site (SRS) in South Carolina, USA, and deter-mined whether flight characteristics were predictable with respect to weather and time variables. We captured birdsat their primary roost and subsequently relocated them via aerial telemetry from 11 February 2002 through 29 Jan-uary 2003. One hundred eighty of 326 locations (55%) for 8 black vultures and 129 of 206 locations (63%) for 5turkey vultures were of flying birds. Black vultures flew at an average altitude of 169 ± 115 (SD) m above groundlevel, whereas turkey vulture flight altitude averaged 163 ± 92 m. Our results contrast with those of previous studiesthat reported less frequent and lower altitude flights. The flight behavior of both species appeared to be influencedminimally by weather and time variables. However, we were unable to construct useful models predicting aspects offlight behavior using the variables we measured (all models had R2 or pseudo R2 values <0.10). We suggest thatother factors, such as food availability, inter- and intra-specific interactions, and physiological demands play a larg-er role in vulture flight behavior than the variables we measured. Our results suggest that the development of birdavoidance strategies by aircraft operators should consider the variability of flight behaviors of black and turkey vul-tures across their ranges. Future research emphases should shift from examinations of the effects of local conditionson flight behavior to the elucidation of factors contributing to differences in flight behavior among regions.

JOURNAL OF WILDLIFE MANAGEMENT 69(2):601–608; 2005

Key words: aviation, bird-aircraft collisions, black vulture, Cathartes aura, Coragyps atratus, flight behavior, military,movements, radiotelemetry, South Carolina, turkey vulture.

Between 1990 and 1998, there were an estimat-ed 22,000 collisions between birds and aircraft inthe United States; such collisions cost $400 mil-lion annually in aircraft repairs. Further, an esti-mated 350 people have been killed in bird-air-craft collisions worldwide (Sodhi 2002). Thus,understanding the causal factors contributing tobird-aircraft collisions and developing solutionsto reduce such collisions are critical challengescurrently facing wildlife managers, civilian avia-tion employees, and military personnel.

The threat of bird-aircraft collisions is especial-ly important for military operations because mili-tary aircraft often fly at higher speeds and loweraltitudes where birds are more likely to beencountered, while civilian aircraft fly at higheraltitudes (DeFusco 1993). Aircraft of the U.S. AirForce (USAF) incur over 2,500 bird strikes peryear (Lovell 1997), and since 1987, 5 USAF air-craft have been totally destroyed and 4 crewmenkilled (Arrington 2003). Although most bird

strikes occur in the airfield environment, a dis-proportionate number of strikes that cause sub-stantial damage to aircraft occur during mid-flight relative to the number of strikes incurredduring mid-flight (Dolbeer et al. 2000). Becausemany of the tactics used by modern aircraft placethe machine and crew in the same airspace at thesame time as large soaring birds, avoidance isthought to be the only viable long-term solutionto this problem (Arrington 2003).

To reduce these losses (i.e., human mortalitiesand monetary costs), the USAF developed a BirdAvoidance Model (BAM) to evaluate the risk ofbird collisions along low-altitude training routesthroughout the United States (Lovell and Dol-beer 1999). The BAM uses historical data on water-fowl, raptor, and vulture distributions to predictthe relative risk of flying predetermined routesthroughout the year. The BAM has been usedextensively by aircrews, flight schedulers, androute planners since its implementation in 1983(Lovell 1997), and it has been shown to success-fully predict the relative strike risk for the speciesincluded in the model (Lovell and Dolbeer 1999). 1 E-mail: [email protected]

J. Wildl. Manage. 69(2):2005602 VULTURE FLIGHT BEHAVIOR • DeVault et al.

Unfortunately, the BAM is a 2-dimensionalmodel and assumes that bird density does notchange with altitude. Thus, its reliability in pre-dicting bird strike hazards may vary with flightaltitude (Arrington 2003). If information on thealtitudinal flight behavior of the most problemat-ic species could be incorporated into the currentBAM, it could elucidate important patterns ofbird density and thus strike probability in thethird dimension and allow for increased flexibili-ty in route planning and training procedures.The inclusion of characteristics of flight behaviorinto the existing BAM would be especially desir-able if bird flight altitudes could be predictedfrom readily obtainable weather variables orother abiotic factors (Arrington 2003).

Black vultures (Coragyps atratus) and turkey vul-tures (Cathartes aura) are appropriate model spe-cies for investigating the possibility of incorporat-ing flight behavior into the existing BAM.Although smaller bird species such as gulls andblackbirds are struck most often by aircraft, vul-tures are the most important avian threat in termsof damage to aircraft and effects on aircraft oper-ations (Dolbeer et al. 2000). However, despite theabundance of black and turkey vultures through-out much of the United States, relatively little isknown about their flight behavior. Until recently(Arrington 2003), only qualitative descriptionsand anecdotal observations were available con-cerning temporal and spatial patterns of flight inblack and turkey vultures (Stewart 1978, Houston1988, Coleman and Fraser 1989, Buckley 1996).

Our primary objective was to characterize theflight behavior of black and turkey vultures at theSavannah River Site in South Carolina, a largeand heavily forested area where both species arecommon. We also sought to identify abiotic fac-tors that influence vulture flight behavior andexamine the extent to which flight behaviorscould be predicted based on the temporal vari-ability of such factors. Finally, we considered howefforts to reduce catastrophic bird strikes by air-craft might be enhanced by implementing knowl-edge of flight behavior and movement patterns oflarge soaring birds like black and turkey vultures.

STUDY AREAThe 78,000 ha Savannah River Site (SRS) is a

limited-access nuclear production and researchfacility located near Aiken, South Carolina. TheSRS is owned and operated by the U.S. Depart-ment of Energy. Approximately 64% of the SRS isplanted in loblolly pine (Pinus taeda), longleaf

pine (P. palustrus), and slash pine (P. elliottii;Workman and McLeod 1990), which are managedfor timber production by the U.S. Forest Service.An additional 15% of the land cover is classified asbottomland hardwood (Workman and McLeod1990). Although most of the SRS is forested, sev-eral industrial areas are located throughout thesite. Elevation ranges from 30 m above sea levelor less on the southwestern portion of the SRSnear the Savannah River to 115 m on the north-ern portion of the site (White and Gaines 2000).

METHODS

Field ProceduresIn 2000 and 2001, we captured 22 black vultures

and 12 turkey vultures, primarily by rocket net. Allindividuals were captured near R-Reactor, anabandoned nuclear reactor located approximately5 km east of the center of the SRS. R-Reactor hasserved as a major communal vulture roost formany years at the SRS. We weighed captured birdsand aged them as adult or juvenile based on thecolor and degree of wrinkling of the head (Kirkand Mossman 1998, Buckley 1999). We attached abackpack-style radio transmitter weighing ∼30 g(Holohil Systems Ltd., Ontario, Canada; modelAI-2B; 164-166 MHz) to each bird. Transmitterswere equipped with position sensors that enabledus to determine remotely whether radiotaggedbirds were oriented horizontally or vertically.Transmitters always emitted the horizontal signalwhen radiotagged birds were in flight. Transmit-ters on perched birds usually emitted the verticalsignal; however, occasionally the horizontal signalwas emitted during feeding or loafing.

A preliminary examination of telemetry errorat the SRS indicated that radio tracking fromground level was not feasible for such wide-rang-ing birds due to extensive forest cover and thelack of an extensive road network in some areas(White and Gaines 2000). Thus, we used fixed-wing aircraft (Cessna 172) equipped with a 2-ele-ment Yagi antenna on each wing strut to locatemarked birds and estimate flight altitudes(DeVault et al. 2003, DeVault et al. 2004). We con-ducted 84 telemetry flights, totaling 175 hrs, from11 Feb 2002 through 29 Jan 2003. One radio-tracker accompanied the pilot on all flights. Weattempted to distribute observation times evenlyover all daylight hours. Occasionally, however,flights were postponed due to rainfall and heavywinds; vultures were generally perched and inac-tive during such times.

J. Wildl. Manage. 69(2):2005 603VULTURE FLIGHT BEHAVIOR • DeVault et al.

During each flight, we systematically searchedthe SRS and a surrounding 15-km buffer zone tolocate as many birds as possible during the pre-arranged flight time (generally 2 hr). Upon detect-ing a radio signal, the observer indicated the gen-eral direction of that signal to the pilot. If thetransmitter emitted a vertical signal, we assumedthat the bird was perched. We flew over birds emit-ting vertical signals and marked their locationswith a handheld GPS unit (DeVault et al. 2003,DeVault et al. 2004). If the transmitter emitted ahorizontal signal (i.e., flying bird), we circled thesignal source widely, beginning at approximately1,000 m above ground level and descending slow-ly, until we visually located the bird. When welocated the bird visually, we flew at the same alti-tude as the bird and recorded its flight altitudeusing the GPS unit. We verified all altitude esti-mates generated by the GPS unit with the aircraft’sonboard altimeter, and we found that the esti-mates were always within 10 m. After we recordedaltitude, we flew directly over the marked bird andrecorded its location with the GPS. If we could notsee the target bird (emitting a horizontal signal)after searching ∼5 min, we first confirmed that thebird was not above the airplane by tilting the wingtips and monitoring signal strength, and we thenproceeded to locate the bird under the assump-tion that it was perched. We did not locate indi-vidual birds more than once per telemetry flight.

Weather DataWeather data were generated by the Westing-

house Savannah River Company’s SavannahRiver Technology Center, Atmospheric Tech-nologies Group. Air temperature (°C), soil tem-perature (°C at 2.5 cm depth), solar radiation(W/m2), vertical wind turbidity (SD), horizontalwind turbidity (SD), and wind speed (km/h)were recorded by an automated weather stationlocated near the center of the SRS every 15 min.Wind turbidity measurements were standarddeviations calculated from individual 1-sec read-ings recorded by bi-directional wind vanes over15-min intervals; they represented the magnitudeof fluctuation (turbidity) about an interval. Meanrelative humidity (%) was averaged for all read-ings at the weather station during 1 day; mini-mum and maximum relative humidity (%) repre-sented the extreme values from all readings atthe weather station during 1 day. We assignedweather data to each vulture location estimatefrom the 15-min time interval that was closest tothe time that the location (and altitude) estimate

was generated. Thus, each vulture location esti-mate was accompanied by weather measurementsthat reflected conditions on the SRS within 7 minof time the estimate was made (except for relativehumidity measurements).

AnalysesEleven black vultures and 6 turkey vultures

apparently left the vicinity of the SRS shortly aftertrapping. Some birds captured during the winterlikely were migrants that returned to more north-ern locations in the spring. Other birds may haveused the R-Reactor only as a secondary roost. Addi-tionally, 2 radiotagged black vultures and 1 radio-tagged turkey vulture died from unknown causesduring the study. We thus restricted our analyses to8 black vultures and 5 turkey vultures that we locat-ed ≥20 times throughout the tracking period.

Because our primary objective was to character-ize flight behavior, we used a variety of descriptivestatistics concerning flight altitudes and time spentflying as opposed to perching. In addition tomeans and SDs, we present maximum flight alti-tudes and the proportion of in-flight locations>150 m above ground level. USAF low-level trainingroutes are usually flown <150 m (Arrington 2003);thus, it is important to characterize vulture flightwith respect to this altitude. We also were interest-ed in describing variability in flight behavior acrossthe year, among individuals, and with respect touse of the R-Reactor (DeVault et al. 2004).

Pearson correlation analyses (SPSS 1999)revealed a considerable degree of correlationamong the weather variables (e.g., 64% of the 36pairwise comparisons were significant at P <0.001; 17% of the r statistics were >0.4). Thus, weconducted a principal components analysis witha varimax rotation on the weather variables toreduce dimensionality (SPSS 1999). Three com-ponents were generated with eigenvalues >1; weretained these for subsequent analyses.

We evaluated whether time of year, time of day,or the principal components generated fromweather variables influenced flight behavior ofeither species. We asked 2 primary questions: (1)which variables influenced the probability ofmarked vultures being perched or in flight (atany altitude), and (2) for flying birds, which vari-ables influenced their flight altitude? We com-bined locations of all individuals by species andconducted analyses separately for each species.To determine whether our measured variablesinfluenced the probability of marked vulturesbeing perched or in flight, we conducted logistic


regression analyses (Norušis 1999) with a binaryresponse variable (perched or flying).

We evaluated all possible models using 5 pre-dictor variables (Julian date, hours before sunset,and 3 principal components generated from theweather variables) with the Akaike’s InformationCriterion (AIC; Akaike 1973). AIC values are use-ful for identifying the most parsimonious modelsthat accurately predict the response variable(Burnham and Anderson 2002). To determinewhether our measured variables influenced theflight altitude of either species, we conducted lin-ear regression analyses using only the locations offlying birds. The response variable in the modelswas flight altitude above ground level (m), andthe predictor variables were the same as in theformer analyses. We also evaluated all possiblemodels of flight altitude with AIC. We used SPSSversion 10.0 (SPSS 1999) for all statistical analyses.

RESULTSWe recorded a total of 532 locations for 13 vul-

tures (8 black and 5 turkey vultures) during thetracking period (Table 1). One hundred eighty of326 locations (55%) for black vultures and 129 of206 locations (63%) for turkey vultures were offlying birds. Considering only locations of flyingbirds, mean flight altitude for black vultures was169 m (SD = 115) above ground level, and themaximum altitude for black vultures was 558 m.Mean flight altitude for turkey vultures was 163 m

(SD = 92), and the maximum altitude for turkeyvultures was 540 m.

Of 180 in-flight locations for black vultures, 94(52%) were over 150 m, while 63 of 129 (49%) obser-vations of flying turkey vultures were over 150 m.Within 50-m altitude bands, black vultures weremost often located in the 151- to 200-m category,whereas turkey vultures were most often located inthe 101- to 150-m category (Fig. 1). There were fluc-tuations in flight altitudes across the year for bothspecies; however, no trend was apparent (Fig. 2).

We documented a substantial level of individualvariation in flight behavior (Fig. 3). For example,the proportion of locations in the perched cate-gory for individual birds ranged from 18% (Indi-vidual E) to 94% (Individual D). Some of the vari-ation in flight behavior between individuals wasapparently due to the home range and roost loca-tions of individual birds. The home ranges ofindividuals E, G, and K did not include R-Reactor(DeVault et al. 2004), and these 3 birds (alongwith individual Y that had too few locations forhome-range analysis; DeVault et al. 2004) had thehighest proportions of locations in the >150-mcategory and the lowest proportions of locationsin the perched category (Fig. 3).

Principal components analysis produced 3component variables with eigenvalues >1,explaining 75% of the variance in the original 9variables (Table 2). WEATHER 1 had high posi-tive factor loadings for the 3 measurements of

Table 1. Flight characteristics of black and turkey vultures captured at the Savannah River Site, South Carolina, and tracked viaaerial telemetry 11 Feb 2002–29 Jan 2003. Mean, SD, and maximum refer to flight altitudes (excluding locations of perched birds)above ground level (m).

Total In-flightIndividuala Ageb locations locations Mean SD Maximum

Black vulturesA Adult 50 32 98 109 491B Adult 42 17 132 77 262C Adult 47 28 162 93 316D Adult 35 2 227 258 410E Adult 40 33 189 90 397F Adult 47 27 176 121 552G Adult 45 30 224 133 558X Juvenile 20 11 208 116 378Total 326 180 169 115 558

Turkey vulturesH Adult 47 24 157 94 354I Juvenile 51 30 170 108 540J Adult 36 18 171 68 310K Adult 50 39 163 91 341Y Adult 22 18 151 87 354Total 206 129 163 92 540

a Individuals A-K (those with >30 locations) were used for home-range analyses in DeVault et al. (2004).b Determined by color and degree of wrinkling of the head (Kirk and Mossman 1998, Buckley 1999).


humidity; WEATHER 2 had high positive factorloadings for air temperature and soil tempera-ture; and WEATHER 3 had high positive factorloadings for vertical wind turbidity, horizontal wind

turbidity, and a high neg-ative factor loading forwind speed. Solar radia-tion was moderately cor-related (negatively) withWEATHER 1 and (posi-tively) with WEATHER 2.

The best model (asindicated by AIC values)from binary logisticregression analyses indi-cated that the probabilityof flying versus perchingwas influenced byWEATHER 2 (i.e., highair, soil temperatures)for black vultures;although, the model hadan R 2 value of only 0.093(Table 3). The bestmodel for turkey vulturesincluded WEATHER 3(i.e., high turbidity, lowwind speed) as a predic-tor variable but was not

statistically significant (P = 0.254). The linearregression models identified WEATHER 2, hoursbefore sunset, and Julian date as variables influ-encing flight altitude for locations of flying birds

(Table 4). However, likethe logistic regressionmodels, the linearregression modelsaccounted for only lowamounts of variation inthe dependent variable(R 2 = 0.076–0.086).

DISCUSSIONVultures exhibited very

different movement pat-terns and flight behav-iors than those reportedfrom other regions. Forexample, we observedblack and turkey vul-tures at the SRS flyingmore frequently thanwas reported previously(DeVault et al. 2004). InPennsylvania and Mary-land, black and turkeyvultures spent only9–12% and 27–33% of

Fig. 1. Flight altitudes of black and turkey vultures at the Savannah River Site, South Caroli-na, USA. Only locations of flying birds are depicted here. Data were gathered via aerialtelemetry 11 Feb 2002–29 Jan 2003.

Fig. 2. Mean flight altitudes for 1 year for black vultures and turkey vultures at the SavannahRiver Site, South Carolina, USA. Numbers above letters signifying months indicate samplesizes (number of in-flight locations) for black vultures (BV) and turkey vultures (TV). Data weregathered via aerial telemetry 11 Feb 2002–29 Jan 2003.


daylight hours flying, respectively (Coleman andFraser 1989). Likewise, turkey vultures spent only26% of daylight hours in flight in Indiana andOhio (Arrington 2003). These figures contrastsharply with those from our study, where 55% ofthe locations for black vultures and 63% of thelocations for turkey vultures were of flying birds.It is generally well accepted that black vulturesspend substantially less time flying than turkeyvultures (Stewart 1978, Coleman and Fraser 1989,Buckley 1996), partly due to their anatomy. Blackvultures have a higher wing-loading than turkeyvultures and must flap their wings more often tostay aloft (Buckley 1999); thus, they are less ener-gy-efficient in the air.

We also found that flight altitudes of SRS vultureswere much different than those reported fromother regions. We found that black and turkey vul-tures exhibited similar flight behaviors, and bothspecies often were located at altitudes >150 mabove ground level (Figs. 1, 3). However, existingliterature suggests that black vultures normally flyat substantially higher altitudes than turkey vul-tures and that turkey vultures usually are foundsoaring below 150 m. For example, in southernTexas, most black vultures (∼60%) foraged at alti-tudes greater than 60 m, whereas most turkey vul-tures (∼55%) foraged below 30 m (Buckley 1996).In Venezuela, 92% of Cathartes vultures (turkey vul-

ture, lesser yellow-head-ed vulture [C. burro-vianus], and greater yel-low-headed vulture [C.melambrotus]) were ob-served flying below 150m, whereas 82% of blackvultures were observedabove 150 m (Houston1988). In Mexico, mostturkey vultures (84%)were observed flying atless than 100 m (Estrella1994), and in Indianaand Ohio, turkey vulturesspent nearly all (97%) oftheir time in flight below150 m (Arrington 2003).Thus, SRS vultures, espe-cially turkey vultures,appear to fly at higheraltitudes than thosereported elsewhere(DeVault et al. 2004).

Why did the vultures weobserved behave differently with regard to theirflight behavior than those in other regions?Undoubtedly there are many factors that influence

Fig. 3. Proportional flight altitudes for individual black vultures (A-G and X) and turkey vultures(H-K and Y) at the Savannah River Site, South Carolina, USA. Data were gathered via aerialtelemetry 11 Feb 2002–29 Jan 2003.

Table 2. Principal components analysis of weather measure-ments recorded at the Savannah River Site, South Carolina,USA, 11 Feb 2002–29 Jan 2003. Varimax rotation was imple-mented after components were extracted to facilitate interpreta-tion of the components. The 3 components (WEATHER 1,WEATHER 2, WEATHER 3) combined explained 74.61% of thevariance in the weather variables. Boldface type highlights vari-ables with high factor loadings (≥0.70) for individual components.

WEATHER 1 WEATHER 2 WEATHER 3

Eigenvalue 2.76 2.54 1.42 % variance

explained 30.61 28.56 15.74 Factor loadingsa

RELHUM 0.96 0.05 –0.02 MINHUM 0.84 0.02 0.01MAXHUM 0.79 0.00 0.05AIRTEMP 0.06 0.94 0.12SOILTEMP 0.04 0.95 0.11SOLRAD –0.48 0.47 0.29HORZTURB –0.09 0.12 0.87VERTTURB –0.14 0.17 0.82WINDSPD –0.34 –0.04 –0.70

a RELHUM = mean daily relative humidity; MINHUM = min-imum daily relative humidity; MAXHUM = maximum daily rela-tive humidity; AIRTEMP = air temperature; SOILTEMP = soiltemperature; SOLRAD = solar radiation; HORZTURB = hori-zontal wind turbidity; VERTTURB = vertical wind turbidity;WINDSPD = wind speed. See Methods for a more detaileddescription of weather variables.


the flight behavior of vultures, such as weather con-ditions and the composition of animal communities(and thus the nature of carrion resources). Howev-er, as is the case with home-range size (DeVault et al.2004), the differences in flight behavior among vul-tures that we examined and those reported fromother studies (Houston 1988, Coleman and Fraser1989, Arrington 2003) probably reflect variations inhabitat structure, and thus feeding opportunities,between regions. For example, black and turkey vul-tures in Pennsylvania and Maryland primarily usedcarcasses of domestic animals generated by thenumerous cattle, hog, poultry, and dairy farmslocated throughout the mostly agricultural land-scape (Coleman and Fraser 1987). Conversely, feed-ing opportunities at the SRS, which is ∼80% forest-ed, are almost certainly fewer and less predictable.To forage adequately, SRS vultures apparently mustrange more widely (DeVault et al. 2004) and soarmore often than those in agricultural landscapes.

Although a difference in habitat structure pro-vides a parsimonious explanation for the tendencyof vultures at the SRS to fly more often than thosein other regions, the factors contributing to thetendency of SRS vultures to fly at higher altitudesthan those studied elsewhere are less obvious. Onepossible explanation concerns the size and con-spicuousness of carrion used by vultures. In Mexi-

co, Estrella (1994)reported that turkey vul-tures tended to forage athigher altitudes in areaswhere carcasses weremore likely to be largeand conspicuous, sug-gesting that low-altitudeflight of turkey vulturesmay be most efficient forfinding small carcasses byolfaction (Stager 1964,Estrella 1994) and thatlarger carcasses are bestlocated by vision fromhigher altitudes. Anecdo-tal observations at theSRS suggest that vulturesthere make extensive useof large road-killed ani-mals (especially white-tailed deer, Odocoileus vir-ginianus).

WEATHER 2 (hightemperature) appearedto influence flight behav-

ior more than the other variables we considered(Tables 3, 4). Temperature (and solar radiation)contributes to thermal column development thatallows for low-cost soaring flight of vultures andother large birds (Kerlinger 1989). Arrington (2003)also found a weak positive relationship betweenflight altitude and air temperature. However, as inArrington’s (2003) study, none of our measuredweather variables appeared to influence flightbehavior of black or turkey vultures to a substantialdegree. Furthermore, we documented noticeablevariation in individual flight behaviors (Fig. 3). Thissuggests that temporal changes (daily or seasonally)in weather conditions at the SRS did not substan-tially alter flight behaviors of resident vultures, eventhough changes in flight behaviors across regionsare apparent. Instead, factors such as daily foodavailability, chick rearing, inter- and intra-speciesinteractions, and physiological demands might playa larger role in vulture flight behaviors than do theweather variables we measured.

MANAGEMENT IMPLICATIONSA comparison of this study to earlier research

(Coleman and Fraser 1989, Arrington 2003)demonstrates that vultures are able to adjust theirmovement and flight behaviors to best cope withlocal conditions, especially those related to habitat

Table 3. Binary logistic regression models of vulture flight behavior (perched or flying) at theSavannah River Site, South Carolina, USA. The models reported here were the best modelsfor each species as indicated by AIC values; they were selected from all possible models using5 predictor variables (WEATHER 1, WEATHER 2, WEATHER 3, hours before sunset, Juliandate). See the text for further description of weather variables. Data were gathered via aerialtelemetry 11 Feb 2002–29 Jan 2003.

Species Nagelkerke R2 Pa % Correctb Variable Coefficient SE

Blackvulture 0.093 <0.001 61.3 WEATHER 2 –0.553 0.121

Turkeyvulture 0.009 0.254 61.6 WEATHER 3 –0.168 0.149

a Evaluated by the model χ2.b % Correct indicates the number of locations (cases) correctly predicted by the logistic

model as flying (zero) or perched (1).

Table 4. Linear regression models of vulture flight altitudes (m above ground level) at theSavannah River Site, South Carolina, USA. The models reported here were the best modelsfor each species as indicated by AIC values; they were selected from all possible modelsusing 5 predictor variables (WEATHER 1, WEATHER 2, WEATHER 3, hours before sunset,Julian date). Only locations of flying birds were used in these analyses. See the text for fur-ther description of weather variables. Data were gathered via aerial telemetry 11 Feb2002–29 Jan 2003.

Species R2 P Variable Coefficient SE

Black 0.086 <0.001 WEATHER 2 21.422 9.996vulture Hours before sunset –14.444 3.740

Turkey 0.076 0.009 WEATHER 2 15.815 8.660vulture Julian date 0.177 0.082


structure and foraging opportunities. Thus, thedevelopment of bird avoidance strategies by aircraftoperators should consider the variability of flightbehaviors of black and turkey vultures across theirranges. For example, in Indiana, Ohio, andVenezuela, 92–97% of turkey vulture locations werebelow 150 m (Houston 1988, Arrington 2003).However, aircraft operators should not assume thatthe risk of striking a vulture is always lower whenthey fly above 150 m because our study demon-strated that vultures potentially can be encounteredregularly at altitudes above 150 m in some regions.

Future research emphases should shift fromexaminations of variation in local conditions onflight behavior (Arrington 2003, this study) to theelucidation of factors contributing to differences inflight behavior among regions. Possible avenues ofsuch research include inter-regional studies of for-aging strategies, diet preferences, home-range char-acteristics, and the use of communal roosts. Inter-regional studies of vulture spatial ecology shouldprovide information that leads to a better overallunderstanding of vulture biology, and ultimately,to improvements in models of bird avoidance.

ACKNOWLEDGMENTSWe thank the staff at Augusta Aviation for their

expert piloting. This project was funded by con-tract DABT63-96-D-0006 between Purdue Universi-ty and the U.S. Air Force Bird/Wildlife AircraftStrike Hazard Team and by the EnvironmentalRemediation Sciences Division of the Office of Bio-logical and Environmental Research, U.S. Depart-ment of Energy, through Financial AssistanceAward No. DE-FC09-96-SR18546 to the Universityof Georgia Research Foundation. A. L. Bryan, D. P.Arrington, W. L. Stephens, K. F. Gaines, and R. A.Kennamer graciously provided logistical support.

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Associate Editor: Bechard.

flight behavior of black and turkey vultures: implications for reducing bird–aircraft collisions

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