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Field studies of avian nocturnal migratory orientation I. interaction of sun, wind and stars as directional cues
Anim. Behav., 1982, 30, 761-767 FIELD
STUDIES
OF AVIAN NOCTURNAL
I. I N T E R A C T I O N
MIGRATORY
ORIENTATION
OF SUN, WIND AND STARS AS
DIRECTIONAL
CUES
BY KENNETH P. ABLE
Department of Biological Sciences, State University of New York, Albany, New York 12222 Abstract. Tracking radar and visual observation techniques were used to observe the orientation of free-flying passerine nocturnal migrants in situations in which potentially usable directional cues were absent or gave conflicting information. When migrants had seen the sun near the time of sunset and/or the stars, they oriented in appropriate migratory directions even when winds were opposed. Under solid overcast skies that prevented a view of both sun and stars, the birds headed downwind in opposing winds and thus moved in seasonally inappropriate directions. The data point to the primacy of visual cues over wind direction, with either sun or stars being sufficient to allow the birds to determine the appropriate migration direction. results in the white-throated sparrow (Zonotrichia albicollis), another short-distance migrant (Bingman & Able 1979). The demonstration of multiple compass cues necessarily complicates investigation of migratory orientation. It is essential to observe the behaviour of migrants under natural free-flight conditions, where one can be certain that the full capabilities of the bird may be expressed. Therefore, I have used radar and visual observation techniques to observe the orientation of free-flying passerine nocturnal migrants in eastern New York in situations where some potentially usable directional cues were absent or gave conflicting information. A preliminary analysis of some early results was presented in Able (1978).
Most songbirds (order Passeriformes) migrate almost exclusively at night. Research has shown them capable of using several cues for compass orientation, including stars, the sun, the earth's magnetic field, and wind direction. Stellar orientation has been reported in a wide variety of migratory birds and its mechanism has been explored in several species. Migratory orientation in European robins (Erithacus rubecula), four species of warblers (genus Sylvia) and the indigo bunting (Passerina cyanea) has been predictably manipulated by shifting the direction of an artificial surrounding magnetic field. Disruption or decrement in orientation has been correlated with natural magnetic disturbance in free-flying nocturnal migrants and gull chicks, while homing ability of pigeons and gulls was altered by attaching magnets or Helmholtz coils to their bodies (see Emlen 1975; SchmidtKoenig 1979; Keeton 1980; Able 1980 for recent reviews of these studies). Nocturnal migrants frequently fly downwind, often in seasonally inappropriate directions, and in some situations wind direction seems to take precedence over other available cues (Gauthreaux & Able 1970; Able 1974a, b). The suggestion that the direction of sunset might serve as an orientation cue for nocturnal migrants emerged from Kramer's (1949) early studies with caged birds and sun compass orientation has been found in three species of nocturnal migrants (Saint-Paul 1953; Able & Dillon 1977). Recently, Moore (1978, 1980) has shown that the orientation of savannah sparrows (Passerculus sandwiehensis) in circular cages was significantly better if they were allowed to see the sun set, and we have replicated those
Methods During 1972 and 1973, data were collected at Millbrook, Dutchess County, New York, with an automatic tracking radar described by Griffin (1973). All other observations were conducted at Berne, Albany County, New York, U.S.A. The automatic tracking radar used there was a type AN/MPQ 10 unit originally manufactured by Sperry Gyroscope for the U. S. Army. The nominal characteristics of this unit are: wavelength 10 cm; peak power 200 kW; pulse length 0.8 gs; pulse repetition rate 1000 Hz, beam width 5 ~ conical; tracking accuracy i 18 m range, 4=_ 0.08~ azimuth and elevation. The azimuth, elevation and slant range of a bird target were recorded every 5 or 10 s, either by storing the values in the memory of a KIM-1 microprocessor (MOS Technology) or by read-
761
ANIMAL
762
BEHAVIOUR,
ing the values into a tape recorder directly from analogue meters on the radar console. Flight directions of nocturnal passerine migrants were sampled with the tracking radar and by visual observations using 20 x binoculars and two 100-W portable ceilometers (Gauthreaux 1969). The flight directions of birds tracked with radar were obtained by computing the resultant vector of all of the 5- or 10-s segments from a given bird. For purposes of this analysis only tracks > 100 m in length of birds flying straight (mean vector length of combined track segments, r > 0.75) and level (average change in altitude < 1 m/s over the length of the track; see Demong & Emlen 1978) courses. Wind data used to calculate radar bird headings were obtained within at least 2 h of each bird track by launching and tracking balloons with the same radar. Similar calculations were done on ceilometer data from nights with no radar tracking, using winds aloft data obtained at 1800 hours at Albany, New York, by the National Weather Service. Only birds observed within 2-3 h following the onset of migration have been used, to help ensure that they initiated migration under the same weather conditions that prevailed at Berne. All statistical analyses follow Batschelet (1965). Mean directions were tested for significance by the Rayleigh test and by the V-test, which asks if a circular distribution is oriented given an a priori expected direction. The length of the mean vector (r) varies between 0 (uniform distribution) and 1.0 (all points in the same direction). Results
Based on current knowledge of the orientation capabilities of passerine nocturnal migrants, I first wished to investigate the behaviour of birds migrating in the absence of visual celestial cues. Table I summarizes data from 214 birds observed on 12 nights when solid, thick cloud cover overspread the region after dawn but before 1500 EST, continued through the observation period, and winds were opposed to the seasonal migration direction in this area (NE in spring, SW in fall). The headings of the birds were nearly always well oriented under these conditions, but because they showed a strong tendency to orient downwind, t h e headings were usually not in directions typical of migration at that season. The headings of all individual birds are plotted with respect to wind direction as a vector diagram in Fig. 1A, and the nightly mean headings under this condition are shown around the periphery.
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3
When plotted with the downwind direction set at 0 ~ the pooled individual headings had a mean of 340" ( N - - 214; P < 0.0001, Rayleigh test) and were significantly oriented by the Vtest (P < 0.0001). The same pattern emerges when the nightly mean headings are examined (mean of means ~- 349 ~ P < 0.01, Rayleigh test; N = 13); they were also significantly oriented by the V-test (P < 0.001). Also shown in Table I are data from 538 birds observed on 32 nights when winds were similarly opposed to the normal migration direction for the season, but skies were clear so that both stars and the sun late in the day were visible to the birds. Unlike the birds that initiated migration under solid overcast, these took up directions appropriate for the season (overall spring mean heading = 19.9~ r = 0.824; P < 0.001, Rayleigh test; overall fall mean heading = 213.5~ r = 0.828; P < 0.001, Rayleigh test). The individual headings and nightly means are plotted with respect to wind direction in Fig. lB. The pooled individual headings yielded a mean direction of 232.8" (r ---- 0.400; P < 0.0001, Rayleigh test; N---- 538) and the mean of the nightly means was 225.1" ( r = 0 . 5 8 0 ; P < 0.001, Rayleigh test; N =-- 33). Neither distribution had a significant component in the downwind direction (V-tests). A statistical comparison of the headings of birds flying in opposing winds under overcast skies versus those flying under clear skies (Fig. 1A versus 1B) showed that the two distributions are significantly different (Watson U 2 test, P < 0.001). In comparably opposed winds the orientation directions of birds flying under clear skies had a significantly smaller downwind component than those flying under solid overcast (hovercast= 0.315; h c l e a r = - - 0 . 3 3 0 ; M a n n Whitney U-test, one-tailed, P < 0.001). On rare occasions solid overcast may commence or end near dusk so that birds can see the sun near the time of sunset, but are unable to see the stars. Data from 82 birds on six nights are available when these sky conditions were accompanied by opposed winds (Table I). The conditions and time course of events on these nights were as follows: (i) 24 May 1975: mostly clear, unseasonably warm day with considerable cumulus build-up in the late afternoon as a cold front approached the area; 0.2 cumulus cover at 1930 hours; cloud cover overspread rapidly with solid overcast before 2130 hours, continuing for the duration of the night. Sunset at 1920 hours.
ABLE: SUN, WIND AND STARS IN MIGRATORY ORIENTAFION (ii) 24 May 1976: partly cloudy and cool day with solid high overcast developing by 1700 hours. Cloud cover becoming thin by 2300 hours. Sunset at 1919 hours. (iii) 1 June i980: solid overcast ensued about 1630; sun itself not directly visible thereafter, but bright area to NW apparent 1830 to 1900 hours; solid overcast with no stars visible for remainder of night. Sunset at 1926 hours. (iv) 15 September 1976: cloud cover preceding a cold front became solid before 1800 hours. Remained solidly overcast for the remainder of the night. Sunset at 1805 hours. (v) 10 October 1980: completely clear and cool day until 1700 hours when solid cloud cover rapidly spread over the area from the SW; solid overcast for most of remainder of night. Sunset at 1721 hours. The behaviour of the birds on these nights was consistent: they were very well oriented in the predicted migratory direction but headed almost directly into the wind (Fig. 1C). Their resulting distribution was significantly different from that of the birds under solid overcast (Fig. 1A) (Watson U 2 test, P < 0.001), but was indistinguishable from that of the birds under clear skies (Fig. 1B). On one night the converse situation occurred, i.e. solid overcast precluded any view of the sun late in the day, but skies cleared after dark and winds were opposed. (vi) 30 September 1979: solid overcast beginning at least by 0800 hours; no rain in local area; warm, humid day; overcast thin and broken by 1930 hours, clear by 2200 hours. Sunset at 1740 hours. On this single occasion, when birds should not have been able to see the sun and migrated in opposed winds under stars, their orientation was virtually identical to the birds that saw sun but no stars. They headed SSW into the wind. Taken together the data support the hypothesis that the passerine migrants sampled from the ambient stream of nocturnal migration were using at least three cues for orientation: stars, the sun and wind direction. A view of the sun or sun and stars appeared to be sufficient to enable the birds to determine the appropriate migration direction. In their absence, the birds resorted to wind as a directional cue. The primacy of celestial cues and the downwind orientation under overcast indicate a hierarchical relationship among these sources of directional information.
763
Discussion Many studies have shown that both migratory birds and homing pigeons rely on several sources of directional information. Only recently have efforts been made to work out the relationships among the cues used by a single individual or species (e.g. Keeton 1971, 1972; Wiltschko & Wiltschko 1975a, b, 1976; Wiltschko et al. 1976; Moore 1978, 1980), and no attempt has been made to examine this problem in migrants in natural free flight. Emlen (1975) emphasized an important distinction between the determination of an orientation direction by a bird at the initiation of migration and the maintenance of that direction during the flight. By using only data obtained during the early evening hours I have tried to ensure that the birds initiated migration under the same weather conditions in which I observed them. All of the days with neither sun nor stars were characterized by very widespread, thick, solid overcast as indicated by examination of surface weather maps and hourly local weather observations. I believe that on these and the clear sky nights, the assumption that the birds took off under conditions similar to those at Berne is adequately fulfilled. On the six nights on which overcast moved in or cleared rapidly around dusk, the situation is obviously more problematical and larger sample sizes are desirable. The efl'ects of cue variability I have observed emphasize the importance of knowing local weather conditions in detail during the hours immediately preceding the initiation of a migratory flight. Few other studies are directly comparable in this regard. Emlen & Demong (1978) and Demong & Emlen (1978) released white-throated sparrows (Zonotrichia albicollis) from boxes carried aloft by balloon and tracked them with radar. Sparrows were able to select correct migratory bearings when released shortly after sunset, when the sun's disc was below the horizon and stars were not yet visible. This result parallels the conclusion from my data that nocturnal passerine migrants can use the sun, the position of sunset, or some related information in lieu of stars in migratory orientation. Emlen & Demong (1978) also released some sparrows under solid overcast skies at night. Under these conditions, the birds took longer to establish a bearing and as a group they were much less well-oriented than birds released under clear skies at night or at sunset. Whereas they showed a northward tendency in
764
ANIMAL
BEHAVIOUR,
30,
3
Table L Flight Orientation of Nocturnal Passerine Migrants in Opposed Winds under Different Conditions of Overcast; Wind Blew Toward Directions Given; r, Length of Mean Vector; h, Downwind Component of Heading Date Spring 7 May 1972 18 May 1976 t 8 May 1980
N
Wind velocity (degrees, m/s)
Mean heading
r
Rayleigh probability
Mean track
h
V probability*
0.508 0.559 0.683
0.01 0.01 0.001
Solid overcast before 1500 EST 15 11 11
186-200 112 150
14 20 24 10 22 14 22 11 16 24
26-50 330--360 260-290 16-40 315-335 315-335 47-75 340-35 315-330 t0-75
6-8 20 7
236.8 ~ 84.4 106.5
0.704 0.631 0.941
0.001 0.01 0.001
221.5 ~ 97.1 121.4
71.5 319.8 270.5 341.7 313.6 328.8 57.6 38.7 185.0 217.4
0.518 0.796 0.633 0.651 0.380 0.607 0.406 0.116 0.660 0.863
0.025 0.001 0.001 0.025 0.05 0.01 0.05 NS 0.001 0.001
48.5 322.3 281.3 16.3 323.7 330.4 56.2 6.0 254.4 208.6
0.416 0.720 0.63i 0.454 0.373 0.606 0.404 0.091 --0.487 --0.862
0.05 0.001 0.001 0.05 0.01 0.001 0.005 NS NS NS
Fall 20 September 1974 16 September 1976 t16 September 1976 13 September 1977 20 September 1977 25 September 1977 20 September 1980 25 September 1980 3 October 1980 15 October 1980 Spring 17 April 1972 29 April 1972 22 May t973 2 May 1975 7 May 1-975 9 May 1975 13 May 1975 16 May 1975 23 May 1976 27 May 1976 "~27 May 1976 12 June 1976 t30 April 1980 ~'20 May 1980 ~22 May 1980 t23 May 1980 "~25 May 1980
6-8 1.5-6 6-7 10-15 12-17 7-14 15-32 15-24 11-14 4-6
Clear skies 28 10 27 14 8 13 29 13 19 19 11 12 22 15 27 25 t0
146-164 10-12 120-140 5-7 130--160 3-9 112-130 15-19 165-197 7-8 125 3 180-215 6 135-150 11-14 112-146 12-17 105-140 13--20 105-140 13-20 160-240 4-8 155 4 250 1.5 105 11 105-125 7.5-9 195--205 12.5
49.2 28.2 11.5 34.5 31.6 20.9 359.4 57.5 23.3 18.3 356.5 337.5 203.4 351.5 34.5 31.5 14.9
0.796 0.623 0.938 0.896 0.735 0.966 0.764 0.836 0.956 0.907 0.990 0.589 0.645 0.920 0.950 0.893 0.614
0.001 0.05 0.001 0.001 0.05 0.001 0.001 0.001 0.001 0.001 0.001 0.05 0.001 0.001 0.001 0.001 0.025
96.4 89.7 35.0 81.2 62.5 35.2 348.3 88.0 64.9 62.4 50.4 317.9 189.7 346.6 70.7 59.4 348.8
--0.217 --0.127 --0.646 0.055 --0.633 --0.235 --0.726 0.073 --0.259 --0.223 --0.582 --0.434 0.471 --0.183 0.317 0.101 --0.612
NS NS NS NS NS NS NS NS NS NS NS NS 0.001 NS 0.01 NS NS
6 18 10 10 8 41 10 18 8 12 18 16 8 17 17 19
300-315 0-20 50-85 325-345 318-338 55-70 70-115 25-60 300-330 310-320 15 360 360 350 360 75-80
198.9 222.9 237.5 281.3 208.1 199.8 253.5 195.2 160.7 258.6 153.2 217.4 246.8 202.6 213.2 168.1
0.914 0.738 0.724 0.788 0.922 0.726 0.896 0.878 0.714 0.872 0.745 0.962 0.615 0.917 0.937 0.666
0.01 0.001 0.01 0.001 0.001 0.001 0.001 0.001 0.05 0.001 0.001 0.001 0.05 0.001 0.001 0.001
205.2 233.7 234.9 286.0 227.6 141.5 222.7 133.5 185.1 270.7 137.0 253.8 256.0 221.0 231.7 125.5
--0.292 --0.620 --0.713 0.466 --0.460 --0.534 --0.847 --0.780 --0.643 --0.483 --0.555 --0.764 --0.242 --0.773 --0.784 0.022
NS NS NS 0.05 NS NS NS NS NS 0.01 NS NS NS INs NS NS
357.5 333,2 325.7
--0.555
NS NS NS
Fall t 4 September 1973 t19 September 1973 t21 September 1973 ~25 September 1973 5 September 1974 10 September 1974 5 October 1974 6 October 1974 4 October 1976 5 October 1976 t 2 September 1978 ~29 September 1978 t17 September 1979 t23 September 1979 ~10 October 1979 t13 October 1979
1-3 5-6 2-4 4-5 4-9 10-17 15-25 12-19 4-9 3-6 7 15 2-3 9-10 8-10 11
Solid overcast beginning at dusk Spring 24 May 1975 24 May 1976 "~ I June 1980
15 14 7
205--250 190 150-155
5-11 10 6.5-8
17.7 350.4 327.7
0.657 0.931 0.916
0.001 0.001 0.001
--0.877 --0.913
ABLE: SUN, WIND AND STARS IN MIGRATORY ORIENTATION Date Fall 15 September 1976 ~15 September 1976 ~10 October 1980
Wind velocity (degrees, m/s)
N 9 7 13
310--350 310-350 360
5-7 5-7 iI
765
Mean heading
r
Rayleigh probability
Mean track
h
V probability*
182.6 179.8 273.9
0.908 0.974 0.760
0.001 0.001 0.001
193.2 188.5 315.8
--0.765 --0.884 0.053
Ns NS Ns
213.4
--0.806
NS
Solid overcast ending after twilight "Fall
30 September 1979
17
350
5
200.5
0.935
< 0.001
*The 'expected' direction used in the V-test was downwind. ~Denotes ceilometer data. Headings for ceilometer data were computed assuming a ground speed of 16 m/s. headings, wind directions were n o t given a n d the possibility t h a t the birds relied on l a n d m a r k s in this coastal locality c a n n o t be eliminated. C o c h r a n et al. (1967) f o u n d d o w n w i n d flight in i n a p p r o p r i a t e directions in four telemetered thrushes (Catharus) t h a t initiated m i g r a t i o n u n d e r solid overcast conditions. A c o m m o n theme in several o f the recent field a n d experimental studies is the a p p a r e n t i m p o r t a n c e o f the sun in the n o c t u r n a l orientation process. M y d a t a imply that either sun o r stars is sufficient to enable the birds to m a k e a m o r e or less a p p r o p r i a t e directional choice. A t present it is n o t clear w h a t the sun-related cue is (the sun itself, glow o f sunset, p o l a r i z a t i o n patterns), h o w it is used (as sun c o m p a s s o r fixed reference point), o r h o w it m i g h t be related to stellar cues in a hierarchy. L a c k i n g the availability o f either sun or stars n e a r the time o f take-off, the birds usually oriented d o w n w i n d . This b e h a v i o u r is m o s t d r a m a t i c a l l y revealed DOWNWIND
when solid overcast is a c c o m p a n i e d b y winds blowing in an u n f a v o u r a b l e direction for migration. W i n d direction, o f course, does n o t by itself yield compass i n f o r m a t i o n ; it m u s t be calibrated b y some other reference. I n the absence o f visual cues, it w o u l d be reasonable to s u p p o s e that birds might use the magnetic c o m p a s s to determine the correct m i g r a t o r y direction. However, the d a t a strikingly suggest that they do n o t usually d o so. This a n o m a l y m a y be related to the W i l t s c h k o s ' (1975a, b, 1976) finding t h a t a p p a r e n t calibration o f the star c o m pass b y the magnetic c o m p a s s m a y take place only periodically (e.g. once every few days). I f this is true, one might n o t expect to see r a p i d a d j u s t m e n t s in o r i e n t a t i o n in response to shortterm changes in cue availability. One o f the consequences o f the orientation b e h a v i o u r I have described is t h a t m i g r a t i o n s in reversed or o t h e r u n u s u a l directions are very rare in this a r e a except u n d e r solid overcast
DOWNWIND
DOWNWIND
o
A,
NO SUN/STARS
B.
SUN AND STARS
o
C.
SUN OR STARS
Fig. 1. The orientation behaviour of free-flying migrants as a function of wind direction. The headings of individual birds are plotted as vectors, with the radius of the circle equal to the greatest number of birds in any 15~ sector. The mean headings of birds on each night are plotted on the periphery of each circle. Open dots denote fall nights, solid dots represent spring nights. Sample sizes are total number of individual birds represented, followed by number of nights. (A) Solid overcast preventing birds from seeing either sun or stars, opposing wind; (B) clear sky, opposing wind; (C) solid overcast commencing or ending near sunset (see text).
766
ANIMAL
BEHAVIOUR,
conditions beginning by midday. While it appears unlikely that any single explanation will accomlt for all migrations in peculiar directions (Richardson 1979), overcast skies and opposed winds are frequent correlates in other areas as well. In this context, it should be noted that the downwind flights in seasonally inappropriate directions reported here were consistently of small magnitude, but the number of individuals engaged in such flights was far greater than the number of birds flying in wrong directions on nights of normal, dense migration. Downwind orientation in inappropriate directions under solid overcast raises a number of questions that cannot be answered directly by these data. Although the number of birds involved is small compared to migration volume under more favourable weather conditions, the question of why birds fly in these directions rather than remaining on the ground is a bothersome one. Several hypotheses might account for this behaviour. Differential wind selection by migrants (Evans 1966; Nisbet & Drury 1967; so-called pseudodrJft of Alerstam 1976) would predict that birds with goals in peculiar directions await these winds to initiate migration, but cannot account for the fact that the behaviour occurs only under solid overcast skies. It could also be argued that the blrds that fly downwind in inappropriate directions represent a group of individuals with faulty orientation mechanisms, i.e. it is a behaviour that should be selected against. This remains a possibility, but arguing against it is the fact that the same phenomenon occurs in spring when only birds that have successfully survived at least one migration are involved. A simpler hypothesis might be that some fraction of migrants adopt downwind orientation whenever visual cues are unavailable. In this region such behaviour would result in forward migration about half the time in spring and less than half the time in fall. The data presented here were obtained over different seasons from an unidentified, large number of species, and from individuals of varying age and experience. We have attempted to control some of these variables in the release experiments described in part II of this paper. Contra Moore (1980, page 702), the results of this field study do point to a hierarchical relationship among cues in this region, at least with respect to the visual cues vis-a-vis wind. It still remains unclear to what extent information from the several sources is integrated when more than one is available.
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3
Acknowledgments I am grateful to D. R. Griffin and the Rockefeller University Center for Field Reasearch for the use of tracking radar facilities during the spring of 1972 and 1973. My tracking radar was obtained with the kind assistance of J. Hughes, Off• of Naval Research, and B. Vonnegut, State University of New York at Albany. R . Zeh helped in renovating, modifying and maintaining the unit. I thank the Research Foundation of State University of New York and the National Science Foundation for support of this research. REFERENCES
Able, K. P. 1974a. Environmental influences on the orientation of freeflying nocturnal bird migrants. Anita. Behav., 22, 224-238. Able, K. P. 1974b. Wind, track, heading and the flight orientation of migrating songbirds. In: Proe. Conf. Biol. Aspects of the Bird/Aircraft Collision Problem (Ed. by S. A. Gauthreaux), pp. 331-357.
Clemson, S. C. Able, K. P. I978. Field studies of the orientation cue hierarchy of nocturnal songbird migrants. In: Animal Migration, Navigation and Homing (Ed. by K. Schmidt-Koenig & W. T. Keeton), pp. 228-238. Heidelberg: Springer-Verlag. Able, K. P. 1980. Mechanisms of orientation, navigation, and homing. In: Animal Migration, Orientation and Navigation (Ed. by S. A. Gauthreaux), pp. 283-373. New York: Academic Press. Able, K. P. & Dillon, P. M. 1977. Sun compass orientation in a nocturnal migrant, the white-throated sparrow. Condor, 79, 393-395. Alerstam, T. 1976. Bird migration in relation to wind and topography. Ph. D. thesis, Universityof Lund, Sweden. Batschelet, E. 1965. Statistical Methods' for the Analysis of Problems in Animal Orientation and Certain Biological Rhythms. Washington, D. C.: A. I. B. S.
Bingman, V. P. & Able, K. P. 1979. The sun as a cue in the orientation of the white-throated sparrow, a nocturnal migrant. Anita. Behav., 27, 621-625. Cochran, W. W., Montgomery, G. G. & Graber, R. R. 1967. Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study. Living Bird, 6, 213-225. Demong, N. J. & Emlen, S. T. 1978. Radar tracking of experimentally released migrant birds. BirdBanding, 49, 342-359. Emlen, S. T. 1975. Migration: orientation and navigation. In: Avian Biology (Ed. by D. S. Farner & J. R. King), Vol. 5, pp. 129-210. New York: Academic Press. Emlen, S. T. & Demong, N. J. 1978. Orientation strategies used by free-flying bird migrants: a radar tracking study. In: Animal Migration, Navigation and Homing (Ed. by K. Schmidt-Koenig & W. T. Keeton), pp. 283-293. Heidelberg: SpringerVerlag. Evans, P. R. 1966. Migration and orientation of passerine night migrants in northeast England. J. Zool., Lond., 150, 319-369.
ABLE: SUN, WIND A N D STARS IN M I G R A T O R Y ORIENTATION Gauthreaux, S. A. 1969. A portable ceilometer technique for studying low-level nocturnal migration. BirdBanding, 40, 309-320. Gauthreaux, S. A. & Able, K. P. 1970. Wind and the direction of nocturnal songbird migration. Nature, Lond., 228, 476-477. Griffin, D. R. 1973. Oriented bird migration in or between opaque cloud layers. Proc. Am.Philos. Soe., 117, 117-141. Keeton, W. T. 1971. Magnets interfere with pigeon homing. Proc. Nat. Aead. Sei., 68, 102-106. Keeton, W. T. 1972. Effects of magnets on pigeon homing. In: Animal Orientation and Navigation, NASA SP-262, pp. 579-594. Keeton, W. T. 1980. Avian orientation and navigation: new developments in an old mystery. In: Acta XVII Congr. Int. Ornithol. (Ed. by R. Nohring), pp. 137-157. Berlin: Deutsche OrnithologenGesellschaft. Kramer, G. 1949. Ober Richtungstendenzen bei der nfichtlichen Zugnnruhe gek~ifigter V6gel. In: Ornithologie als biologische Wissenschaft, Festsehrift Erwin Stresemann (Ed. by E. Mayr & E. Schuz), pp. 269-283. Heidelberg: Carl Winter. Moore, F. R. 1978. Sunset and the orientation of a nocturnal migrant bird. Nature, Lond., 274, 154-156. Moore, F. R. 1980. Solar cues in the migratory orientation of the savannah sparrow, Passerculus sandwiehensis. Anim. Behav., 28, 684-704.
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Nisbet, I. C. T. & Drury, W. H. 1967. Orientati#n of spring migrants studied by radar. Bird-Banding, 38, 173-186. Richardson, W. J. 1979. Reverse migration in spring over New Brunswick and Nova Scotia. Abstract 66; Abstracts, 97th meeting, Am. Ornithol. Union. Saint-Paul, U. von. 1953. Nachweiss der Sormenorientierung bei n~ichtlieh ziehenden VOgeln. Behaviour, 6, 1-7. Schmidt-Koenig, K. 1979. Avian Orientation and Navigation. Berlin: Springer-Verlag. Wiltschko, W. & Wiltschko, R. 1975a. The interaction of stars and magnetic field in the orientation system of night migrating birds. I. Autumn experiments with European warblers ( G e n . Sylvia). Z. 7~erpsychol., 37, 337-355. Wiltschko, W. & Wiltschko, R. 1975b. The interaction of stars and magnetic fields in the orientation system of night migrating birds. II. Spring experiments with European robins (Erithacus rubecula). Z. Tierpsychol., 39, 265-282. Wiltschko, W. & Wiltschko, R. 1976. Interrelation of magnetic compass and star orientation in night migrating birds. J. comp. Physiol., 109, 91-100. Wiltscblco, W., Wiltschko, R. & Keeton, W. T. 1976. Effects of a 'permanent' clock-shift on the orientation of young homing pigeons. Behav. Ecol. Sociobiol., 1, 229-243.
(Received 12 August 1981, revised 14 December 1981 ; MS. number: A2704)
STUDIES
OF AVIAN NOCTURNAL
I. I N T E R A C T I O N
MIGRATORY
ORIENTATION
OF SUN, WIND AND STARS AS
DIRECTIONAL
CUES
BY KENNETH P. ABLE
Department of Biological Sciences, State University of New York, Albany, New York 12222 Abstract. Tracking radar and visual observation techniques were used to observe the orientation of free-flying passerine nocturnal migrants in situations in which potentially usable directional cues were absent or gave conflicting information. When migrants had seen the sun near the time of sunset and/or the stars, they oriented in appropriate migratory directions even when winds were opposed. Under solid overcast skies that prevented a view of both sun and stars, the birds headed downwind in opposing winds and thus moved in seasonally inappropriate directions. The data point to the primacy of visual cues over wind direction, with either sun or stars being sufficient to allow the birds to determine the appropriate migration direction. results in the white-throated sparrow (Zonotrichia albicollis), another short-distance migrant (Bingman & Able 1979). The demonstration of multiple compass cues necessarily complicates investigation of migratory orientation. It is essential to observe the behaviour of migrants under natural free-flight conditions, where one can be certain that the full capabilities of the bird may be expressed. Therefore, I have used radar and visual observation techniques to observe the orientation of free-flying passerine nocturnal migrants in eastern New York in situations where some potentially usable directional cues were absent or gave conflicting information. A preliminary analysis of some early results was presented in Able (1978).
Most songbirds (order Passeriformes) migrate almost exclusively at night. Research has shown them capable of using several cues for compass orientation, including stars, the sun, the earth's magnetic field, and wind direction. Stellar orientation has been reported in a wide variety of migratory birds and its mechanism has been explored in several species. Migratory orientation in European robins (Erithacus rubecula), four species of warblers (genus Sylvia) and the indigo bunting (Passerina cyanea) has been predictably manipulated by shifting the direction of an artificial surrounding magnetic field. Disruption or decrement in orientation has been correlated with natural magnetic disturbance in free-flying nocturnal migrants and gull chicks, while homing ability of pigeons and gulls was altered by attaching magnets or Helmholtz coils to their bodies (see Emlen 1975; SchmidtKoenig 1979; Keeton 1980; Able 1980 for recent reviews of these studies). Nocturnal migrants frequently fly downwind, often in seasonally inappropriate directions, and in some situations wind direction seems to take precedence over other available cues (Gauthreaux & Able 1970; Able 1974a, b). The suggestion that the direction of sunset might serve as an orientation cue for nocturnal migrants emerged from Kramer's (1949) early studies with caged birds and sun compass orientation has been found in three species of nocturnal migrants (Saint-Paul 1953; Able & Dillon 1977). Recently, Moore (1978, 1980) has shown that the orientation of savannah sparrows (Passerculus sandwiehensis) in circular cages was significantly better if they were allowed to see the sun set, and we have replicated those
Methods During 1972 and 1973, data were collected at Millbrook, Dutchess County, New York, with an automatic tracking radar described by Griffin (1973). All other observations were conducted at Berne, Albany County, New York, U.S.A. The automatic tracking radar used there was a type AN/MPQ 10 unit originally manufactured by Sperry Gyroscope for the U. S. Army. The nominal characteristics of this unit are: wavelength 10 cm; peak power 200 kW; pulse length 0.8 gs; pulse repetition rate 1000 Hz, beam width 5 ~ conical; tracking accuracy i 18 m range, 4=_ 0.08~ azimuth and elevation. The azimuth, elevation and slant range of a bird target were recorded every 5 or 10 s, either by storing the values in the memory of a KIM-1 microprocessor (MOS Technology) or by read-
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ANIMAL
762
BEHAVIOUR,
ing the values into a tape recorder directly from analogue meters on the radar console. Flight directions of nocturnal passerine migrants were sampled with the tracking radar and by visual observations using 20 x binoculars and two 100-W portable ceilometers (Gauthreaux 1969). The flight directions of birds tracked with radar were obtained by computing the resultant vector of all of the 5- or 10-s segments from a given bird. For purposes of this analysis only tracks > 100 m in length of birds flying straight (mean vector length of combined track segments, r > 0.75) and level (average change in altitude < 1 m/s over the length of the track; see Demong & Emlen 1978) courses. Wind data used to calculate radar bird headings were obtained within at least 2 h of each bird track by launching and tracking balloons with the same radar. Similar calculations were done on ceilometer data from nights with no radar tracking, using winds aloft data obtained at 1800 hours at Albany, New York, by the National Weather Service. Only birds observed within 2-3 h following the onset of migration have been used, to help ensure that they initiated migration under the same weather conditions that prevailed at Berne. All statistical analyses follow Batschelet (1965). Mean directions were tested for significance by the Rayleigh test and by the V-test, which asks if a circular distribution is oriented given an a priori expected direction. The length of the mean vector (r) varies between 0 (uniform distribution) and 1.0 (all points in the same direction). Results
Based on current knowledge of the orientation capabilities of passerine nocturnal migrants, I first wished to investigate the behaviour of birds migrating in the absence of visual celestial cues. Table I summarizes data from 214 birds observed on 12 nights when solid, thick cloud cover overspread the region after dawn but before 1500 EST, continued through the observation period, and winds were opposed to the seasonal migration direction in this area (NE in spring, SW in fall). The headings of the birds were nearly always well oriented under these conditions, but because they showed a strong tendency to orient downwind, t h e headings were usually not in directions typical of migration at that season. The headings of all individual birds are plotted with respect to wind direction as a vector diagram in Fig. 1A, and the nightly mean headings under this condition are shown around the periphery.
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When plotted with the downwind direction set at 0 ~ the pooled individual headings had a mean of 340" ( N - - 214; P < 0.0001, Rayleigh test) and were significantly oriented by the Vtest (P < 0.0001). The same pattern emerges when the nightly mean headings are examined (mean of means ~- 349 ~ P < 0.01, Rayleigh test; N = 13); they were also significantly oriented by the V-test (P < 0.001). Also shown in Table I are data from 538 birds observed on 32 nights when winds were similarly opposed to the normal migration direction for the season, but skies were clear so that both stars and the sun late in the day were visible to the birds. Unlike the birds that initiated migration under solid overcast, these took up directions appropriate for the season (overall spring mean heading = 19.9~ r = 0.824; P < 0.001, Rayleigh test; overall fall mean heading = 213.5~ r = 0.828; P < 0.001, Rayleigh test). The individual headings and nightly means are plotted with respect to wind direction in Fig. lB. The pooled individual headings yielded a mean direction of 232.8" (r ---- 0.400; P < 0.0001, Rayleigh test; N---- 538) and the mean of the nightly means was 225.1" ( r = 0 . 5 8 0 ; P < 0.001, Rayleigh test; N =-- 33). Neither distribution had a significant component in the downwind direction (V-tests). A statistical comparison of the headings of birds flying in opposing winds under overcast skies versus those flying under clear skies (Fig. 1A versus 1B) showed that the two distributions are significantly different (Watson U 2 test, P < 0.001). In comparably opposed winds the orientation directions of birds flying under clear skies had a significantly smaller downwind component than those flying under solid overcast (hovercast= 0.315; h c l e a r = - - 0 . 3 3 0 ; M a n n Whitney U-test, one-tailed, P < 0.001). On rare occasions solid overcast may commence or end near dusk so that birds can see the sun near the time of sunset, but are unable to see the stars. Data from 82 birds on six nights are available when these sky conditions were accompanied by opposed winds (Table I). The conditions and time course of events on these nights were as follows: (i) 24 May 1975: mostly clear, unseasonably warm day with considerable cumulus build-up in the late afternoon as a cold front approached the area; 0.2 cumulus cover at 1930 hours; cloud cover overspread rapidly with solid overcast before 2130 hours, continuing for the duration of the night. Sunset at 1920 hours.
ABLE: SUN, WIND AND STARS IN MIGRATORY ORIENTAFION (ii) 24 May 1976: partly cloudy and cool day with solid high overcast developing by 1700 hours. Cloud cover becoming thin by 2300 hours. Sunset at 1919 hours. (iii) 1 June i980: solid overcast ensued about 1630; sun itself not directly visible thereafter, but bright area to NW apparent 1830 to 1900 hours; solid overcast with no stars visible for remainder of night. Sunset at 1926 hours. (iv) 15 September 1976: cloud cover preceding a cold front became solid before 1800 hours. Remained solidly overcast for the remainder of the night. Sunset at 1805 hours. (v) 10 October 1980: completely clear and cool day until 1700 hours when solid cloud cover rapidly spread over the area from the SW; solid overcast for most of remainder of night. Sunset at 1721 hours. The behaviour of the birds on these nights was consistent: they were very well oriented in the predicted migratory direction but headed almost directly into the wind (Fig. 1C). Their resulting distribution was significantly different from that of the birds under solid overcast (Fig. 1A) (Watson U 2 test, P < 0.001), but was indistinguishable from that of the birds under clear skies (Fig. 1B). On one night the converse situation occurred, i.e. solid overcast precluded any view of the sun late in the day, but skies cleared after dark and winds were opposed. (vi) 30 September 1979: solid overcast beginning at least by 0800 hours; no rain in local area; warm, humid day; overcast thin and broken by 1930 hours, clear by 2200 hours. Sunset at 1740 hours. On this single occasion, when birds should not have been able to see the sun and migrated in opposed winds under stars, their orientation was virtually identical to the birds that saw sun but no stars. They headed SSW into the wind. Taken together the data support the hypothesis that the passerine migrants sampled from the ambient stream of nocturnal migration were using at least three cues for orientation: stars, the sun and wind direction. A view of the sun or sun and stars appeared to be sufficient to enable the birds to determine the appropriate migration direction. In their absence, the birds resorted to wind as a directional cue. The primacy of celestial cues and the downwind orientation under overcast indicate a hierarchical relationship among these sources of directional information.
763
Discussion Many studies have shown that both migratory birds and homing pigeons rely on several sources of directional information. Only recently have efforts been made to work out the relationships among the cues used by a single individual or species (e.g. Keeton 1971, 1972; Wiltschko & Wiltschko 1975a, b, 1976; Wiltschko et al. 1976; Moore 1978, 1980), and no attempt has been made to examine this problem in migrants in natural free flight. Emlen (1975) emphasized an important distinction between the determination of an orientation direction by a bird at the initiation of migration and the maintenance of that direction during the flight. By using only data obtained during the early evening hours I have tried to ensure that the birds initiated migration under the same weather conditions in which I observed them. All of the days with neither sun nor stars were characterized by very widespread, thick, solid overcast as indicated by examination of surface weather maps and hourly local weather observations. I believe that on these and the clear sky nights, the assumption that the birds took off under conditions similar to those at Berne is adequately fulfilled. On the six nights on which overcast moved in or cleared rapidly around dusk, the situation is obviously more problematical and larger sample sizes are desirable. The efl'ects of cue variability I have observed emphasize the importance of knowing local weather conditions in detail during the hours immediately preceding the initiation of a migratory flight. Few other studies are directly comparable in this regard. Emlen & Demong (1978) and Demong & Emlen (1978) released white-throated sparrows (Zonotrichia albicollis) from boxes carried aloft by balloon and tracked them with radar. Sparrows were able to select correct migratory bearings when released shortly after sunset, when the sun's disc was below the horizon and stars were not yet visible. This result parallels the conclusion from my data that nocturnal passerine migrants can use the sun, the position of sunset, or some related information in lieu of stars in migratory orientation. Emlen & Demong (1978) also released some sparrows under solid overcast skies at night. Under these conditions, the birds took longer to establish a bearing and as a group they were much less well-oriented than birds released under clear skies at night or at sunset. Whereas they showed a northward tendency in
764
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Table L Flight Orientation of Nocturnal Passerine Migrants in Opposed Winds under Different Conditions of Overcast; Wind Blew Toward Directions Given; r, Length of Mean Vector; h, Downwind Component of Heading Date Spring 7 May 1972 18 May 1976 t 8 May 1980
N
Wind velocity (degrees, m/s)
Mean heading
r
Rayleigh probability
Mean track
h
V probability*
0.508 0.559 0.683
0.01 0.01 0.001
Solid overcast before 1500 EST 15 11 11
186-200 112 150
14 20 24 10 22 14 22 11 16 24
26-50 330--360 260-290 16-40 315-335 315-335 47-75 340-35 315-330 t0-75
6-8 20 7
236.8 ~ 84.4 106.5
0.704 0.631 0.941
0.001 0.01 0.001
221.5 ~ 97.1 121.4
71.5 319.8 270.5 341.7 313.6 328.8 57.6 38.7 185.0 217.4
0.518 0.796 0.633 0.651 0.380 0.607 0.406 0.116 0.660 0.863
0.025 0.001 0.001 0.025 0.05 0.01 0.05 NS 0.001 0.001
48.5 322.3 281.3 16.3 323.7 330.4 56.2 6.0 254.4 208.6
0.416 0.720 0.63i 0.454 0.373 0.606 0.404 0.091 --0.487 --0.862
0.05 0.001 0.001 0.05 0.01 0.001 0.005 NS NS NS
Fall 20 September 1974 16 September 1976 t16 September 1976 13 September 1977 20 September 1977 25 September 1977 20 September 1980 25 September 1980 3 October 1980 15 October 1980 Spring 17 April 1972 29 April 1972 22 May t973 2 May 1975 7 May 1-975 9 May 1975 13 May 1975 16 May 1975 23 May 1976 27 May 1976 "~27 May 1976 12 June 1976 t30 April 1980 ~'20 May 1980 ~22 May 1980 t23 May 1980 "~25 May 1980
6-8 1.5-6 6-7 10-15 12-17 7-14 15-32 15-24 11-14 4-6
Clear skies 28 10 27 14 8 13 29 13 19 19 11 12 22 15 27 25 t0
146-164 10-12 120-140 5-7 130--160 3-9 112-130 15-19 165-197 7-8 125 3 180-215 6 135-150 11-14 112-146 12-17 105-140 13--20 105-140 13-20 160-240 4-8 155 4 250 1.5 105 11 105-125 7.5-9 195--205 12.5
49.2 28.2 11.5 34.5 31.6 20.9 359.4 57.5 23.3 18.3 356.5 337.5 203.4 351.5 34.5 31.5 14.9
0.796 0.623 0.938 0.896 0.735 0.966 0.764 0.836 0.956 0.907 0.990 0.589 0.645 0.920 0.950 0.893 0.614
0.001 0.05 0.001 0.001 0.05 0.001 0.001 0.001 0.001 0.001 0.001 0.05 0.001 0.001 0.001 0.001 0.025
96.4 89.7 35.0 81.2 62.5 35.2 348.3 88.0 64.9 62.4 50.4 317.9 189.7 346.6 70.7 59.4 348.8
--0.217 --0.127 --0.646 0.055 --0.633 --0.235 --0.726 0.073 --0.259 --0.223 --0.582 --0.434 0.471 --0.183 0.317 0.101 --0.612
NS NS NS NS NS NS NS NS NS NS NS NS 0.001 NS 0.01 NS NS
6 18 10 10 8 41 10 18 8 12 18 16 8 17 17 19
300-315 0-20 50-85 325-345 318-338 55-70 70-115 25-60 300-330 310-320 15 360 360 350 360 75-80
198.9 222.9 237.5 281.3 208.1 199.8 253.5 195.2 160.7 258.6 153.2 217.4 246.8 202.6 213.2 168.1
0.914 0.738 0.724 0.788 0.922 0.726 0.896 0.878 0.714 0.872 0.745 0.962 0.615 0.917 0.937 0.666
0.01 0.001 0.01 0.001 0.001 0.001 0.001 0.001 0.05 0.001 0.001 0.001 0.05 0.001 0.001 0.001
205.2 233.7 234.9 286.0 227.6 141.5 222.7 133.5 185.1 270.7 137.0 253.8 256.0 221.0 231.7 125.5
--0.292 --0.620 --0.713 0.466 --0.460 --0.534 --0.847 --0.780 --0.643 --0.483 --0.555 --0.764 --0.242 --0.773 --0.784 0.022
NS NS NS 0.05 NS NS NS NS NS 0.01 NS NS NS INs NS NS
357.5 333,2 325.7
--0.555
NS NS NS
Fall t 4 September 1973 t19 September 1973 t21 September 1973 ~25 September 1973 5 September 1974 10 September 1974 5 October 1974 6 October 1974 4 October 1976 5 October 1976 t 2 September 1978 ~29 September 1978 t17 September 1979 t23 September 1979 ~10 October 1979 t13 October 1979
1-3 5-6 2-4 4-5 4-9 10-17 15-25 12-19 4-9 3-6 7 15 2-3 9-10 8-10 11
Solid overcast beginning at dusk Spring 24 May 1975 24 May 1976 "~ I June 1980
15 14 7
205--250 190 150-155
5-11 10 6.5-8
17.7 350.4 327.7
0.657 0.931 0.916
0.001 0.001 0.001
--0.877 --0.913
ABLE: SUN, WIND AND STARS IN MIGRATORY ORIENTATION Date Fall 15 September 1976 ~15 September 1976 ~10 October 1980
Wind velocity (degrees, m/s)
N 9 7 13
310--350 310-350 360
5-7 5-7 iI
765
Mean heading
r
Rayleigh probability
Mean track
h
V probability*
182.6 179.8 273.9
0.908 0.974 0.760
0.001 0.001 0.001
193.2 188.5 315.8
--0.765 --0.884 0.053
Ns NS Ns
213.4
--0.806
NS
Solid overcast ending after twilight "Fall
30 September 1979
17
350
5
200.5
0.935
< 0.001
*The 'expected' direction used in the V-test was downwind. ~Denotes ceilometer data. Headings for ceilometer data were computed assuming a ground speed of 16 m/s. headings, wind directions were n o t given a n d the possibility t h a t the birds relied on l a n d m a r k s in this coastal locality c a n n o t be eliminated. C o c h r a n et al. (1967) f o u n d d o w n w i n d flight in i n a p p r o p r i a t e directions in four telemetered thrushes (Catharus) t h a t initiated m i g r a t i o n u n d e r solid overcast conditions. A c o m m o n theme in several o f the recent field a n d experimental studies is the a p p a r e n t i m p o r t a n c e o f the sun in the n o c t u r n a l orientation process. M y d a t a imply that either sun o r stars is sufficient to enable the birds to m a k e a m o r e or less a p p r o p r i a t e directional choice. A t present it is n o t clear w h a t the sun-related cue is (the sun itself, glow o f sunset, p o l a r i z a t i o n patterns), h o w it is used (as sun c o m p a s s o r fixed reference point), o r h o w it m i g h t be related to stellar cues in a hierarchy. L a c k i n g the availability o f either sun or stars n e a r the time o f take-off, the birds usually oriented d o w n w i n d . This b e h a v i o u r is m o s t d r a m a t i c a l l y revealed DOWNWIND
when solid overcast is a c c o m p a n i e d b y winds blowing in an u n f a v o u r a b l e direction for migration. W i n d direction, o f course, does n o t by itself yield compass i n f o r m a t i o n ; it m u s t be calibrated b y some other reference. I n the absence o f visual cues, it w o u l d be reasonable to s u p p o s e that birds might use the magnetic c o m p a s s to determine the correct m i g r a t o r y direction. However, the d a t a strikingly suggest that they do n o t usually d o so. This a n o m a l y m a y be related to the W i l t s c h k o s ' (1975a, b, 1976) finding t h a t a p p a r e n t calibration o f the star c o m pass b y the magnetic c o m p a s s m a y take place only periodically (e.g. once every few days). I f this is true, one might n o t expect to see r a p i d a d j u s t m e n t s in o r i e n t a t i o n in response to shortterm changes in cue availability. One o f the consequences o f the orientation b e h a v i o u r I have described is t h a t m i g r a t i o n s in reversed or o t h e r u n u s u a l directions are very rare in this a r e a except u n d e r solid overcast
DOWNWIND
DOWNWIND
o
A,
NO SUN/STARS
B.
SUN AND STARS
o
C.
SUN OR STARS
Fig. 1. The orientation behaviour of free-flying migrants as a function of wind direction. The headings of individual birds are plotted as vectors, with the radius of the circle equal to the greatest number of birds in any 15~ sector. The mean headings of birds on each night are plotted on the periphery of each circle. Open dots denote fall nights, solid dots represent spring nights. Sample sizes are total number of individual birds represented, followed by number of nights. (A) Solid overcast preventing birds from seeing either sun or stars, opposing wind; (B) clear sky, opposing wind; (C) solid overcast commencing or ending near sunset (see text).
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BEHAVIOUR,
conditions beginning by midday. While it appears unlikely that any single explanation will accomlt for all migrations in peculiar directions (Richardson 1979), overcast skies and opposed winds are frequent correlates in other areas as well. In this context, it should be noted that the downwind flights in seasonally inappropriate directions reported here were consistently of small magnitude, but the number of individuals engaged in such flights was far greater than the number of birds flying in wrong directions on nights of normal, dense migration. Downwind orientation in inappropriate directions under solid overcast raises a number of questions that cannot be answered directly by these data. Although the number of birds involved is small compared to migration volume under more favourable weather conditions, the question of why birds fly in these directions rather than remaining on the ground is a bothersome one. Several hypotheses might account for this behaviour. Differential wind selection by migrants (Evans 1966; Nisbet & Drury 1967; so-called pseudodrJft of Alerstam 1976) would predict that birds with goals in peculiar directions await these winds to initiate migration, but cannot account for the fact that the behaviour occurs only under solid overcast skies. It could also be argued that the blrds that fly downwind in inappropriate directions represent a group of individuals with faulty orientation mechanisms, i.e. it is a behaviour that should be selected against. This remains a possibility, but arguing against it is the fact that the same phenomenon occurs in spring when only birds that have successfully survived at least one migration are involved. A simpler hypothesis might be that some fraction of migrants adopt downwind orientation whenever visual cues are unavailable. In this region such behaviour would result in forward migration about half the time in spring and less than half the time in fall. The data presented here were obtained over different seasons from an unidentified, large number of species, and from individuals of varying age and experience. We have attempted to control some of these variables in the release experiments described in part II of this paper. Contra Moore (1980, page 702), the results of this field study do point to a hierarchical relationship among cues in this region, at least with respect to the visual cues vis-a-vis wind. It still remains unclear to what extent information from the several sources is integrated when more than one is available.
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Acknowledgments I am grateful to D. R. Griffin and the Rockefeller University Center for Field Reasearch for the use of tracking radar facilities during the spring of 1972 and 1973. My tracking radar was obtained with the kind assistance of J. Hughes, Off• of Naval Research, and B. Vonnegut, State University of New York at Albany. R . Zeh helped in renovating, modifying and maintaining the unit. I thank the Research Foundation of State University of New York and the National Science Foundation for support of this research. REFERENCES
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Clemson, S. C. Able, K. P. I978. Field studies of the orientation cue hierarchy of nocturnal songbird migrants. In: Animal Migration, Navigation and Homing (Ed. by K. Schmidt-Koenig & W. T. Keeton), pp. 228-238. Heidelberg: Springer-Verlag. Able, K. P. 1980. Mechanisms of orientation, navigation, and homing. In: Animal Migration, Orientation and Navigation (Ed. by S. A. Gauthreaux), pp. 283-373. New York: Academic Press. Able, K. P. & Dillon, P. M. 1977. Sun compass orientation in a nocturnal migrant, the white-throated sparrow. Condor, 79, 393-395. Alerstam, T. 1976. Bird migration in relation to wind and topography. Ph. D. thesis, Universityof Lund, Sweden. Batschelet, E. 1965. Statistical Methods' for the Analysis of Problems in Animal Orientation and Certain Biological Rhythms. Washington, D. C.: A. I. B. S.
Bingman, V. P. & Able, K. P. 1979. The sun as a cue in the orientation of the white-throated sparrow, a nocturnal migrant. Anita. Behav., 27, 621-625. Cochran, W. W., Montgomery, G. G. & Graber, R. R. 1967. Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study. Living Bird, 6, 213-225. Demong, N. J. & Emlen, S. T. 1978. Radar tracking of experimentally released migrant birds. BirdBanding, 49, 342-359. Emlen, S. T. 1975. Migration: orientation and navigation. In: Avian Biology (Ed. by D. S. Farner & J. R. King), Vol. 5, pp. 129-210. New York: Academic Press. Emlen, S. T. & Demong, N. J. 1978. Orientation strategies used by free-flying bird migrants: a radar tracking study. In: Animal Migration, Navigation and Homing (Ed. by K. Schmidt-Koenig & W. T. Keeton), pp. 283-293. Heidelberg: SpringerVerlag. Evans, P. R. 1966. Migration and orientation of passerine night migrants in northeast England. J. Zool., Lond., 150, 319-369.
ABLE: SUN, WIND A N D STARS IN M I G R A T O R Y ORIENTATION Gauthreaux, S. A. 1969. A portable ceilometer technique for studying low-level nocturnal migration. BirdBanding, 40, 309-320. Gauthreaux, S. A. & Able, K. P. 1970. Wind and the direction of nocturnal songbird migration. Nature, Lond., 228, 476-477. Griffin, D. R. 1973. Oriented bird migration in or between opaque cloud layers. Proc. Am.Philos. Soe., 117, 117-141. Keeton, W. T. 1971. Magnets interfere with pigeon homing. Proc. Nat. Aead. Sei., 68, 102-106. Keeton, W. T. 1972. Effects of magnets on pigeon homing. In: Animal Orientation and Navigation, NASA SP-262, pp. 579-594. Keeton, W. T. 1980. Avian orientation and navigation: new developments in an old mystery. In: Acta XVII Congr. Int. Ornithol. (Ed. by R. Nohring), pp. 137-157. Berlin: Deutsche OrnithologenGesellschaft. Kramer, G. 1949. Ober Richtungstendenzen bei der nfichtlichen Zugnnruhe gek~ifigter V6gel. In: Ornithologie als biologische Wissenschaft, Festsehrift Erwin Stresemann (Ed. by E. Mayr & E. Schuz), pp. 269-283. Heidelberg: Carl Winter. Moore, F. R. 1978. Sunset and the orientation of a nocturnal migrant bird. Nature, Lond., 274, 154-156. Moore, F. R. 1980. Solar cues in the migratory orientation of the savannah sparrow, Passerculus sandwiehensis. Anim. Behav., 28, 684-704.
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Nisbet, I. C. T. & Drury, W. H. 1967. Orientati#n of spring migrants studied by radar. Bird-Banding, 38, 173-186. Richardson, W. J. 1979. Reverse migration in spring over New Brunswick and Nova Scotia. Abstract 66; Abstracts, 97th meeting, Am. Ornithol. Union. Saint-Paul, U. von. 1953. Nachweiss der Sormenorientierung bei n~ichtlieh ziehenden VOgeln. Behaviour, 6, 1-7. Schmidt-Koenig, K. 1979. Avian Orientation and Navigation. Berlin: Springer-Verlag. Wiltschko, W. & Wiltschko, R. 1975a. The interaction of stars and magnetic field in the orientation system of night migrating birds. I. Autumn experiments with European warblers ( G e n . Sylvia). Z. 7~erpsychol., 37, 337-355. Wiltschko, W. & Wiltschko, R. 1975b. The interaction of stars and magnetic fields in the orientation system of night migrating birds. II. Spring experiments with European robins (Erithacus rubecula). Z. Tierpsychol., 39, 265-282. Wiltschko, W. & Wiltschko, R. 1976. Interrelation of magnetic compass and star orientation in night migrating birds. J. comp. Physiol., 109, 91-100. Wiltscblco, W., Wiltschko, R. & Keeton, W. T. 1976. Effects of a 'permanent' clock-shift on the orientation of young homing pigeons. Behav. Ecol. Sociobiol., 1, 229-243.
(Received 12 August 1981, revised 14 December 1981 ; MS. number: A2704)