Environmental influences on the orientation of free-flying nocturnal bird migrants

Anita. Behav.,1974, 22, 224-238

ENVIRONMENTAL INFLUENCES ON THE ORIENTATION OF FREE-FLYING NOCTURNAL B1RD MIGRANTS BY KENNETH P. ABLE*

Department of Zoology and Institute of Ecology, University of Georgia, Athens, Georgia 30601 Abstract. Autumn nocturnal bird migration was investigated using radar and visual observations.

Multivariate analyses assessed the influence of environmental variables on orientation. Two categories of birds were distinguished: (1) shorebirds and waterfowl migrating in flocks; and (2) passerine birds flying singly. These two classes of migrants employ different orientation mechanisms. Landbirds selectively flew with the wind, regardless of its direction or speed or whether the skies were clear or overcast. Shorebirds and waterfowl flew in directions independent of wind in light or moderate winds. The accuracy of passerine orientation was not correlated with any of the variables examined. The passerine orientation observed cannot be accounted for by stellar orientation, but is e~plicable via orientation on the basis of wind direction. The long-distance migratory and homing flights of birds provide one of the most puzzling cases of animal orientation and navigation. Twice each year, in spring and autumn, millions of birds engage in goal-directed migratory flights carrying them between their wintering and breeding grounds. For the majority of migratory landbirds (Order Passeres), these flights take place at night, beginning and terminating under cover of darkness. In spite of over three decades of intensive field and laboratory study, even the environmental cues used in these oriented movements remain in dispute. Extensive experimental investigations of migratory orientation began when Kramer (1949) demonstrated that a bird in migratory condition showed oriented locomotor activity when placed in a circular cage from which it could see only the sky. The numerous subsequent studies using caged birds, particularly those of Sauer (1957) and Emlen (1967a, b), have shown convincingly that these birds orient their nocturnal locomotor restlessness ('Zugunruhe') o n the basis of stars. At the same time, other research on caged migrants and in the field has suggested that additional information (e.g. terrestrial magnetism) may also be used for orientation (Merkel & Fromme 1958; Merkel & Wiltschko 1965; Wiltschko 1968; Wiltschko & Wiltschko 1972; Keeton 1969, 1971; Southern 1970). These results suggest the existence of a hierarchy of usable directional cues. Virtually all studies with caged nocturnal migrants have shown that the orientation of the *Present address: Department of Biological Sciences, State University of New York at Albany, Albany, New York 12203, U.S.A.

birds deteriorates when stars are obscured, and they therefore predict that in the field birds would either become disoriented or would not migrate on overcast nights. Field studies of nocturnal migration using radar, though inconsistent in many of their results, have repeatedly demonstrated that very large numbers of birds often migrate under solid cloud cover and that the birds maintain their flight orientation under these conditions. This implies that directional information in addition to that available in a cage is used by the bird in flight. This study concerns the migratory orientation of free-flying nocturnal migrants in the southeastern United States in autumn. It attempts to resolve some of the inconsistencies between laboratory and field studies and to test the validity of some of the hypotheses generated by work with caged migrants. Methods I used two basic techniques in gathering the data presented in this paper: (1) radar surveillance of migratory movements, and (2) direct visual observations of nocturnal migrants. The radar information was collected using Weather Surveillance Radars (WSR-57) maintained by the National Weather Service. The characteristics of this equipment and its use in the study of bird migration have been described by Gauthreaux (1970). On the plan position indicator (PPI) of the WSR-57, echoes from bird targets at night are of two main types. At low altitudes, usually less than 1500 m, fine misty echoes often completely saturate the screen (Able 1970). Individual echoes in a display of this type are ephemeral

224

ABLE: NOCTURNAL BIRD ORIENTATION and usually so numerous during migration seasons that they are impossible to track individually. Visual observations (to be described below) have consistently confirmed that the fine grainy echoes are produced by small passerine birds flying individually or in very loose aggregations. In addition, the seasonal and diel timing of this echo display on the radar screen correlates exactly with the timing of landbird migration (Gauthreaux 1971). The other echo type seen on WSR-57 radar at night consists of discrete dot echoes. These echoes usually occur at higher altitudes than the grainy landbird display (frequently up to 4000 m, or more; Able 1970), have fast air speeds, and can be tracked on the radar screen for several kilometres. On time exposures of the PPI these echoes produce clear streaks from which their direction and speed can be determined. Opening the camera shutter for the final revolution of the radar antenna yields a brighter dot at the downflight end of the photographic trace. The lengths of the tracks were measured on the films, yielding the ground distance travelled by the flock during the exposure. These measurements yield ground speeds directly, and air speeds were computed by subtracting the wind vector for the appropriate altitude. The altitudes of the flocks were determined from the radar photographs (Able 1970). The brightness and size of these echoes indicate that they are produced by flocked birds, and their air speed, altitude and seasonal timing suggest that they are primarily returns from flocks of shorebirds and waterfowl. I used two main methods to gather orientation data: The Orientation of Passerine Nocturnal Migrants Because of the problems inherent in obtaining information on the flight directions of passerine nocturnal migrants displayed on WST-57 radar, I made visual observations using the portable ceilometer described by Gauthreaux (1969). I used two units with the beams of their 100-W bulbs superimposed to increase illumination and enhance the detection of higher birds. Observations were made as described by Gauthreaux, using a 20 • 60 telescope or binocular. On a clear night, nearly all passerine birds flying below about 750 m can probably be seen. This range of altitude contains the majority of passerine birds on most nights (Able 1970; Bellrose 1971). Cloud cover reduces the contrast between the illuminated underside of the bird

225

and the background, and under these conditions some birds are probably missed. Atmospheric haze drastically reduces the number of birds seen. However, it was nearly always possible to obtain an adequate sample of passerine flight directions when migrations of average magnitude for the latitude were under way. I usually made ceilometer watches for 30 to 45 min during each hour as long as the migration traffic rate remained high enough to yield adequate samples (twenty or more birds). The changing quantity of migration during the night is such that it is usually increasingly difficult to obtain meaningful ceilometer data after 01.00 hours unless a large migration is under way (Gauthreaux 1971; Newman 1956; Lowery 1951). The data were read into a portable tape recorder as clock-face co-ordinates and later converted to flight direction azimuths for each bird (Gauthreaux 1969). The Orientation of Flocked Migrants Shorebirds and waterfowl are rarely observed with the portable ceilometer. However, because they produce dot echoes on WSR-57 radar, I obtained quantitative information on the flight directions of these birds directly from the radar screen. On nights when substantial numbers of dot echoes were present on radar, I took 3-min time exposures of the radar screen (nine revolutions of the radar antenna). The moving dot echoes produced streaks on the photographs. I determined the flight direction and ground speed of every dot echo in selected altitudinal zones by examining the photographs under a dissecting microscope, The data were collected on ninety nights during the autumns of 1969 and 1970. During August, September and October 1969, I made observations at the Weather Bureau station at Lake Charles, Louisiana, approximately 40 km inland from the coast of the Gulf of Mexico. During July, August, September and October 1970, I made similar observations at the Weather Bureau station at Athens, Georgia. All weather data used in this study were gathered at these two stations. The statistical analyses of flight orientation were done according to Batschelet (1965). The multivariate analyses were done with computer programmes developed at the University of Georgia. Multivariate analyses of variance and discriminant function analysis were done using MUDAID and multiple correlations were computed with STEP-WIZE ECLUSION (see Morrison 1967 for details of these analyses).

226

ANIMAL

BEHAVIOUR,

Other statistical procedures will be discussed where appropriate. All times are given in local standard time. The weather variables and reference mnemonics are listed and described in Appendix I. The Orientation Migrants

Results o f Free-Flying

1

Table I. Multivariate F-values from Analysis of Passerine Orientation

Single variable effects

Passerine

For each of 94 hr on sixty-eight nights I computed the mean flight direction, length of the mean vector and angular deviation around that mean from the flight tracks of the individual passerine birds observed with the portable ceilometer. Measurements of wind directions aloft were taken only three times during the period of interest (18.00, 24.00 and 06.00 hours). Therefore, for purposes of this analysis, I have customarily used two flight direction samples per night, one early in the evening (19.00 or 20.00 hours), close to the time of the radiosonde balloon release at 18.00 hours, the other (23.00 or 24.00 hours) near the time of the pibal sounding (visually tracked balloon) at midnight. Rarely, I have included a sample taken late in the night (e.g. 03.00 hours) if there was no change in wind direction between midnight and the 06.00 hours radiosonde. The hourly mean flight directions were analysed with respect to twelve environmental variables (Appendix I). I used a step-wise exclusion multiple correlation programme to examine the relationship of landbird flight directions to the other circular variables (surface wind direction, WIND; 305-m wind direction, W-AL; and the orientation of surface isobars, Iso). Since circular distributions cannot be compared with ordinal variates in the context of correlation or regression, I divided the birdflight directions into eight groups containing 45 ~ each (0 ~ to 45 ~, 46 ~ to 90 ~, and so on) and tested these groups against the non-circular variables in a series of one-way classification multivariate analyses of variance. The F-values from this analysis are given in Table I. The three, directional variables all showed significant correlations with the flight orientation of the birds (Table II). The two wind variables have a coefficient of multiple correlation with bird flight direction equal to 0.972 with a coefficient of determination (r 2) of 0.945. These data are plotted in Figs 1 and 2, and show that passerine birds consistently flew downwind, regardless of the direction in which the wind was blowing.

22,

SVEL

0"82

VEL

0 "97

FiRS

1"26

CLDS

0"76

C-I-IT

0'85

TEMP

1"88

DLTT

1"58

aP

2"32*

DLBP

1 "68

SU~:

0"76

S

0 "72

*P<0"05. Table II. Matrix of Simple Correlations Among the Circular Variables Used in the Analysis of Passerine Orientation

DIR WIND W-AL ISO

D1R

WIND

W-AL

ISO

1.00

0.980

0-943

0.590

1"00

0"968

0"568

1"00

0"617 1"00

Naturally, the agreement between mean flight direction and wind direction is not perfect. The mean deviation of flight directions from surface wind directions was 17.5~ from 305-m winds it was 20.8 ~. Much of this deviation, particularly in the case of 305-m winds, is probably due to inaccuracies in wind direction measurements. Other deviations are due to changes in the flight altitude of the birds sampied. In this regard, the maximum deviations recorded are illuminating. The greatest differences between mean track directions and surface wind directions were 103 ~ and 54 ~, in samples taken 24.00 to 01.00 hours and 20.00 to 21.00 hours, respectively. At these times of the night, migrants are flying at relatively high altitudes (Able 1970), and the deviations from 305-m winds in the same samples were only 13 ~ and 4 ~

ABLE: NOCTURNAL BIRD ORIENTATION

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Fig. 1. A plot of the hourly mean flight directions of passerine birds on the corresponding direction toward which surface winds were blowing. 36O

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Fig. 2. A plot of the hourly mean flight directions of passerine birds on the corresponding direction toward Which 305-m winds were blowing. respectively. Alternatively, the maximum deviations from 305-m winds were 92 ~ and 74 ~ in samples taken 19.00 to 20.00 hours and 03.00 to 04.00 hours, respectively. The corresponding deviations from surface winds were 8 ~ and 9 ~ respectively, and these samples were taken at times of the night when birds fly at low altitudes. Controlling for these sources of error in the data would undoubtedly raise the correlations even more.

227

The multivariate analysis showed that the eight flight-direction categories contained significant differences in terms of only one additional weather variable, barometric pressure, (BP; F = 2-32; P<0.05). Barometric pressure is predictably related to broad weather patterns and is correlated with wind direction. In a two-way classification multivariate analysis of variance with bird directions and wind directions divided into eight groups, a significant interaction was found between bird direction and wind direction in terms of m, ( F = 3.19; P<0.05). This interaction obscures the interpretation of the barely significant F-value of Be in terms of the eight directional groups in the one-way analysis. The directions of surface and 305-m winds, explaining 94.5 per cent of the variability in the flight directions of passerine birds, are clearly the only variables of importance included in this analysis. But wind direction occasionally shifts markedly during the course of a night's migration. Major wind shifts occurred on only five occasions during this study. The data are presented in Table III and illustrated in Fig. 3. All shifts in the hourly mean flight directions of the birds are significant as indicated (parametric two-sample F-test; Batschelet 1965). In addition, the direction and magnitude of the shifts in bird direction are significantly correlated with the direction and magnitude of the shifts in wind direction ( r 2 = 0 . 7 7 0 ; P<0.001). Because my nightly observations were made at a single locality, the birds observed before and after the wind changes were different individuals and may have belonged to different populations of migrants flying in different air masses. Nonetheless, the migrants were moving downwind in each air mass, and as the air masses changed over the station (sometimes abruptly) the direction of migration changed accordingly. The occurrence of the downwind flight behaviour I have described is not influenced by the velocity of the wind. The air speeds of small migrant landbirds average about 13 m per second (summary in Eastwood 1967; Tucker & Schmidt-Koenig 1971). My data on the flight directions of passerines represent surface wind speeds ranging from 1.5 to 12.7 m per second (x-----2.61 • 0.103) and 305-m wind speeds from 2.3 to 16.5 m per second (x = 7..06 • 0.734). It is apparent that the birds are usually flying with the wind when its speed is only a

ANIMAL

228

BEHAVIOUR,

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Fig. 3. Vector diagrams showing passerine flight directions on five nights when wind direction shifted. 6-7 Aug: A, 20.15 to 20.45; B, 00.20 to 00.50; 28-29 Aug: C, 20.15 to 20.50; D, 00.15 to 00-45; 17-18 Sept: E, 19.10 to 19.40; F, 21.15 to 21-45; 22-23 Sept: G, 19.15 to 19-45; H, 00.15 to 00.45; 26-27 Sept: I, 22-15 to 22.50; J, 10.01 to 01.50. The vector diagrams are plotted so that the radius equals the greatest number of birds in any 7.5 ~ sector. The open and solid triangles pointing into the circles represent the directions of surface and 350-m winds, respectively. The numbers are the wind velocities in knots. Mean flight directions are indicated by the solid triangles pointing out from the circles. T a ~ e H I . Flight Directions of Passerine Birds in Relation to Shifts in Wind Direction

Date

Time

(hours)

Mean flight direction

N

Wind Time direction* (hours)

Mean flight direction

N

Wind direction

F

1969 6- 7 Aug

20.00

349 ~

136

340 ~

24.00

299 ~

34

270 ~

27"4t

28-29 Aug

20.00

309

76

300

24.00

270

13

260

14"8t

17-18 Sept

19.00

350

16

340

21.00

77

10

70

21.6t

22-23 Sept

19.00

63

163

40

00.00

133

75

140

30-7t

1970 26-27 Sept

22.00

3

14

360

01.00

43

4

25

4-8~

*Wind direction is given as the direction towards which the wind was blowing. tP<0-001. *P<0"05.

ABLE: NOCTURNAL BIRD ORIENTATION fraction of their air speeds and are not, therefore, being passively drifted by the wind. Most of the migrations took place under clear skies, but the downwind flight behaviour of the birds was the same under clear and overcast skies. The birds did not deviate significantly from a downwind direction when a full or nearly full moon was present. I conclude, then, that passerine birds, once aloft on a migratory flight, somehow determine the direction in which the wind is blowing and selectively fly with it, regardless of its direction or speed and even under clear skies. Radar surveillance at both stations provided good qualitative information on the flight direction of passerines. While the radar and ceilometer may not sample the same birds in all cases, the radar directions consistently confirmed in general the flight directions revealed by the ceilometer. At Lake Charles, radar observations gave no indication that birds were influenced by the coastline of the Gulf. When winds were appropriate, large migrations moved out over the Gulf; careful observations indicated that there was no tendency for the birds to turn either at the coastline or offshore. Wind patterns were such, however, that many flights in southwesterly directions, paralleling the coast, did occur (Able 1972).

The Orientation of Flocked Nocturnal Migrants Does the downwind flight behaviour characteristic of passerine nocturnal migrants also occur in other types of birds which migrate at night 9. To answer this question, I collected flight orientation data on the shorebird and waterfowl echoes on the radar screen at night. Table IV contains these data. I usually sampled the directions of dot echo movement between 18.00 and 3000 m, in altitudinal zones of 305 to 610 m. Dot echoes were infrequently seen above or below this altitudinal range. The wind directions and speeds are those at the altitudes of the birds, taken at 18.00 or 24.00 hours. These data are plotted in Fig. 4. No significant correlation was found between the track direction of the bird flocks and wind direction when the wind speed was below 26 m per second ( r = 0 . 4 8 2 ; r 2 = 0 . 2 3 2 ) . Wind speeds in excess of 26 m per second were accompanied by downwind flights (r2 = 0.949), but in most of these cases, the winds were blowing in a direction obviously favourable to the migration of the birds. Indeed, it is difficult to obtain data on the flight orientation of shorebirds and

229

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Fig. 4. A plot of the mean flight direction of samples of shorebird and waterfowl echoes on the corresponding directions toward which the winds at their altitudes were blowing. Solid circles indicate winds of less than 26 m per second; open circles indicate wind speeds greater than or equal to 26 m per second. The data are given in Table IV. waterfowl over a broad spectrum o f wind directions because they appear to select favourable winds for their specific goal-directed flights. On nights when winds o f moderate or high velocity were blowing in directions counter to the normal seasonal migration of these birds, few dot echoes were seen on radar. Examination o f Table IV reveals that the angular deviations of the mean dot echo directions tend to be fairly large. In fact, the flight dispersions of these flocks are significantly greater than those of the hourly passerine samples (Mann-Whitney U-test; P<0.001). The angular deviations computed for samples o f dot echoes varied independently o f wind speed and cloud cover. These are further indications that the flight directions of these birds are influenced more by their migratory goals than by ambient environmental conditions. The flight track of a bird or flock of birds is the vector resultant of the birds' heading and air speed and the direction and velocity of the wind at the birds' altitude. By employing the Triangle of Velocities to remove the influence of the wind from the track directions of the birds (Evans 1966), I computed headings for each sample of flock echoes (only samples taken in winds of less than 26 m per second were used). It has ~been postulated that if birds fly on a goal-directed

230

ANIMAL

BEHAVIOUR,

22,

1

Table IV. The Fright Directions of Shorebirds and Waterfowl

Night 1969 9-10 Sept 17-18 Sept 18-t9 Sept 18-19 Sept 19-20 Sept 20-21 Sept 22-23 Sept 1- 2 Oct 2- 3 Oct 4- 5 Oct 8- 9 Oct 10-11 Oct 13-14 Oct 13-14 Oct 14-15 Oct 14-15 Oct 21-22 Oct 21-22 Oct 22-23 Oct 23-24 Oct 23-24 Oct

Wind direction and velocity at altitude of birds

195 ~ 278 288 298 260 260 142 140 360 303 360 18 162 170 160 155 195 220 160 118 118

11 knots 12 16 15 10 9 7 10 22 18 12 24 23 36 28 25 15 14 9 7 7

320 185 255 285 85 80 10 220

15 4 7 6 6 7 7 6

No. of echoes

Vector resultant of flight directions

Angular deviation

Azimuth Length in % 45 61 60 32 55 50 51 36 24 27 37 13 34 36 34 37 26 35 21 30 27

168 ~ 213 229 252 208 138 175 182 69 252 167 52 156 148 160 t60 202 183 119 43 30

77"4 87 "8 73 "4 64 "0 71 "9 49 "3 71 "7 89 "2 40-5 94"9 66 "9 97.6 93 "4 95"6 95"8 96.1 81.1 53 "8 61 "0 50-0 84.6

39 ~ 28 42 49 43 58 43 27 62 18 47 13 21 17 17 16 35 55 51 57 32

14 32 9 12 24 26 15 27

189 229 246 128 129 198 190 186

66.8 43.5 99 -2 61 "7 72"3 79 "5 71-6 60.1

47 61 7 50 43 37 43 51

1970

18-19 July 27-28 July 7- 8 Aug 11-12 Aug 16-17 Aug 19-20 Aug 21-22 Aug 26-27 Aug

t r a c k a n d therefore alter their headings so as to c o m p e n s a t e for the drifting effects o f the wind, the s p r e a d in headings will exceed the s p r e a d in tracks. Conversely, i f t h e y fly a t r a c k which is the resultant o f a fixed h e a d i n g ( p r e s u m a b l y directed t o w a r d the m i g r a t o r y goal) a n d the direction a n d speed o f the w i n d (i.e. a r e drifted b y the wind) the spread in tracks will exceed t h a t in headings. I n m y d a t a no significant difference existed between the spread in tracks (angular deviation, s = 37.2 ~ a n d headings (s = 35"1 ~ c o m p u t e d f o r the flocks. It does n o t a p p e a r , therefore, t h a t the birds c o m p o s i n g these flocks were altering their headings in o r d e r t o fly o n a goal-directed track. I then c o m p u t e d a m e a n t r a c k direction for all flocks flying in winds with an easterly c o m p o n e n t

(201 "6 ~) a n d c o m p a r e d it with the m e a n for all those flying in winds with a westerly c o m p o n e n t (161.5~ A significant difference exists between these two means ( F = 4.2; P < 0 . 0 5 ) , suggesting that the birds are either subject to drift b y the wind o r select winds t h a t are particularly favourable for flights t o w a r d their m i g r a t o r y goals. W i t h the field techniques used in this study, two classes o f n o c t u r n a l bird migrants were distinguishable. I have d e m o n s t r a t e d t h a t these two r a t h e r b r o a d categories o f birds utilize different in-flight o r i e n t a t i o n mechanisms. Previous l a b o r a t o r y a n d field studies have failed to reveal this difference, a n d it is possible t h a t other differences in orientation could be detected if finer categories o f m i g r a n t s (species groups a n d age classes) could be s e p a r a t e d by field techniques.

231

ABLE: NOCTURNAL BIRD ORIENTATION Table V. Matrix of Simple Correlations Among the Variables used in the Analysis of Angular Deviation SVEL SVEL

1.00

VEL

0.365 1.00

VEL CLDS

CLDS

C-HT

SUMK

TgMP

DLTT

BP

DLBP

---0.014

0.016

--0.066

--0.275

----0.376 --0.035

0.296

0.098

0.039

--0.031

--0.155

--0.194

---41.232

0.106

1.00

---0-445

0.125

---0.007 --0.015

--0.091

0.052

1.00

0.069

----0.108 --0.116

0.264

0.130 ---0.114

C-HT

1.00

$OMK TEldP

0.029

0.103

--0.014

1.00

0.575

----0.225 --0.198

1.00

--0.216

.--0.495

1.oo

0.435

DLTT BP

1.00

DLBP

--0.080

-4).295

--0.157

--0.017

The Accuracy of Orientation

The angular deviation of each hourly mean flight direction based on ceilometer samples of songbirds provides an index of the tightness of the flight orientation of the birds around that mean. The same data set used for the analysis of passerine flight directions was used to examine the dispersion in flight orientation. The ceilometer samples exhibited consistently tight orientation. The ninety-four samples analysed had a mean angular deviation of 23.7 ~ Jcl-1 ~ and a maximum of 58.5 ~ The correlation matrix of weather and bird variables is given in Table V. Although several are barely significant, a step-wise exclusion multiple correlation analysis showed that the three best variables in combination (TEa_V, VEL, and DLTT) explained only 25.6 per cent of the variance in angular deviation (multiple correlation coefficient, r = 0.506). The dispersion in flight directions might be predicted, a priori, to be influenced by several variables (DIR, WIND, SVEL, VEL), some of which are circular and could not be included in the correlation analysis. The relationship of angular deviation to these variables was analysed using a series of two-way classification multivariate runs. The F-values obtained from these data are presented in Table VI. None of the single variable effects or interactions was significant. Since the visibility of stars has b e e n considered very important in the nocturnal orientation process, I compared further the angular deviations of hourly mean flight directions under

0.034

0.408

0 . 1 0 8 --0.200

--0.025

Table VI. Multivariate F-values From Analysis of Angular Deviation

Single variable effects SVEL

0"259

WL

0"278

W-AL

0"642

WL

0"066

WIND

0"732

SVEL

0"104

DIR

0"564

WIND

1"002

Interactions

0"037

1"065 0-885

I "050

12 and 57 degrees of freedom. clear skies (cloud cover 0]10 or 1/10 for at least 3 hr prior to observation; N = 71) with those under heavily overcast skies (9/10 or 10/10 cloud cover for at least 3 hr prior to observation; N-----20). The mean dispersion of the flight directions of the birds under overcast (mean = 22.1 o ~= 1.8 ~ was not significantly different from that under clear skies (mean = 21.6 ~ ~ 2-2~ There appears to be no tendency for the orientation of the birds to deteriorate when stars are not visible. My data indicate no tendency for angular deviation to change in any regular pattern during the night. Although it is frequently

232

ANIMAL BEHAVIOUR,

difficult to obtain adequate samples with the portable ceilometer late in the night, I have hourly information for fifteen complete nights. Apparently random hour-to-hour fluctuations in flight dispersion occur, but no consistent timedependent increase or decrease is evident. The results indicate that the quality of passerine flight orientation is approximately the same from night to night and hour to hour. Neither the magnitude and direction of migration nor any of thirteen environmental variables significantly influenced the dispersion of passerine flight directions. Discussion The Orientation of Nocturnal Migrants Previous studies investigating the orientation of nocturnal bird migrants have used two basic approaches: (1) experimental studies'with circular orientation cages in which the oriented hopping behaviour of birds (primarily passerines) m migratory condition ('Zugunruhe') was monitored; and (2) radar techniques to record the flight directions of individual birds and groups of migrants under natural conditions. Studies of the orientation of nocturnal migrants began with Kramer's (1949) demonstration that a bird in 'Zugunruhe' when placed in a circular cage under the night sky showed intense locomotor activity which was oriented in the proper seasonal migratory direction. Nocturnal orientation by many species of caged birds under natural skies has been confirmed by numerous subsequent workers (Matthews 1968). Sauer (1957) demonstrated that the orientation of nocturnal migrants could be duplicated under the artificial sky of a planetarium. This work and the extensive experiments conducted by Emlen (1967a, b) have shown beyond doubt that the birds in their cages were orienting on the basis of star patterns. Recently Emlen (1969a, 1970) has shown that visual exposure to the night sky during the juvenile period is important for the development of migratory orientation in at least one species and that this maturation process in which stellar cues come to be associated with a directional reference system is based on the axis of celestial rotation. Planetarium experiments involving blocking of portions of the sky (Emlen 1967b) and directional training (Wallraff 1960) suggested that stellar orientation is based on 'Gestalt' stimuli provided by star patterns. Emlen's results pointed to the circumpolar area within 35 ~ of Polaris

22,

1

as important, if not essential, to the stellar orientation mechanism. These studies with caged nocturnal migrants, both under natural skies and in the planetarium, have been consistent in their implication of stars in the orientation mechanism. When clouds obscured the stars or the planetarium projector was turned off, the hopping of the birds became random. However, data indicating that birds may obtain directional information from subtle environmental cues are becoming more convincing (older work reviewed by Matthews 1968). Merkel and his associates (Merkel & Fromme 1958; Fromme 1961; Merkel, Fromme & Wiltschko 1965; Merkel & Wiltschko 1965) conducted extensive experiments which seem to demonstrate a statistical orientation of 'Zugunruhe' in chambers lacking visual directional cues. This weak orientation can be predictably altered by experimentally changing the directionality of normal-intensity artificial magnetic fields (Wiltschko 1968) and it appears that the birds determine directions from information about the inclination and axial direction of the magnetic field lines (Wiltschko & Wiltschko 1972). While the results cannot be related directly to nocturnal orientation, Keeton (1971) reported that magnets glued to the backs of homing pigeons disrupted their performance, though not in a predictable manner. Southern (1970) showed that naturally occurring disturbances in the earth's magnetic field were correlated with a deterioration in the orientation of ring-billed gull (Larus delawarensis) chicks in an outdoor circular arena. Griffin (1969) recently speculated on a variety of other potential orientation and navigation mechanisms. The recent results from experimental studies seem to suggest a hierarchy of usable directional cues. However, results obtained with caged birds are relevant only to the extent that they can be applied to free-flying migrants. My results are explicable in this context. The data reported in this study demonstrate that passerine birds orient their flight downwind once they are aloft on a night's migration. This behaviour occurs under clear skies as well as overcast and is unaltered in the presence of the moon. Therefore, orientation on the basis of wind direction apparently takes precedence over stellar orientation under natural free-flight conditions. The influence of wind on migratory orientation has been the subject of controversy among students of migration using radar. The argument has centred on whether birds fly on a goal-

ABLE: NOCTURNAL BIRD ORIENTATION directed track, altering their headings to correct for the drifting effects of the wind, or fly a track which is the vector resultant of a fixed heading (presumably directed toward the migratory goal) and the direction and velocity of the wind. Among radar workers, Bellrose & Graber (1963), Bellrose (1967), Drury & Nisbet (1964), Nisbet & Drury (1967), and Evans (1966) have supported the compensation for drift hypothesis, and Lack (1969) has recently re-interpreted his data as supporting this theory. Eastwood (1967), Gauthreaux (1968), Steidinger (1968), and Parslow (1969) have claimed that nocturnal migrants are passively drifted by wind. The proponents of flight on a fixed track with correction for wind drift have observed night-to-night differences in the flight directions of the birds, usually correlated with different wind directions. Nisbet & Drury (1967) and Evans (1966) proposed that these differences resulted from selection of favourable winds by different populations of migrants on the basis of their different goal directions. The frequency of northward autumn flights which I observed precludes the possibility that all the flights were directed toward the birds' migratory destinations and that the night-to-night differences in direction reflect population-specific preferred migratory tracks as proposed by Nisbet & Drury (1967) and Evans (1966). Since the ultimate destinations of birds are never precisely known, the matter of whether birds fly on fixed tracks or fixed headings will remain a problem, and neither hypothesis can be proven from radar data. However, the downwind orientation of passerine birds that I have observed is not wind drift. In downwind flight, track and heading are the same and the question of drift by the wind or correction for drift does not arise. Since the downwind flights of passerines reported here usually took place in wind speeds considerably less than the air speeds of the birds, they are not being passively drifted by wind. Flying downwind under these conditions represents an active behavioural process. Nearly all landbirds observed with the portable ceilometer have flown straight paths through the narrow light beam, and their headings and tracks were the same. Individual birds flying at angles to the wind are obvious because their headings and tracks differ and they appear to move laterally (or crab) across the field of observation. This behaviour was observed very rarely. Waterfowl and shorebird flocks frequently move at angles to the wind. Removing the wind

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vector from the resultant flight track yields the heading of the bird, or in this case, the average heading of the individuals comprising the flock. A comparison of the spread in headings and tracks has been used by several workers to determine if birds correct for wind drift (Evans 1966; Drury & Nisbet 1964; Nisbet & Drury 1967). It is assumed that if the spread in headings exceeds the spread in tracks, the birds are altering their headings in such a manner as to correct for the drifting influence of the winds and to maintain their goal-directed tracks. My data on headings and tracks provide no evidence that shorebirds and waterfowl do this. In fact, the flight directions of flocks flying in westerly winds were significantly different from their directions in easterly ones. While differences of this sort have been used to support the contention that the birds are drifted by wind (Eastwood 1967), the data just as convincingly support the hypothesis of favourable wind selection by different populations of migrants (Nisbet & Drury 1967; Evans 1966). My data show that flocks of shorebirds and waterfowl do not exhibit downwind flight of the type described for landbirds. The great spread in flight directions on any one night suggests that they orient in a goal-directed manner. However, they may use the wind as a directional cue, but orient their flight at some angle to its direction on the basis of their migratory goals. On the other hand, they may utilize the sun, stars, topographic features or some combination of cues in their orientation. This question will require experimental investigation. It is sufficient now to point out that the orientation mechanisms and their ecological consequences are very different in these two rather broad categories of nocturnal migrants. Previous field studies did not detect the two modes of flight orientation demonstrated by my data, probably because of the difficulty in identifying bird types on radar. This may account for some of the disparity between my results and those of earlier workers. On the surveillance radars used in most of these studies, it is very difficult to obtain information on the flight directions of landbirds. With some equipment and in certain geographic localities reliable identification of the bird types responsible for the displays is not possible (Gauthreaux & Able 1970, 1971). These problems are compounded by the fact that most of the radars used in migration studies have vertical beam dimensions of about 30 ~ (Eastwood 1967). For

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this reason they have virtually no altitudinal discrimination and compress onto their screens bird targets from a tremendous height range. It is thus impossible to compare flight directions observed on the radar screen with the corresponding winds at various altitudes. Most of these studies also suffered from a lack of reliable information on winds aloft and several have applied gradient or geostrophic winds to all strata sampled with fan beam radars (but note the poor correlation between W-AL and ~so, Table II). These workers have generalized that all birds utilize the same in-flight orientation mechanisms and all respond to wind in the same manner. But it is likely that the flight direction information collected from these radars frequently represents some admixture of the flight orientation of different types of birds flying in different atmospheric conditions at their respective altitudes. The frequent observations of reversed migrations (general movements in directions roughly opposite to that normal for the season) provide the only hint of the existence of downwind orientation contained in previous radar work. Such movements have been noted in most radar studies (particularly Lack 1963a, b; Drury & Keith 1962; Nisbet & Drury 1967; Parslow 1969) and have consistently been associated with winds opposed to the normal seasonal migratory direction. The greater frequency and variety of movements in inappropriate directions that I observed during this study were very likely a function of the greater frequency of southerly winds in the southeastern United States in autumn (Able 1972). The notion that passerine migrants selectively fly downwind under certain conditions is not new. Analysis of banding data and field observations of grounded nocturnal migrants at Fair Isle, Scotland, led Williamson (1955, 1969) to propose that the occasional arrival of large numbers of migrants which do not normally pass through Great Britain was a result of downwind flight over the North Atlantic. He restricted the occurrence of this behaviour to weather conditions (e.g. fog) in which visual cues were absent. Since Williamson's hypothesis was inferred from field observations of grounded migrants, his data provide no direct evidence of downwind orientation, although the weather systems accompanying these arrivals support his conclusions. Vleugel (1959, 1962) proposed that both nocturnal and diurnal migrants orient their flight by maintaining a constant angle to

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the wind. The direction of movement is initially determined from the sun or stars. Additional indirect evidence of wind-influenced movements of birds is provided by numerous reports of migrants at localities farremoved from their normal routes of movement. Sources such as Audubon FieM Notes (now American Birds) contain voluminous records of such vagrants. Of particular importance are occurrences of migrants north of their normal breeding ranges in autumn, and these are numerous. Baird et al. (1959) were able to correlate the arrival of numbers of southern and western birds on the north-east Atlantic coast with southwesterly winds. These data are explicable in terms of downwind orientation. Recent reports in the literature (Drury & Nisbet 1964; Steidinger 1968) described the interesting observation that the direction of passerine migration shifted gradually in a clockwise direction during the course of the night. These shifts were presumed to be independent of changes in wind direction. Consistent changes in direction are intriguing because they could shed light on the basic mechanisms of orientation used by birds. In each case, the shift was less than 20 ~ during the course of the night. Very precise information on wind direction is required to demonstrate that such small changes are independent of changes in the wind and neither of these reports contains convincing information of winds aloft. Careful examination of my ceilometer data revealed no case of a regular shift in bird direction that was not accompanied by a similar shift in winds or change in the altitude of the birds. On nights when the wind direction remained nearly the same, small counterclockwise shifts in orientation were as frequent as clockwise changes, and no consistent pattern existed. While this problem merits further investigation, the evidence presented thus far in support of spontaneous hour to hour shifts is not convincing. Observations of flight disorientation under natural conditions are surprisingly few. Among radar workers, only Lack (1962a) and Drury & Nisbet (1964) have described convincing cases, all of which were associated with some combination of rain, overcast and fog. In contrast to the behaviour of birds in orientation cages, overcast by itself has never been shown to be sufficient to produce disorientation among freeflying migrants. Well-oriented migrations under solid cloud cover have been reported by

ABLE: NOCTURNAL BIRD ORIENTATION nearly all radar workers, and Cochran, Montgomery & Graber (1967) tracked telemetered thrushes (Catharus spp.) under solid overcast and showed that they maintained oriented flight even when their migrations were initiated under these conditions. Observations of this sort have provided the most important incongruity between field and cage studies of nocturnal orientation. Oriented flight under solid overcast is explicable in terms of orientation on the basis of wind direction, a cue not available to birds in orientation cages. I have never observed disorientation during my work with the portable ceilometer. On the contrary, the analysis of my data has shown that the tightness of orientation of passerine migrants is uninfluenced by any of the environmental variables considered. This is a surprising result, since several of these factors might, a priori, be expected to influence the spread in flight directions of the birds. For example, one might predict that when wind velocity is very low, the expression of slightly different directional preferences by individual birds would result in a greater spread in flight directions. This would seem especially likely when the downwind flight behaviour of the birds results in movements in seasonally abnormal directions. However, neither wind speed, wind direction, nor flight direction is associated with a consistent change in the tightness of passerine orientation. Southern's (1969, 1970) data on the orientation of ring-billed gull chicks indicate that a decline in the quality of orientation might be correlated with natural magnetic storms. Although no severe magnetic disturbances occurred during my study (maximum = 5 K, see Appendix I), no increase in angular deviation was associated with several instances of moderate activity. Although the mechanisms used by passerine birds to orient their flight on the basis of wind direction are unknown, there is at least a theoretical basis which can account for the behaviour. By maximizing ground speed and minimizing lateral drift using a fixed point of reference such as the ground, downwind orientation can be accomplished. The majority of migratory flights take place under conditions which permit an adequate view of topographic features and the generally low altitude of passerine migration (Able 1970) would facilitate this process. It would be difficult, however, to guage the direction of very low-speed winds by this means alone. Nisbet (1955) and Bellrose (1967) discussed the turbulence structure of the atmos-

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phere and pointed out that a bird could, in theory, determine the direction and speed of the wind from its characteristic gust structure. Wind gusts are typically elongated and oriented parallel to the wind direction. In addition, the velocity of the air contained in the gusts is apparently asymmetrical, being greatest at the leading edge of the gust and decreasing gradually in the trailing edge. On the basis of these accelerations, a bird could determine wind direction without reference to any visual cues. This turbulence structure is most characteristic of the lower atmospheric layers (e.g. below 1000 m) where the majority of passerine migration takes place. In fact, Bellrose (1971) presented some evidence that birds were more concentrated in a layer of mild turbulence immediately below a temperature inversion aloft than in more stable layers above and below the turbulent stratum. Furthermore, the conditions associated with observations of disorientation under natural conditions (i.e. rain and fog) are precisely those in which visual input from the ground and the gust structure of the wind are most likely to be disrupted (Bellrose 1967). The existence of different flight strategies among diverse bird types is intuitively reasonable on the basis of the different aerodynamic properties of large and small birds. Pennycuick (1969) demonstrated that a wind of given speed has a much greater influence on the potential flight range of a slow-flying bird than a fastflying one. Thus, smaller (and hence, slower) migrants derive a greater relative advantage from tailwinds, and conversely are hampered more by headwinds, than larger (faster) birds. Accordingly, crosswinds deflect slower birds through a greater angle than faster ones. This enables shorebirds and waterfowl to fly at large angles to winds of moderate speed and still achieve nearly the flight range they could accomplish in still air. In faster winds they tended either to move downwind (when winds were blowing in favourable directions), or their echoes were absent from the radar screen (when winds were unfavourable). Consistently flying with the wind can be advantageous to the small migrant bird in several ways, Most important is the energetic saving. Tucker (1971) recently showed that the long, over-water flights known to be made by migrating landbirds would require them to perform near their physiological limits without assistance from the wind. Abeam winds would malke the flight risky and headwinds would often be

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fatal. The potential flight range of a bird may frequently be halved by a headwind or increased by 50 per cent or even doubled by a tailwind (if the wind speed is equal to the bird's air speed). Tucker's theoretical predictions are supported by field data from the northern coast of the Gulf of Mexico. Migrants embarking on a trans-Gulf crossing in spring are nearly always aided by a strong tailwind, frequently of greater speed than the birds'. Migrants landing near the coast under these conditions are often bulging with fat reserves. On the other hand, when infrequent cold fronts penetrate the Gulf and the birds meet headwinds during the water crossing, many die on offshore oil rigs or in the water; and those which reach the coast are extremely emaciated (Gauthreaux 1971). During flights over land, energy reserves probably rarely reach critically low levels because the birds can terminate their flights at nearly any point, but downwind flight can be of great importance in over-water migrations. Downwind flight also insures that birds will often escape potentially dangerous weather situations. In the northern hemisphere in spring, north winds usually indicate the passage of a cold front and impending colder weather. A reversed migration under these conditions enhances the probability of avoiding the inclement weather that could prove fatal to a small insectivorous bird. In autumn, the same response will move birds out o f an area with the passage of a cold front. Natural selection would seem to favour downwind flight under these circumstances. Northward flights in warm air masses in autumn, while seemingly energetically wasteful, may serve some function that outweighs the energetic cost. At this time one can only speculate on the possible adaptive roles of such flights. All of my observations on migratory orientation have been made in autumn in the southeastern United States on birds migrating over land. Although there is some evidence that the behaviour of the birds may be similar in eastern Canada (Richardson 1971), it would be dangerous to extend my results to other seasons and geographic areas at this time. For example, there is considerable evidence that several species of small passerine migrants (e.g. blackpoll warbler, Dendroica striata, Cape May warbler, D. tigrina, Connecticut warbler, Oporornis agilis) depart from New England on southbound flights that carry them over the Atlantic Ocean at least to the West Indies, and perhaps

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non-stop to South America (Nisbet 1970). The behaviour of these species, at least when engaged in these long over-water flights, may be different from the overland migrants I observed. For example, the birds may proceed into headwinds to make a landfall, particularly when nearing their goals or after proceeding a considerable distance on a migration over the ocean. Further investigation will be required to determine if there are differences between age classes or species of passerines in terms of orientation behaviour. For passerine birds, perhaps the most important consequence of downwind flight is that they rather frequently fly in directions (e.g. northward in autumn) which are inexplicable in terms of their ultimate migratory destinations. This is not in accord with the known ability of birds to navigate precisely, and the multitude of observations showing that even small landbirds return faithfully to specific breeding and wintering localities (Matthews 1968). However, several factors enhance the efficiency of downwind flight in moving a bird toward its migratory destination (Able 1972). First, average winds blow roughly in the migratory directions of the birds so that even if a bird flew every night, taking numerous detours, it would undergo a net movement toward its destination. Second, some birds select winds blowing in the proper direction, thus reducing lateral or reverse movements (Able, in preparation). Third, when they are available, birds select certain synoptic weather situations in which to migrate (Nisbet & Drury 1967; Able, in preparation). In autumn, the heaviest flights occur in the high pressure areas behind cold fronts, the conditions most likely to enable a long-distance downwind flight in good flying weather. The known precision of landbird navigation cannot be accounted for on the basis of wind orientation alone. It is hard to reconcile the frequent flights in wrong directions with the map and compass sense necessary for goaldirected movement (Kramer 1953). Perhaps nocturnal migrants make corrections during the daylight hours for the displacements that obviously occur at night.

Appendix I: The Weather Variables 1. W~ND. The direction, in degrees, toward which surface winds were blowing at the time of observation. 2. W-AL. The direction, in degrees, toward

ABLE: NOCTURNAL BIRD ORIENTATION which winds 305 m a b o v e the surface were blowing at 18.00 o r 24.00 hours. 3. SWL. T h e speed o f surface winds in knots. 4. VEL. T h e speed o f 305-m winds in knots. 5. CLDS. C l o u d a m o u n t s m e a s u r e d h o u r l y on a scale f r o m 0 to 10 (0 = clear; 10 = solid overcast) were s u m m e d over the 2 hr preceding a ceilometer watch a n d the h o u r d u r i n g which the d a t a were collected (3-hr summation). 6. C-nT. The m e a n altitude o f the base o f the clouds d u r i n g the 3 hr as with CLDS. T h e maxim u m m e a s u r e d c l o u d height r e c o r d e d was 9000 m a n d small a m o u n t s o f high cirrus clouds o n which height measurements are not m a d e were a s s u m e d to be at this altitude. 7. TEMP. Surface t e m p e r a t u r e m e a s u r e d at 18.00 hours. 8. DLTT. The 24-hr change in surface temperature at 18.00 hours. 9. BP. Surface b a r o m e t r i c pressure m e a s u r e d at 18.00 hours. 10. DLBP. T h e 24-hr change in b a r o m e t r i c pressure at 18.00 hours. 11. sutaK. A n index o f fluctuations in the earth's magnetic field, m e a s u r e d as K-indices (1 K = 28.57 g a m m a = 0.0002857 gauss) on a scale f r o m 0 to 9, where 5 represents m o d e r a t e disturbance, 8 o r 9 indicates severe s t o r m activity. 12. ISO. T h e direction, in degrees, o f g r a d i e n t o r geostrophic winds which parallel surface isobars.

Acknowledgments This study has been stimulated b y the w o r k a n d ideas o f Sidney A. G a u t h r e a u x . I a m grateful to h i m for his assistance d u r i n g all phases o f the work. I t h a n k C a r l W. H e l m s for advice on d a t a analysis a n d for criticizing a p r e l i m i n a r y draft o f the manuscript. H e r m a n H. Shugart, Dr Rolf Bargmann and Mrs Thelma Richardson gave indispensable advice on statistical analyses a n d c o m p u t e r p r o g r a m s . W i t h o u t the gracious assistance o f the N a t i o n a l W e a t h e r Service personnel at L a k e Charles a n d Athens, the field w o r k w o u l d n o t have been possible. D u r i n g the course o f this study I was s u p p o r t e d by G r a n t 70-1879 f r o m the A i r F o r c e Office o f Scientific Research to D r G a u t h r e a u x a n d b y a fellowship f r o m the Institute o f Ecology, University o f Georgia.

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Nisbet, I. C. T. (1970). Autumn migration of the blackpoll warbler: Evidence for long flight provided by regional survey. Bird-Banding, 41, 207-240. Nisbet, I. C. T. & Drury, W. H. (1967). Orientation of spring migrants studied by radar. Bird-Banding, 38, 173-186. Parslow, J. L. F. (1969). The migration of passerine night migrants studied by radar, lbis, 111, 48-79. Pennycuick, W. J. (1969). The mechanics of bird migration. Ibis, 111, 48-79. Richardson, W. J. (1971). Spring migration and weather in eastern Canada: A radar study. Am. Birds, 25, 684-690. Sauer, E. G. F. (1957). Die Sternenorientierungnachtlich ziehender Grasmucken (Sylvia atricapilla, borin, und curruca). Z. TierpsychoL, 14, 29-70. Southern, W. E. (1969). Orientation behavior of ringbilled gull chicks and fledglings. Condor, 71, 418--425. Southern, W. E. (1970). Influences of disturbances in the earth's magnetic field on ring-billed gull orientation. Am. ZooL, 10, 292-293. Steidinger, P. (1968). Radarbeobachtungen fiber die Richtung und deren Streuung beim nachlichen Vogelzug im Schweizerischen Mittelland. Orn. Beob., 65, 197-226. Tucker, V. A. (1971). Flight energetics in birds. Am. Zool., 11, 115-124. Tucker, V. A. & Schmidt-Koenig, K. (1971). Flight speeds of birds in relation to energetics and wind directions. Auk, 88, 97-107. Vleugel, D. A. (1959). Ober die wahrscheinlichste Method der Wind-Orientierung ziehender Buchfinken. Ornis Fennica, 36, 78-88. Vleugel, D. A. (I 962). ~lber nachtlichen Zug von Drosseln und ihre Orientierung. Vogelwarte, 21, 307-313. Wallraff, H. G. (1960). Does celestial navigation exist in animals? CoM Spring Harb. Symp. Quant. BioL, 25, 451--461. Williamson, K. (1955). Migrational drift. Acta XI Intern. Ornith. Congr., 179-186. Williamson, K. (1969). Weather systems and bird movements. Q. J. Roy. Meteorol. Soc., 95, 414423. Wiltschko, W. (1968). 13ber den Einfluss statischer Magnetfeld die Zugorientierung der Rotkehlchen ( Erithacus rubecula). Z. TierpsychoL, 25, 537-558. Wiltschko, W. & Wiltschko, R. (1972). Magnetic compass of European robins. Science, N.Y., 176, 62-64. (Received 15 February 1972; revised 8 June 1972; second revision 18 August 1972; MS. number: A1295)