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IBIS 1 3 8 : 667-674

The use of thermals by soaring migrants YOSSI LESHEM’ 81YORAM YOM-TOVZ.* Society for the Protection of Nature in Israel, 4 HaShfellu Street, Tel Aviv, lsrael Department of Zoology, Tel Aviv University, Ramat Aviv 69978, lsrael

The use of thermals during the spring and autumn migration across Israel by four species of soaring birds (White Pelican Pelecanus onocrotalus, White Stork Ciconia ciconia, Lesser Spotted Eagle Aquila pomarina and Honey Buzzard Pernis apiworus) was studied by monitoring them with a motorized glider, light aircraft and radar. This is the first study in which soaring migrants have been followed in flight for any length of time and their flight performance has been recorded directly. The buds flew in a n average height band between 344 and 1123 m above ground level. Altitude increased from the morning towards noon and decreased again in the afternoon. Average velocities were 29.2 km/h. 38.7 km/h, 50.9 km/h and 45.2 km/h for White Pelicans, White Storks, Lesser Spotted Eagles and Honey Buzzards, respectively. Atmospheric conditions had a major effect on flight velocity. White Storks showed a positive correlation between the flight velocity and the height between the base and top of the thermals. In White Pelicans, there was a correlation between velocity and mean height, Wing load (body m a d w i n g area) was positively related to the climbing time in thermals and negatively related to the mean height used by a species. There was also a positive, but not significant, relationship between wing load and velocity. Soaring birds appreciably extend the distance covered in migration in relation to the straight line from their breeding to wintering grounds (by 48-91%). The increased distance, caused through circumventing sea areas, ranged between 22-34%. while the increase resulting from soaring accounted for an additional 22-57% of the route. Several factors determine the migration altitude in birds, including optimal wind direction and the optimal ambient temperature for cooling the body, conserving water and conserving energy (Bruderer & Steidinger 1972, Torre-Bueno 1978, Berthold 1993). Kerlinger (1989) suggested that factors influencing migration altitude for soaring birds are related to geographical location. topography (over sea or land, along coasts and mountain ranges). time (night or day, or hour of the day), weather and type of flight (active or passive). Active flying birds use height bands which may vary during flight in relation to one or more of the above-mentioned factors (Gauthreaux 1970. Able 1973). Relatively few data have been published on the flight altitude of migrating birds and even fewer on soaring birds. Early data were gathered mainly by ground observers, and their usefulness was questionable because many observers failed to provide sample size or altitude averages and variation related to weather, time of day and season (Kerlinger 1989). Since the 1970s. most data on the altitude of soaring bird migration and factors affectingit were gathered by radar (Evans & Lathbury 1973, Kerlinger 1989. Kerlinger 81 Gauthreaux 1984. 1985a,b), but some studies also used ground observers (Kerlinger et a!. 1985).

Medium- and large-sized birds tend to use thermals and other rising air currents in order to conserve energy. They gain height by flying in circles, and on reaching a certain altitude, they glide in the desired direction. Pennycuick (1972) was the first to use a glider in order to measure the height attained by soaring birds (vultures), and Hopkins et al. (1975) used a motorized glider in order to estimate the altitude reached by six migrating raptor species: however, their estimates were based on a small number of flights. NO systematic study has previously been carried out in Israel to define the migration altitude of birds and to determine the factors affecting it. In the present study we gathered data on the altitude of soaring birds during migration and related our data to environmental data as well as to morphometricdata on the birds.

METHODS The study was carried out in Israel during four autumn (August-November) and four spring (March-May) migration seasons from 1986 to 1989. Two main methods were employed the birds were followed in the air by a motorized glider and on the ground by radar. Occasionally a light aircraft was used.

Author for correspondence.

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Tracking by motorized glider A two-seater motorized glider (Ogar, SZD-45A: PZL,Poland) equipped with a 1700 cc, 65 hp Limbach engine was used. The engine and the propeller were located behind the wing, which enabled the glider to take off and fly without the help of a motorized aeroplane. Its specifications were: wingspan-17.7 m. body length-7.95 m. body height-1.72 m. minimum and maximum velocities (with engineb-82 and 225 kmlh. respectively. The two observers had a significantly wider angle of view compared with the light aircraft. The glider had a 30-1 petrol tank, which allowed a flying time of 4 h, covering a distance of 400 km. In order to double the time in the air. a n extra 30-1 petrol tank was installed. enabling 8 h of continuous flight with the motor running. which, combined with gliding hours, allowed up to 11 hours continuously in the air. The glider instruments were used to measure the flock flight velocity and altitude at any time while in thermals or gliding. The direction of progress of gliding flocks was measured by the glider’s compass. Data were recorded on a tape recorder or by completing standard forms prepared in advance. For each flock followed. the thermal coordinates. as well as the altitude and the time the first bird entered the thermal, were recorded. When the first birds left the thermal and commenced gliding, the time. altitude and new coordinates were again recorded. Communication with ground observers and radar operators was maintained by using the glider’s communications system, and after the first year, a portable telephone and a Motorola radio transmitter were added. These systems enabled communication with the ground observer network, which provided directions of movement of the flock in the morning. Binoculars (Zeiss 10 x 40B)were used to identify unusual individuals, such as unidentified raptors joining the flock. The flock to be followed was located by two methods: by the ground observers the night before, or, in real time, by the Ben-Gurion International Airport radar. One of the ground observer coordinators searched a n area near the borders of Israel by vehicle each evening to locate roosting flocks of raptors. White Storks Ciconia ciconia or White Pelicans Pelecanus onocrotalus. Over time, it was noted that certain sites were preferred by the buds. When a flock of more than 500 individuals was located, a tracking flight was planned for the following day, and, at Erst light. the observers’ vehicle would join the flock. An attempt was made to reach the site by powered glider about a quarter of an hour before the expected t a k e 4 time of the flock and to maintain position above the flock to the point when it left the borders of Israel. In autumn, this meant flocks reached the Egyptian border and crossed into Sinai, and in spring, they flew into Lebanon. In cases in which the migrating flock was located by the radar at Ben-Gurion Airport, the aircontroller guided the glider to the flock. Raptors allowed close observation from the start, and storks got used to the presence of the glider after approximately 1 h of joint flight.

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Table 1. The number of days and hours spent following soaring migrants with the motorized glider: There were 158 days and 730 h of flight -

Species White Stork

Season Spring Autumn

White Pelican

Spring Autumn

Honey Buzzard

Spring

Lesser Spotted Eagle

Autumn Spring Autumn

No. of

No. of

days

hours

60 17 3 28 13 25 1 11

290 90 17 154 46

93

7 33

Only pelicans remained frightened, forcing us to fly at 50100 m from the flock in order to observe their natural behaviour. From the moment the aircraft joined a soaring flock in a thermal on days with reasonable to good gliding conditions, the pilot turned off the engine, in order to cause minimal disturbance to the migrating buds. There were 158 successful flights with the glider. totalling 730 h (Table 1).

Tracking by light aircraft A single-engine, five-seater light aircraft, Cessna 206, 300 hp, was used. The observers (excluding the pilot) located flocks visually or with binoculars. Observations were recorded on a tape recorder and analysed later. This aircraft had certain disadvantages: the engine noise frightened the birds and sometimes caused the flocks to disperse, it was too fast to follow a specific flock continuously and its large turning radius was a problem. The aircraft flew parallel to and slightly above the migrating birds, at a distance of 200-300 m. When the buds glided out of the thermals, forming long, trailing lines, their direction of progress was noted as the aircraft overtook the flock because of its greater speed. Principal migratory routes were thereby tracked and mapped. Data on base and top altitudes of the thermal were gathered with the aircraft altimeter. This method also worked well when flying against the direction of the flock. but in this case the velocity relative to the birds was much greater and it was more difficult to map routes. In all. 29 Cessna flights took place, totalling 84 h. Although the Cessna was not appropriate for continuous tracking of a specific flock, it provided a good tool for locating principal migration routes and obtaining a general picture of migration routes, especially on peak migration days. Tracking by radar The Airport Surveillance Radar (ASR-8. Texas Instruments Inc.) located in Ben Gurion Airport in central Israel was used. This radar located loose flocks of about 100 raptors

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(Honey Buzzard Pernis apivorus size and above) at distances of up to 73 km. When flocks climbed to 1300 m, the radar located them even at 80 km, i.e. the radar enabled us to locate migrating birds west of the mountain tops in central Israel but not east of them in the Jordan (Rift) Valley. The radar operated during daytime for 6 days a week for the entire spring and autumn migration seasons. and the radar screen was photographed every 15-20 min. The radar operators also noted wind conditions and the intensity and direction of migration.

Data Data were summarized for all thermals in which migrants were followed by either the glider or light aircraft in spring and summer. Those thermals for which only partial or doubtful data were obtained were not included in the calculations. For five species of soaring migrants, mean flight velocity was calculated for every hour of the day, as well as the mean minimum and mean maximum height of the birds in thermals. These data enabled calculation of the average height band of each species for the time they spent in the air.

The extension of migration routes as a consequence of soaring The actual distance covered by a soaring migrant is composed of the total distance covered while climbing thermals and gliding between them and is greater than the horizontal (aerial) distance measured between the point of departure and h a 1 landing. In addition, soaring birds circumvent large areas of water, so their route is considerably longer than the direct distance between breeding and wintering sites. The actual distances covered by four species which were followed regularly by the glider were calculated using data on the average number of thermals each species climbed per hour and per flightday. The climb rate in thermals (m/s) was calculated for each flock by dividing the time difference from the moment the first individuals in the flock started circling in a thermal (the thermal base) to the moment the fist individuals left the thermal (the thermal top) and started gliding towards the next thermal by the vertical distance between the thermal base and top altitudes (taken by the glider’s altimeter). The distance travelled by the birds within a thermal, while circling in it, was also measured using the glider’s data. The direct, straight line distance from the centre of breeding range to the center of wintering range was determined (by calculating the mean between two extreme longitudes and latitudes of both areas) for the four species tracked by motorized glider, as well as their migration routes taken from Cramp and Simmons (1980). These distances were measured on a scaled globe. For each species, the average progress velocity and the average number of flight-hours per day were calculated (from recorded take offs and landings of different species and continuous tracking days with the glider). These data en-

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abled the calculation of the average daily flight range for each species and the extra distance covered by soaring birds because of various factors.

RESULTS Relationships between altitude, velocity, height band and time of day Altitude

For all species, the average altitude of flight varied during the day as a direct function of the thermal conditions (Fig. 1).Altitude increased from the morning towards noon and decreased in the afternoon. Similar results were reported by Kerlinger and Gauthreaux (1985a.b). Kerlinger et al. (1985) tracked altitude changes during the day by radar and ground observers and found that the migration altitude of raptors rose from 0 to 100 m between 07.00 and 08.00 h to 850 m, and then decreased between 13.00 h and 14.00 h. Means for the base and top altitudes of thermals for the species studied in Israel are given in Table 2. Daily flight velocity

Table 2 presents the average progress in kilometres per hour according to the time of day for each of the species tracked by the motorized glider. The averages were 29.2 km/h. 38.7 km/h, 50.9 k d h and 45.2 km/h for White Pelicans, White Storks, Lesser Spotted Eagles and Honey Buzzards, respectively (Table 2). Few comparative data exist in the literature. Pennycuick (1972) followed wintering White Storks in Tanzania and one flock of White Pelicans, using a motorized glider, and found that the average velocities were 43.9 km/h and 45 km/h, respectively. These velocities are 13% and 35% greater than those found in the present study. The reasons for these differences are probably that Pennycuick joined flocks in the middle of the day and so did not measure the slower velocities typical of the morning hours: his sample sizes were also smaller than ours. Pennycuick (1972) also followed four resident vulture species in East Africa by motorized glider. In the New World, Hopkins et al. (1975) and Welch (1987) measured the progress of a number of migrating raptors. following them by motorized glider. In six flights over distances between 40 km and 150 km, flocks of Broad-winged Hawks Buteo platypterus had velocities of 3744 km/h. Kerlinger (1989) measured the gliding velocity between thermals of nine migrating raptor species using radar and ground observations and recorded a n average velocity of 55 km/h for the Red-tailed Hawk Buteo jamaicensis. The data gathered in the United States are similar to those gathered by us in Israel for Lesser Spotted Eagles (51 km/ h) and Honey Buzzards (45 km/h). Velocity changes during the day are shown in Figure 2, which is representative of a typical flight day of a flock of White Pelicans in autumn (12 October 1987). The pelicans

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WHITE PELICAN (n=467)

WHITE STORK (n=1059) VEL0CIl-V

50 40 14W

30 20

1Mo-

10 n

--

01 8

1

9

:

10

.

11

, 12

, 13

.

14

, 15

,

16

!

17

4 8

,

9

.

10

.

11

.

12

.

13

.

14

,

15

.

:

18

17

E

5 -

rq

LESSER-SPOTTED EAGLE (n=78)

HONEY BUZZARD (n=215)

J

I

FIgure 1. Average altitude at the base and top of thermals (height band) and average progress velocity (km/h) during the day for four species of migratory soaring birds. Means and standard deviations are present-

i

10

ii

i2

i3

li

i5

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covered a total of 241 km in horizontal distance over a period of 8 h 14 min. at velocities ranging from 16 km/h to 42 km/h. Lower velocities were recorded during the morning and afternoon and higher ones at noon. The effects of unusual atmospheric conditions on migration height and velocity are shown in the following accounts of two migration days. (1) On 16 May 1987, when soaring conditions were poor, we followed a flock of 164 White Pel-

icans in the western Negev. They took off at about 09.00 h but landed and took off three more times until 10.07 h. and between this time and 12.00 h they went through ten thermals, all below about 250 m above ground. At 12.00 h they climbed to almost 700 m. where they remained for 1 h, when unfavorable conditions again prevailed. On this day the mean flight velocity of the pelicans was 20 km/h. only 51% of the average. (2) On 3 April 1988, when soaring

Table 2. Average daily altitude ( 2 s . d ) and daily progress velocity (2s.d.) of four species of soaring migrants

Species White Pelican White Stork Lesser Spotted Eagle Honey Buzzard

Base of thermal (m) . 344 2 463 2 567 2 836 2

175 209 201 211

Top of thermal (m) 562 2 713 2 871 2 1123 2

186 221 184 225

Height band

(m)

Velocity (Wh)

n

218 250 304 287

29.2 2 9.1 38.7 2 9.6 50.9 t 6.7 45.2 t 9.0

467 1059 78 215

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(MI 2500--

2000--

1500--

1000--

500--

0 -

-

0

1

2

3

4

5

6

7

8

g

Total 8 1 2 hours Flighlhoun

25km

Figure 2. The route and altitude in relation to ground level of a flock of White Pelicans followed by glider on 12 October 1987. Each dot represents a point where the first birds Started to climb or descend in a thermal. Time of day and average velocities are given on the abscissa.

conditions were mixed. we followed a flock of White Storks. For the first 4 h (until 11.30 h). the.flock advanced at a velocity of 29 km/h, only 74% of the mean rate, and did not climb to more than 250 m above ground. At 11.30 h, they appeared to locate cumulus clouds above the mountain range, climbed to 1550 m above ground level and doubled their speed (to 57 km/h, 47% above the average velocity), crossing Samaria within 72 min in two long glidks of 36 km and 19 km, respectively, after which they returned to an average speed (Fig. 3). White Storks showed a positive correlation between the hourly flight velocity and the height of the base and top altitudes of the thermals ( r = 0.84, d.f. = 1059 and r = 0.75. d.f. = 467. respectively. P C 0.01 for both), and for the White Pelican there was a correlation between velocity and mean height (r = 0.66, P C 0.05). In Lesser Spotted Eagles and Honey Buzzards. no significant correlation was found between altitude and velocity. The only comparable data known to us are those of Michev and Simeonov (1988). who followed White Storks in autumn along the Bulgarian coast of the Black Sea during 1979-1983 with ground observers and radar. They found

that the average rate of progress was 38.5 km/h. The storks travelled for a n average of 8 h 48 min each day, thus covering a distance of 336 km/day. Our figures (39 km/h, 9 h and 348 km/day) are almost identical.

The effect of bird morphology on flying performance Body mass, wingspan and wing area are important factors in soaring (Table 3). Wing load (body masslwing area) is positively related to climbing time in thermals, with relatively heavier species ascending more slowly (r = 0.99, d.f. = 2, P C 0.02), and negatively related to mean height band used by a species ( r = 0.96, d.f. = 2, P < 0.04) (Fig. 4). There is also a positive. although not significant, relationship between wing load and velocity ( r = 0.88, d.f. = 2). However, the sample size of Lesser Spotted Eagles was small (n = 78). so it is possible that the mean for its average velocity is imprecise and that it is actually faster than the Honey Buzzard, as would be expected from allometric considerations. The above relationships show that larger birds (which have larger wing loads) climb slowly in ther-

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Total 6 4 7 hours 1 29.1

2 29.1

5 56.9

4 30.3

3 26.6

6

45.8

7 27

Flighthours Velocity (kph)

Figure 3. The route and altitude in relation to ground level of a flock of White Storks followed by glider on 3 April 1988. Each dot represents a point where the first birds started to climb or descend in a thermal. Time of day and average velocities are given on the abscissa.

mals and travel at lower heights and at lower velocity between thermals than do smaller birds. The net result is that large birds travel more slowly than do small ones. We have refrained from going into greater detail in comparing the performance of the various soaring birds because the species compared do not migrate at the same time, when thermal conditions would be identical. Nevertheless, it appears Table 3 . Bodg mass. wingspan, wing area and wing load o j lour species of soaring migrants. Wing parameters were taken jollowing the methods recommended by Penngcuick (1 989) Jrom specimens in the Tel Aviv University Zoological Museum

Body Species

(kg)

Wingspan (m)

White Pelican

10.5 3.4 1.4 0.8

3.1 2.0 1.5 1.3

mass

White Stork Lesser Spotted Eagle Honey Buzzard

Wing Wing area load (m2) (kglm') 1.2 0.5 . 0.3 0.3

8.4 6.3 4.4 3.0

1000 -

-; E

800-

n 5

.?

600-

-

400 -

200 -

2

4

6

8

Wing load (kglm 2)

Figure 4. The relationships between wing load. average height band and average climbing time in thermals for four species of soaring migrants. Stars are averages of soaring time in thermals.

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Table 4. The extension of migration routes from the centre of breeding to wintering grounds for four species of soaring migrants, Number of dailyflight hours arefrom Leshem and Yom-Tov (1 996), minimal number of days of migration was calculated by dividing the average distance travelled from the centre of the breeding range to that of the wintering grounds by the daily flight distance over Israel. For calculation of the centre of breeding and wintering grounds see text

White Pelican Mean soaring t i e (hlday) Mean velocity (kmlh) Mean no. thermals per hour Daily flight distance over Israel (km) Minimum migration duration (days) Climbing time in a thermal (min) Climbing velocity in thermal (km/h) Extension of route (%) caused by Flying from thermal to thermal Gliding between thermals Climbing in thermals Total extension of route (%) caused by using thermals Shortest route between breeding and wintering grounds

(W

White Stork

Lesser Spotted Eagle

Honey Buzzard

7.5 29.2

9 38.7

7.5 50.9

10 45.2

4.6

5.6

5.3

5.7

249

348

381

446

21

23

21

23

4.6

3.4

2.9

2.2

31

30

28

25

25.6

12.5

10.7

22.2

2.4

3.4

2.5

5.5

29.1

20.9

9.3

6.5

57.1

36.8

22.5

34.2

3885

6660

6500

8325

Extension (%) caused by circumventing seas

34.2

21.6

25.6

23.0

Total extension (%)

91.3

58.4

48.1

57.2

that the average altitude band of all species lies between 2 18 and 3 14 m. a n d differences between the various species result chiefly from variations i n the thermal base altitude.

The extension of migration routes as a consequence of soaring Soaring birds significantly extend the distance covered in migration (by 48-91%: Table 4). The extension caused by circumventing seas ranges between 22% and 34% and that

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caused by travelling by soaring accounts for an additional extension of 22-57% of the route. We thank the many observers who took part in the migration surveys and particularly the people who organized the ground surveys: Yaron Bazar, Ron Beer, Shai Bliblau. Ehud Dovrat. Kobi Merom. Hadoram Shirihai and Ariel Zoval. We thank Eli Peretz and Michael Pinkus. Ait Company. for flying the motorized glider; the kibbutzim that hosted the volunteers; the workers of the Raptor Information Center, Haim AMa. Ofer Bahat, Mirl Galam. Shlomit Greenbloom, Tamar and Ron Frumkin. Ran Lapid and Yotam Regev: Ian Newton and Uriel Safriel for their comments on the manuscript and the Israeli Airport Authority for allowing the use of the radar of the Ben Gurion Airport. This study was financed by the Israel Ministry of Sciences, the Israeli Air Force, the Interuniversity Ecology Fund and the Society for the Protection of Nature in Israel. We are grateful for their generous support.

REFERENCES Able, K.P. 1973. The role of weather variables and flight direction in determining the magnitude of nocturnal bird migration. Ecology 54: 1031-1041. Berthold. P. 1993. Bird Migration. A general survey. Oxford Oxford University Press. Bruderer. B. & Steidinger. P. 1972. Methods In quantitative and qualitative analysis of bird migration with tracking radar. NASA Spec. Publ. SP 262: 151-167. Cramp, S. & Simmons. K.E.L. 1980. The Birds of the Western Palearctic, Vols 1 & 2. Oxford Oxford University Press. Evans, P.R. & Lathbury, G.W. 1973. Raptor migration across the straits of Gibraltar. Ibis 115: 572-585. Gauthreaux. S.A. 1970. Weather radar quantification of bird migration. BioScience 2 0 17-20. Hopkins. D.A.. Mersereau. G.S. & Welch, W.A. 1975. The Report of the Smirnof Hawk Patrol. Windsor. Conn.: Connecticut Audubon Council Inc. Kerlinger, P. 1989. Flight Strategies of Migrating Hawks. Chicago, Ill.: Chicago University Press. Kerlinger. P. & Gauthreaux. S.A. 1984. Flight strategies of Sharp shinned Hawks during migration over land. Anirn. Behav. 32: 1021-1028. Kerlinger. P.& Gauthreaux. S.A. 1985a. Flight behavior of raptors during spring migration in south Texas studied with radar and visual observations. J, Field Ornithol. 56: 394-402. Kerlinger. P. & Gauthreaux, S.A. 1985b. Seasonal timing. geographical distribution and flight behaviour of Broad-winged Hawks during spring migration in south Texas: A radar and visual study. Auk 102: 735-743. Kerlinger, P.. Bingman. V.P. & Able, K.P. 1985. Comparative flight behavior of migrating hawks studied with tracking radar during migration In central New York. Can. J. 2001. 63: 755-761. Leshem, Y. & Yom-Tov, Y. 1996. The magnitude and timing or migration by soaring raptors, pelicans and storks over Israel. Ibis 138: 188-203. Michev. T. & Simeonov. P. 1988. Der Herbstung des Weisstorchs (Ciconia ciconia) Entang Der Bulgarischen Schwanmeerkuste. Walsrode: Proc. Int. Symp. White Stork 281-296. Pennycuick. CJ. 1969. The mechanics of migration. Ibis 111: 525-556. Pennycuick. C.J. 1972. Soaring behaviour and performance of

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some East African buds observed from a motorglider. Ibis 114: 178-218. Pennycuick. C.J. 1978. Mechanics of flight. In Farrier, D.S.& King. J.R. (eds) Avian Biology, 5th ed.: 1-75. New York Academic Press. Pennycuick, C.J. 1989. Flight Performance. Oxford Oxford University Press.

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Torre-Bueno, J.R. 1978. Evaporative cooling and water balance during flight in birds. J. Exp. Biol. 75: 231-236. Welch, B. 1987. Hawks at My Wing Tips. Thorndike. Maine: North Country Press.

Submitted 12 June 1995: revision accepted 11 August 1995