SUNSET AND THE ORIENTATION BEHAVIOUR OF MIGRATING BIRDS

1. Migratory birds integrate information from a wide array of environmental sources. As our knowledge of migratory orientation depends heavily upon the results of cage-experiments with nocturnal migrants, it is essential that the results of these cage studies be interpreted in the light of field observations of migratory behaviour and experiments with free-flying migrants. When this is done, the impression emerges that night-migrating birds integrate directional information prior to departure, probably during the transition between daylight and darkness. At this time, information gained from the sun, in conjunction with other references, becomes especially valuable. 2. Despite intensive work with a few species, how migrants integrate information in the selection and maintenance of a direction is not well understood. The relationship between magnetic stimuli and solar cues at sunset in the selection process, for example, remains to be resolved, as does the contribution of skylight polarization patterns at sunset. Once a migratory heading is selected, birds probably use the stars or winds aloft to maintain that direction. How migrants integrate information is largely a matter of unravelling the complex causal relations among the different environmental stimuli that serve as orientation cues. Imagine a hypothetical migrant that departs on a migratory flight around the time of sunset. Given the uncertain relationship among variables (orientation cues) that might influence her migratory orientation, a path diagram is a useful device for displaying graphically the pattern of causal relations among the set of variables (see Fig. 1). This technique is adopted from path analysis, which is a statistical method developed by Sewall Wright for studying the direct and indirect causal relations among variables (see Kerlinger & Pedhazur, 1973). The pattern depicted in the figure is less a specific model of causal relations than it is a summary of possible relationships among the several cues based on current understanding. Causal flow in this ‘model’ is unidirectional, i.e. at any given point in time a variable cannot be both a cause and an effect of another variable. For example, variable 3 is dependent on variables 1 and/or 2, and is one of the independent variables in relation to variable 5 (orientation of migratory activity). Although the value of path analysis to the study of migratory orientation may be largely heuristic at this point, ‘one virtue of the method is that in order to apply it the researcher is required to make explicit the theoretical framework within which he operates’ (Kerlinger & Pedhazur, 1973). For instance, path diagrams (and path analysis, to the degree that correlations between variables can be specified) would help researchers study (i) the apparent redundancy built into the orientation process (see Fig. 1), (ii) alternative or competing causal models of orientation and navigation, or (iii) the ontogenetic changes that affect the relationship among orientation variables. Imagine, for example, how path coefficients might change in value with migratory experience. 3. Migrants probably redetermine preferred directions soon after landing or shortly before their next departure rather than while aloft. Cage-orientation results as well as observations of free-flying migrants suggest that solar-related information is involved in the morning orientation of ongoing migratory flight and possibly the re-determination of direction following night-time displacement. 4. Evidence is not clear on whether migrants respond to sunset by constant-angle orientation (menotaxis) or constant-azimuth orientation. 5. How migrants correctly identify sunset as a reference stimulus is an unresolved question. Identification might be based upon the characteristic spectral distribution of sunset, its pattern of illumination, or some other feature, such as the characteristic pattern of skylight polarization at sunset. 6. Several lines of evidence suggest that migrants learn to use the setting sun and associated skylight features as orientation cues. 7. The setting sun functions not only as a source of directional information but also as an environmental stimulus that influences the likelihood of migratory activity.     

Integration of environmental stimuli in the migratory orientation of the savannah sparrow (Passerculus sandwichensis)

Two ‘cue-conflict’ experiments were designed to evaluate the role of (1) solar cues at sunset and stars, and (2) solar cues at sunset and geomagnetic stimuli, in the migratory orientation of the savannah sparrow (Passerculus sandwichensis). A sunset and stars experiment exposed birds in the experimental group to a mirror-reflected sunset followed by an unmanipulated view of stars. Experimental birds shifted their migratory activity in accordance with the setting sun despite exposure to a normal night sky. The sunset and geomagnetism experiment exposed birds in the experimental group to a simultaneous shift in both the position of sunset and the earth's magnetic field. Again experimentals shifted their activity in accordance with the setting sun rather than the artificially shifted magnetic field. Savannah sparrows probaly use stars as celestial landmarks to maintain a preferred direction and do not reorient their activity when exposed to an alternative cue once a direction is established. Moreover, savannah sparrows with experience of migration do not require geomagnetic information in order to use the solar cues available at sunset to select a migratory direction.     

Solar cues in the migratory orientation of the Savannah sparrow, Passerculus sandwichensis

Results clearly implicate the setting sun as a critical source of directional information in the migratory orientation of the savannah sparrow, Passerculus sandwichensis. Savannah sparrows allowed a view of both sunset and stars displayed oriented behaviour in biologically meaningful directions during spring and fall seasons. When the same individuals were denied a view of sunset, and tested under the stars alone, disorientation characterized their behaviour. Furthermore, birds allowed a view of sunset, but tested under ‘overcast’ night skies (no stars visible), displayed well-oriented behaviour indicating the sufficiency of sunset. Experiments in which the migrant's internal chronometer was shifted suggested a fixed-angle (menotactic) response to the sunset cue rather than a time-compensating compass mechanism. I believe stars are valuable to this migrant as celestial reference points. Orientational information gained at the time of sunset is transferred to stars on a nightly basis. The relationship between solar and stellar cues is apparently hierarchical in the savannah sparrow. Information necessary to select the appropriate migratory direction is gained from the primary cue, the setting sun, while maintenance of that heading is dependent on a secondary cue, probably the stars.     

Mechanisms of dusk orientation in white-throated sparrows (Zonotrichia albicollis): Clock-shift experiments

Summary Several species of night migrating birds, especially North American emberizines, exhibit markedly different orientation behaviour when tested in circular cages under clear skies at dusk as compared with tests performed after complete darkness. During the period between sunset and the first appearance of stars, birds tend to show high levels of well-oriented hopping; birds deprived of exposure to clear skies at dusk hop less and their activity is usually not oriented. There is evidence that visual cues available during the dusk period, but not later, are responsible for this difference, but details of the orientation mechanisms involved are unclear. We performed 3-h fast and slow clock shifts on white-throated sparrows (Zonotrichia albicollis) to address two questions concerning migratory orientation at dusk: (1) Is the better orientation of sparrows tested at dusk a function of the visual cues available at that time, or does it result from circadian changes in motivation?; and (2) Is the dusk orientation based on a time-compensated sun compass?Sparrows subjected to a 3-h slow clock shift were tested with controls on clear, moonless nights beginning immediately after lights-off in the clock shift room and thus about 3.5 h after local sunset. Individuals of both groups performed poorly oriented hopping typical of tests performed after complete darkness. The pooled data from each group were not significantly oriented. These results show that the visual cues available shortly after sunset, not temporal changes in the motivation of the birds, are responsible for the qualitative differences in orientation.Birds exposed to a 3-h fast clock shift were tested with controls on clear evenings between sunset and the first appearance of stars. Both groups showed well-oriented hopping. The mean direction of the pooled tests of controls was 325°, a typical spring orientation direction for this species. The mean direction of the pooled tests of the clock shifted birds (274°) was significantly different from that of controls and the 51° counterclockwise shift is consistent with that predicted by a time-compensated sun compass model.     

You Can Get There from Here: Responses to Simulated Magnetic Equator Crossing by the Bobolink (Dolichonyx oryzivorus)

The avian magnetic compass works as an inclination compass. Instead of using the polarity of the magnetic field to determine direction, birds use the inclination of the dip angle. Consequently, transequatorial migrants have to reverse their response to the magnetic compass after crossing the magnetic equator. When confronted with an artificial magnetic field that reverses the vertical component of the magnetic field, migrants such as the bobolink reverse their headings relative to magnetic north even in the presence of visual cues such as stellar patterns. Bobolinks, which breed in temperate North America and winter in temperate South America, were tested in a planetarium under fixed star patterns in a series of magnetic fields incremented each night from the natural field in the northern hemisphere through an artificial horizontal field to an artificial southern hemisphere magnetic field. The birds maintained a constant heading throughout the experiment and did not reverse direction after the simulated crossing of the magnetic equator as previous experiments predicted. In nature, this response would have meant continuation of their migration flight across the equator and into the opposite hemisphere. The switch from “equatorward” orientation to “poleward” orientation is probably triggered by experience with a horizontal magnetic field and/or visual cues. The ability to maintain an accurate heading while crossing the magnetic equator may be based on the use of visual cues such as the stars.     

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

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 explicable via orientation on the basis of wind direction.     

The Direction of Celestial Rotation Affects the Development of Migratory Orientation in Pied Flycatchers,Ficedula hypoleuca

The migratory direction in young passerine migrants is based on innate information, with the geomagnetic field and celestial rotation as references. To test whether the direction of celestial rotation is of importance, hand-raised pied flycatchers in Latvia were exposed during the premigratory period to a planetarium rotating in different directions. During autumn migration, when their orientation behavior was recorded in the local geomagnetic field in the absence of celestial cues, birds that had been exposed to a sky rotating in the natural direction showed a unimodal preference of their south-westerly migratory direction. Birds that had been exposed to a sky rotating in the reversed direction, in contrast, showed a bimodal preference of an axis south-west-north-east. Their behavior was similar to that of pied flycatchers that had been raised without access to celestial cues. In Latvia, the magnetic field alone allows only orientation along the migratory axis, and celestial rotation enables birds to select the correct end of this axis. Our findings show that the direction of rotation is of crucial importance: celestial rotation is effective only if the stars move in the natural direction.     

Reed warbler orientation: initiation of nocturnal migratory flights in relation to visibility of celestial cues at dusk

We used radiotelemetry to investigate the time of migratory flight initiation relative to available celestial orientation cues and departure direction of a nocturnal passerine migrant, the reed warbler, Acrocephalus scirpaceus, during autumn migration. The study was carried out at Falsterbo, a coastal site in southwest Sweden. The warblers initiated migration from times well after local sunset and well into the night, corresponding to sun elevations between -4 degrees and -35 degrees, coinciding with the occurrence of stars at night. They departed in the expected migratory direction towards south of southwest with a few initiating migration in reverse directions towards northeast to east. Flight directions under overcast conditions (7-8/8) were more scattered than under clear sky conditions (0-4/8). There were fewer clouds on departure nights than on nights when the birds did not initiate migration. For birds staying longer than one night at stopover the horizontal visibility was higher and precipitation was less likely on departure nights than on the previous night. The results show that the visibility of celestial cues, and stars in particular, are important for the decision to initiate migration in reed warblers. However, cloud cover, horizontal visibility and precipitation might be correlated with other weather variables (i.e. wind or air pressure) that are also likely to be important for the decision to migrate. Copyright 2001 The Association for the Study of Animal Behaviour.     

Optimal use of wind by migrating birds: combined drift and overcompensation

Migrating birds may save flying time by allowing themselves to be partially drifted by strong winds at high altitude and correcting for the displacement at low altitude under relatively weaker winds. This behaviour will be favourable with strong upper winds and with wind direction approx. 30 ° to 90 ° in relation to the goal direction (following side winds). Radar observations of drift in high altitude bird migration and visual records of low altitude overcompensation are compatible with the optimal flight behaviour of migrants at high and low altitude, respectively, as predicted from this hypothesis.     

SPRING MIGRATION OVER PUERTO RICO AND THE WESTERN ATLANTIC, A RADAR STUDY

Migration over Puerto Rico was recorded by time-lapse filming of the display of a long-range surveillance radar on 40 days and 37 nights in the period 2 March-29 May 1971. Moderate density movements occurred every night; low density movements occurred on most days. Many birds, primarily passerines, took off from Puerto Rico each evening at 20–45 minutes after sunset.
Almost all birds flew to the west, NW or north. Birds were seen approaching from the direction of the Windward Islands and Venezuela, over Puerto Rico, and departing towards the Bahamas and eastern coast of the U.S. Uni- and multivariate analyses showed that the number of birds departing W-N each evening was positively correlated with following winds.
There is less night-to-night variation in the amount of migration at Puerto Rico than in eastern North America. However, this is apparently the result of less variable weather in the tropics, not the result of any lesser degree of meteorological selectivity by the migrants.
The tracks of the birds were correlated with wind direction. Birds moved WNW-NW with NE side winds but NW-NNW with SE following winds. The tracks were rarely exactly downwind. The variance amongst the directions of individual birds at any given time was usually small and not correlated with cloud cover or magnetic disturbances. The estimated headings of the birds varied from day to day in a pattern suggesting adjustment of headings to compensate at least partially for lateral wind drift.
In autumn many birds approach Puerto Rico from the north or even east of north; in spring few birds moved in the opposite directions. This difference in routes takes advantage of prevailing wind patterns.     

The Orientation Behaviour of Chaffinches, Fringilla coelebs, Caught during Active Migratory Flight, in Relation to the Sun

The few orientation studies that have been carried out with day-migrating birds show that they are able to use solar and magnetic orientation cues for orientation. Previous orientation experiments in Emlen funnels have been carried out either with hand-raised birds or with birds caught during resting periods at stop-over sites. The aim of our study was to test whether birds caught during active flight show a higher concentration of migratory activity in the seasonally appropriate migratory direction in the funnels than birds that had not experienced migration just before the funnel experiments. The topography at the alpine pass Col de Bretolet at the border of Switzerland and France allowed us to capture birds during active migratory flight. These birds were in full migration disposition. Orientation experiments with chaffinches suggested an influence of the sun because chaffinches did not orient in the seasonally expected direction, but probably showed positive phototaxis towards the light of the sun at the opposite side of the funnel. Chaffinches tested under overcast conditions oriented to the north-west which probably was a 'nonsense' orientation and not a reverse migration or compensatory behaviour. We conclude that freshly caught birds are too stressed to show appropriate orientation when tested immediately after catching.     

Convergent patterns of long-distance nocturnal migration in noctuid moths and passerine birds

Vast numbers of insects and passerines achieve long-distance migrations between summer and winter locations by undertaking high-altitude nocturnal flights. Insects such as noctuid moths fly relatively slowly in relation to the surrounding air, with airspeeds approximately one-third of that of passerines. Thus, it has been widely assumed that windborne insect migrants will have comparatively little control over their migration speed and direction compared with migrant birds. We used radar to carry out the first comparative analyses of the flight behaviour and migratory strategies of insects and birds under nearly equivalent natural conditions. Contrary to expectations, noctuid moths attained almost identical ground speeds and travel directions compared with passerines, despite their very different flight powers and sensory capacities. Moths achieved fast travel speeds in seasonally appropriate migration directions by exploiting favourably directed winds and selecting flight altitudes that coincided with the fastest air streams. By contrast, passerines were less selective of wind conditions, relying on self-powered flight in their seasonally preferred direction, often with little or no tailwind assistance. Our results demonstrate that noctuid moths and passerines show contrasting risk-prone and risk-averse migratory strategies in relation to wind. Comparative studies of the flight behaviours of distantly related taxa are critically important for understanding the evolution of animal migration strategies.     

Das Orientierungssystem der V?gel I. Kompa?mechanismen

Zusammenfassung V?gel stellen den Bezug zum Ziel indirekt über ein externes Referenzsystem her. Der Navigationsproze? besteht deshalb aus zwei Schritten: zun?chst wird die Richtung zum Ziel als Kompa?kurs festgelegt, dann wird dieser Kurs mit Hilfe eines Kompa?mechanismus aufgesucht. Das Magnetfeld der Erde und Himmelsfaktoren werden von den V?gel als Kompa? benutzt. In der vorliegenden Arbeit werden der Magnetkompa?, der Sonnenkompa? und der Sternkompa? der V?gel in ihrer Funktionsweise, ihrer Entstehung und ihrer biologischen Bedeutung vorgestellt. Der Magnetkompa? erwies sich als Inklinationskompa?, der nicht auf der Polarit?t, sondern auf der Neigung der Feldlinien im Raum beruht; er unterscheidet „polw?rts“ und „?quatorw?rts“ statt Nord und Süd. Er ist ein angeborener Mechanismus und wird beim Vogelzug und beim Heimfinden benutzt. Seine eigentliche Bedeutung liegt jedoch darin, da? er ein Referenzsystem bereitstellt, mit dessen Hilfe andere Orientierungsfaktoren zueinander in Beziehung gesetzt werden k?nnen. Der Sonnenkompa? beruht auf Erfahrung; Sonnenazimut, Tageszeit und Richtung werden durch Lernprozesse miteinander verknüpft, wobei der Magnetkompa? als Richtungsreferenzsystem dient. Sobald er verfügbar ist, wird der Sonnenkompa? bei der Orientierung im Heimbereich und beim Heimfinden bevorzugt benutzt; beim Vogelzug spielt er, wahrscheinlich wegen seiner Abh?ngigkeit von der geographischen Breite, kaum eine Rolle. Der Sternkompa? arbeitet ohne Beteiligung der Inneren Uhr; die V?gel leiten Richtungen aus den Konfigurationen der Sterne zueinander ab. Lernprozesse erstellen den Sternkompa? in der Phase vor dem ersten Zug; dabei fungiert die Himmelsrotation als Referenzsystem. Sp?ter, w?hrend des Zuges, übernimmt der Magnetkompa? diese Rolle. Die relative Bedeutung der verschiedenen Kompa?systeme wurde in Versuchen untersucht, bei denen Magnetfeld und Himmelsfaktoren einander widersprechende Richtungs-information gaben. Die erste Reaktion der V?gel war von Art zu Art verschieden; langfristig scheinen sich die V?gel jedoch nach dem Magnetkompa? zu richten. Dabei werden die Himmelsfaktoren umgeeicht, so da? magnetische Information und Himmelsinformation wieder im Einklang stehen. Der Magnetkompa? und die Himmelsfaktoren erg?nzen einander: der Magnetkompa? ersetzt Sonnen- und Sternkompa? bei bedecktem Himmel; die Himmelsfaktoren erleichtern den V?geln das Richtungseinhalten, zu dem der Magnetkompa? offenbar wenig geeignet ist. Magnetfeld und Himmelsfaktoren sollten deshalb als integrierte Komponenten eines multifaktoriellen Systems zur Richtungsorientierung betrachtet werden.

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The orientation system of birds — I. Compass mechanisms

Summary Because of the large distances involved, birds establish contact with their goal indirectly via an external reference. Hence any navigation is a two-step process: in the first step, the direction to the goal is determined as a compass course; in the second step, this course is located with a compass. The geomagnetic field and celestial cues provide birds with compass information. The magnetic compass of birds, the sun compass the star compass and the interactions between the compass mechanisms are described in the present paper. Magnetic compass orientation was first demonstrated by testing night-migrating birds in experimentally altered magnetic fields: the birds changed their directional tendencies according to the deflected North direction. The avian magnetic compass proved to be an inclination compass: it does not use polarity; instead it is based on the axial course of the field lines and their inclination in space, distinguishing “poleward” and “equatorward” rather than North and South. Its functional range is limited to intensities around the local field strength, but this biological window is flexible and can be adjusted to other intensities. The magnetic compass is an innate mechanism that is widely used in bird migration and in homing. Its most important role, however, is that of a basic reference system for calibrating other kinds of orientation cues. Sun compass orientation is demonstrated by clock-shift experiments: Shifting the birds' internal clock causes them to misjudge the position of the sun, thus leading to typical deflections which indicate sun compass use. The analysis of the avian sun compass revealed that it is based only on sun azimuth and the internal clock; the sun's altitude is not involved. The role of the pattern of polarized light associated with the sun is unclear; only at sunset has it been shown to be an important cue for nocturnal migrants, being part of the sun compass. The sun compass is based on experience; sun azimuth, time of day and direction are combined by learning processes during a sensitive period, with the magnetic compass serving as directional reference. When established, the sun compass becomes the preferred compass mechanism for orientation tasks within the home region and homing: in migration, however, its role is minimal, probably because of the changes of the sun's arc with geographic latitude. The star compass was demonstrated in night-migrating birds by projecting the northern stars in different directions in a planetarium. The analysis of the mechanism revealed that the internal clock is not involved; birds derive directions from the spatial relationship of the star configurations. The star compass is also established by experience; the directional reference is first provided by celestial rotation, later, during migration, by the magnetic compass. The relative importance of the various compass mechanisms has been tested in experiments in which celestial and magnetic cues gave conflicting information. The first response of birds to conflicting cues differs considerably between species; after repeated exposures, however, the birds oriented according to magnetic North, indicating a long-term dominance of the magnetic compass. Later tests in the absence of magnetic information showed that celestial cues were not simply ignored, but recalibrated so that they were again in agreement with magnetic cues. The magnetic compass and celestial cues complement each other: the magnetic field ensures orientation under overcast sky; celestial cues facilitate maintaining directions, for which the magnetic compass appears to be ill suited. In view of this, the magnetic field and celestial cues should be regarded as integrated components of a multifactorial system for directional orientation.

     

Nocturnal migratory songbirds adjust their travelling direction aloft: evidence from a radiotelemetry and radar study

In order to fully understand the orientation behaviour of migrating birds, it is important to understand when birds set their travel direction. Departure directions of migratory passerines leaving stopover sites are often assumed to reflect the birds'' intended travel directions, but this assumption has not been critically tested. We used data from an automated radiotelemetry system and a tracking radar at Falsterbo peninsula, Sweden, to compare the initial orientation of departing songbirds (recorded by radiotelemetry) with the orientation of songbird migrants in climbing and level flight (recorded by radar). We found that the track directions of birds at high altitudes and in level flight were more concentrated than the directions of departing birds and birds in climbing flight, which indicates that the birds adjust their travelling direction once aloft. This was further supported by a wide scatter of vanishing bearings in a subsample of radio-tracked birds that later passed an offshore radio receiver station 50 km southeast of Falsterbo. Track directions seemed to be more affected by winds in climbing compared with level flights, which may be explained by birds not starting to partially compensate for wind drift until they have reached cruising altitudes.     

Compass orientation and possible migration routes of passerine birds at high arctic latitudes

The use of celestial or geomagnetic orientation cues can lead migratory birds along different migration routes during the migratory journeys, e.g. great circle routes (approximate), geographic or magnetic loxodromes. Orientation cage experiments have indicated that migrating birds are capable of detecting magnetic compass information at high northern latitudes even at very steep angles of inclination. However, starting a migratory journey at high latitudes and following a constant magnetic course often leads towards the North Magnetic Pole, which means that the usefulness of magnetic compass orientation at high latitudes may be questioned. Here, we compare possible long‐distance migration routes of three species of passerine migrants breeding at high northern latitudes. The initial directions were based on orientation cage experiments performed under clear skies and simulated overcast and from release experiments under natural overcast skies. For each species we simulated possible migration routes (geographic loxodrome, magnetic loxodrome and sun compass route) by extrapolating from the initial directions and assessing a fixed orientation according to different compass mechanisms in order to investigate what orientation cues the birds most likely use when migrating southward in autumn. Our calculations show that none of the compass mechanisms (assuming fixed orientation) can explain the migration routes followed by night‐migrating birds from their high Nearctic breeding areas to the wintering sites further south. This demonstrates that orientation along the migratory routes of arctic birds (and possibly other birds as well) must be a complex process, involving different orientation mechanisms as well as changing compass courses. We propose that birds use a combination of several compass mechanisms during a migratory journey with each of them being of a greater or smaller importance in different parts of the journey, depending on environmental conditions. We discuss reasons why birds developed the capability to use magnetic compass information at high northern latitudes even though following these magnetic courses for any longer distance will lead them along totally wrong routes. Frequent changes and recalibrations of the magnetic compass direction during the migratory journey are suggested as a possible solution.     

A RADAR STUDY OF THE AUTUMN MIGRATION OF WOOD PIGEONS COLUMBA PALUMBUS IN SOUTHERN SCANDINAVIA

The migration of Wood Pigeons in southern Scandinavia was studied from 21 September to 10 October 1971 and from 16 September to 15 November 1972 using radar stations supplemented with observations from an aircraft and a network of ground observers. By far the largest quantities of Wood Pigeons migrated after cold front passages with northwesterly to northeasterly tailwinds. Most birds departed on a few days, apparently as a consequence of strong preference for tailwind situations. With northwesterly winds a proportionately high migratory activity was recorded in the Kattegatt area. With northeasterly winds activity was higher in the Baltic area. This allowed the Wood Pigeons to make maximal use of the tailwind component, and their ground speed usually exceeded 80 km/h. The calculated mean air speed was 60 km/h. Their dependence on tailwind was particularly strong when the birds were engaging in long sea-crossings, such as across the Kattegatt. Different coastlines affected the geographical pattern of migration in different ways. Frequently Wood Pigeon flocks flew almost parallel to the coast but some distance off shore, until they finally departed. The deflective force of coastlines was greatest when the birds' ground speed was low, that is, under headwind conditions or in calm weather. Mean track directions measured over two areas in northern Skane, called Inland W and Inland E, situated about 60 km apart, differed by 11, those over the western area being directed more to the south than those over the eastern. No significant correlation with wind directions was found in these areas. Combining data from the whole land area, however, track directions were found to vary from day to day in significant correlation to the wind direction. Mean track directions over the Baltic agreed with those over Skane, but both differed significantly from those over the Kattegatt. Both over the Baltic and over the Kattegatt directions were significantly correlated with wind directions, and showed greater variation than track directions over land. Daily track differences over the Baltic resulted both from differences taking place over the land, and from real wind deflection (drift). Both over the land and over the sea heading directions were correlated with wind directions, suggesting compensatory efforts on the part of the birds. On three days extensive fog covered much of the study area. Wood Pigeons continued to migrate, but certain aberrations in their behaviour were noted. Over land migration was relatively heavier in the west with northwesterly winds and in the east with northeasterly winds. The correlation demonstrated between wind direction and the mean track direction was based upon the fact that populations with different inherent primary directions made up different proportions of the migrating cohorts under different wind conditions (pseudo-drift). The incomplete compensation for wind deflection over the sea is ascribed to the lack of visual orientation cues. The more accurate orientation possible over land suggests one reason for the birds' reluctance to flights across the open sea. When mean track directions of Wood Pigeons in different parts of southern Scandinavia were related to the migratory goals of these birds, it was found that they have to change their primary direction in the course of their journey from breeding to wintering areas.     

Navigational ability in the domestic fowl (Gallus gallus domesticus)

Little is known about the navigational abilities of domestic fowl. The question of how chickens represent and orient in space becomes relevant when they are kept in non-cage systems. Since the sun is known to be the dominant spatial organiser in other diurnal bird species, we started our investigation of the chicken’s spatial abilities by subjecting them to a food-searching task with the sun as the only consistent visual cue. In an additional experiment we tried to rule out the use of auditory cues in finding a food reward.

Eight ISA Brown chicks were housed in outdoor pens. A separate test arena comprised an open-topped, opaque-sided wooden octagon (2 m wide and 1.5 m high). Eight goal boxes with food pots were attached to each of the arena sides; a wooden barrier inside each goal box prevented the birds from seeing the food pot before entering. After habituation we tested during five daily 5 min trials whether the chicks were able to find food in a systematically allocated goal direction. Food residue in every foot pot controlled for the use of olfactory cues and no external landmark cues were visible. Every day each box was unpredictably moved to a randomly assigned side of the arena and the side to face north was also randomly allocated, to prevent the chicks from using cues other than the sun’s position. Circular statistics were used to determine whether birds moved in a non-random direction and if so, if they significantly oriented goalwards. The results showed that seven of the eight birds moved significantly in the goal direction. It seems likely that the chicks used the sun to orient. Due to weather constraints only four chicks received the same treatment on a new location, to rule out the use of auditory cues. Two of these four chicks significantly moved in the goal direction.

The results from our experiments show that domestic chicks use spatial memory to orient towards a hidden goal. Moreover, their orientation is most likely to be based on sun cues opening up the possibility that the sun compass may dominate even in this ancestrally predominantly ground-living forest bird.     

THE NIGHTLY INITIATION OF PASSERINE MIGRATION IN SPRING: A DIRECT VISUAL STUDY

The nightly initiation of migration of passerine birds was studied during a spring season in coastal Louisiana. A horizontally-directed portable ceilometer placed on a tower illuminated birds as they took off from or landed in the trees. Daily censuses were conducted to supplement nocturnal observations. Local weather conditions were continuously monitored at the site of the study. Appreciable numbers of migrants landed in the woods on the coast almost every day, the first birds usually appearing between 10.30 and 11.00 hrs, and the last about 16.00 hrs. On two occasions birds were seen landing after dark. The migrants usually became quiet by about 18.00 hrs and the first bird was usually seen leaving the trees just over an hour later. The exodus usually peaked between 19.10 and 19.15 hrs, 40 to 45 minutes after sunset. The time of initiation of nocturnal migration was not significantly affected by immediate weather factors. The duration of the exodus varied widely, and was related to the number of individual birds in the census area. On some occasions grounded migrants stayed overnight, but only when there had been an active cold front over the Gulf. No local weather conditions examined were found to be directly inhibitory to nocturnal migration. Birds departed singly and without preliminary activity. Wind direction influenced the initial orientation of the migrants. With southerly winds, 57% of the birds seen flying across the beam initially headed toward the Gulf, while with northerly winds only 19% initially flew in this direction.     

Wind drift,compensation, and the use of landmarks by nocturnal bird migrants

In this paper we describe fall nocturnal migration at three localities in eastern New York, one adjacent to the Hudson River, the other two 30 km to the west in a topographically more uniform area. Migrants at both study areas moved southwest in winds not out of the west and were, therefore, seemingly unaffected by the river. In west winds, however, birds away from the river moved south-southeast whereas those in the vicinity of the river flew a track west of south paralleling the river. In addition, a relative increase in the number of migrants along the river compared to away was observed in west winds as birds presumably became concentrated near the river. We conclude that on most autumn nights migrants passing through this area have a preferred track direction toward the southwest and in strong winds from the west and northwest they are drifted. Upon reaching the vicinity of the Hudson River, some birds alter their headings yielding a track direction that closely parallels the river resulting in at least a partial compensation for wind drift. No alternative hypothesis is consistent with all the data.     

Shifted magnetic fields lead to deflected and axial orientation of migrating robins,Erithacus rubecula,at sunset

The migratory orientation of the robin was tested in shifted magnetic fields during the twilight period after sunset, under clear skies and under simulated total overcast. The horizontal direction of the geomagnetic field was shifted 90° to the right or left in relation to the local magnetic field, without changing either the intensity of the field or its angle of inclination. Experiments were conducted during both spring and autumn, with robins captured as passage migrants at the Falsterbo and Ottenby bird observatories in southern Sweden as test subjects. Generally, the orientation of robins was affected by magnetic shifts compared to controls tested in the natural geomagnetic field. Autumn birds from the two capture sites differed in their responses, probably because of different migratory dispositions and body conditions. The robins most often changed their orientation to maintain their typical axis of migration relative to the shifted magnetic fields. However, preferred directions in relation to the shifted magnetic fields were frequently reverse from normal, or axial rather than unimodal. These results disagree with suggested mechanisms for orientation by visual sunset cues and with the proposed basis of magnetic orientation. They do, however, demonstrate that the geomagnetic field is involved in the sunset orientation of robins, probably in combination with additional visual or non-visual cues that contribute to establish magnetic polarity.