Magnetic Cues Research Articles

Monarch butterflies (Danaus plexippus L.) use a magnetic compass for navigation.

Fall migratory monarch butterflies, tested for their directional responses to magnetic cues under three conditions, amagnetic, normal, and reversed magnetic fields, showed three distinct patterns. In the absence of a magnetic field, monarchs lacked directionality as a group. In the normal magnetic field, monarchs oriented to the southwest with a group pattern typical for migrants. When the horizontal component of the magnetic field was reversed, the butterflies oriented to the northeast. In contrast, nonmigratory monarchs lacked directionality in the normal magnetic field. The results are a direct demonstration of magnetic compass orientation in migratory insects.

Orientation in pied flycatchers: the relative importance of magnetic and visual information at dusk

We investigated the orientation of juvenile pied flycatchers,Ficedula hypoleuca, during autumn migration in south Sweden using orientation cage experiments, to study the relative importance of visual and magnetic information at sunset. We performed cage tests under 12 experimental conditions that manipulated the geomagnetic and visual sunset cues available for orientation: natural clear skies in the local or a vertical magnetic field; simulated total overcast in the local or a vertical magnetic field; natural pattern of skylight polarization and directional information from stars screened off, with the sun’s position as normal or shifted 120° anticlockwise with mirrors; reduced polarization in the local or a vertical magnetic field; directions of polarization (e-vector) NE/SW and NW/SE, respectively, in the local or a vertical magnetic field. The pied flycatchers were significantly oriented towards slightly south of west when they could use a combination of skylight and geomagnetic cues. The mean orientation was significantly shifted along with the deflection of the sunset position by mirrors. Reduced polarization had no significant effect on orientation either in the local, or in a vertical, magnetic field. The birds tended to orient parallel with the axis of polarization, but only when the artificial e-vector was aligned NW/SE. The mean orientation under simulated total overcast in a vertical, and in the local, magnetic field was not significantly different from random. It is difficult to rank either cue as dominant over the other and we conclude that both visual and magnetic cues seem to be important for the birds’ orientation when caught and tested during active migration.

After-Effects of Exposure to Conflicting Celestial and Magnetic Cues at Sunset in Migratory Silvereyes Zosterops l. lateralis

Migratory birds can use celestial and magnetic cues to locate their migratory direction. The relative importance of the two cue systems was analyzed by testing Australian Silvereyes for their orientation behavior in a cue-conflict situation during spring at dusk. Control birds tested under the natural sky in the local geomagnetic field headed in their southerly migratory direction. The experimental birds, tested under the natural sky in a magnetic field with magnetic North turned to 2400 WSW, followed the shift in magnetic North and preferred ENE directions that corresponded to magnetic South. Subsequently, all birds were tested under the sky in the absence of magnetic information. Both groups continued in their previous direction, control birds south and experimental birds northeast, by celestial cues alone. Tested indoors with the magnetic field as the only cue, both groups headed southward. These findings indicate a dominant role of magnetic cues: in case of conflicting cues, the birds maintained their magnetic heading and recalibrated the celestial cues on the basis of the altered magnetic field. This is in contrast to recent findings with Savannah Sparrows Passerculus sandwichensis; one possible reason for these differences may be that the Savannah Sparrows were tested in their breeding area at high latitudes where celestial cues might generally be of greater importance, whereas our Silvereyes were tested at lower latitudes where the magnetic field appears to play a more important role.

Interaction of Magnetic and Celestial Cues in the Migratory Orientation of Passerines

Cage experiments with various species of migrating passerines indicate two distinct phases in migratory orientation in which magnetic and celestial cues interact in different ways. The first phase during the premigratory period involves the conversion of genetically coded information into an actual compass course. Celestial rotation and the geomagnetic field serve as external references. Celestial rotation alone, mediated by rotating stars at night or by the changing pattern of polarized light during the day, provides a reference direction away from its center, corresponding to geographic South. Population-specific deviations from this reference direction appear to be coded only with respect to the magnetic field. Both cue systems interact to produce the population-specific migratory course. In the role of providing the reference direction, celestial rotation dominates over the magnetic field during this first premigratory phase. Magnetic South, however, can also serve as reference direction when celestial cues are not available. The second phase involves orientation during migration, once the course is set and the birds have left their breeding area. Migrants en route have several options to locate this course, namely a star compass, sunset cues and the magnetic field, and they seem to make use of them all. During this phase, however, the magnetic compass dominates over celestial cues, as indicated by cue-conflict experiments. In case of conflict, the directional significance of stellar cues and sunset cues is adjusted to be in agreement with the ambient magnetic field. Migratory orientation is thus based on an integrated system of celestial and magnetic cues that reverse their dominant role between the premigratory phase and actual migration. The reasons may lie in the changes in the sky and in the magnetic field migrants experience when travelling from higher to lower latitudes towards their winter quarters.

Redstarts,Phoenicurus phoenicurus, can orient in a true-zero magnetic field

I tested the migratory orientation of redstarts in a true-zero magnetic field to elucidate the importance to this species of access to either magnetic or celestial cues. I also tested the validity of the assumption on which all funnel experiments are based: that what we observe in an orientation funnel reflects what the bird would do if actually migrating. In a set of funnel experiments, I tested 47 night-migrating redstarts caught during their first autumn migration. Each bird was tested once under each of four experimental conditions in a semi-randomized block-design. Upon completion of the final funnel tests the birds were fitted with light indicators and released. The results showed that redstarts can find the migratory direction on the basis of access to either celestial or magnetic cues. Thus, when celestial cues were available they could orient in a true-zero magnetic field. In addition, a starry sky facilitated high migratory activity as well as a clearly directed orientation. The release experiments showed that the vanishing bearings on nights with no wind were in good agreement with the direction of the funnel activity, but that the vanishing bearings were strongly influenced even by light wind.

Lacustrine sockeye salmon return straight to their natal area from open water using both visual and olfactory cues.

Mechanisms of the amazing ability of salmon to migrate a long distance from open water to natal streams for spawning are still unknown. Lacustrine sockeye salmon (Oncorhynchus nerka) in Lake Toya offers an excellent model system for studying the orientation mechanism in open water, because mature fish return to the natal area with a high degree of accuracy. First we examined the percentage of fish returning to the natal area after they were released 7 km south of the natal area. Forty percent of control male mature fish and 25% of the fish blinded by injection of a mixture of carbon toner and corn oil into the eyeball were captured in the natal area within 5 days. Forty-four percent of fish with brass rings (control) and 31% of fish with NdFe magnetic rings which interfere with the magnetic cue were captured in the natal area within 3 days. These experiments suggested that, although the number of blinded fish captured in the natal area was less than that of the controls, the difference was not statistically significant. In the fish captured in the natal area within 3 or 5 days, fish which found the natal area using their olfactory cue after random swimming for a long time and returned to that area may be included. Hence we tracked fish telemetrically using an ultrasonic tracking system, and found that mature males released at a long distance (3.6 or 6.8 km) from the natal area swam straight to the vicinity of the natal area. Interference of the magnetic cue by the attachment of a magnetic ring did not affect their direct return. Blockage of the visual cue caused them to move randomly. These data suggest that lacustrine sockeye salmon return straight to the vicinity of the natal area using their visual cue and finally reach the exact homing point using their olfactory cue.

Magnetic versus celestial cues: cue-conflict experiments with migrating silvereyes at dusk

To assess the relative importance of celestial and magnetic cues for orientation at dusk, Australian silvereyes, Zosterops l. lateralis, were subjected to artificial magnetic fields under the natural evening sky, beginning 30 min before sunset. Control birds tested in the local geomagnetic field preferred their normal south-southwesterly migratory direction. Birds tested in a magnetic field with north deflected counterclockwise to 240°WSW showed northeasterly tendencies from the first test onward. Birds subjected to a corresponding clockwise deflection to 120°ESE, in contrast, first showed southerly directions, but from the 7th test onward shifted towards the northwest. Hence, both experimental groups followed the shift in magnetic north, one immediately, the other after a delay. When the birds were later tested in a vertical magnetic field without directional information, the two experimental groups continued in the direction they had preferred in the artificial magnetic fields, presumably by celestial cues alone. This indicates that they had not simply ignored celestial cues, but had recalibrated them according to the altered magnetic fields. The reasons for the initial difference between the two experimental groups remain unclear. Delayed responses to deflections of magnetic north have also been observed in previous studies. They appear to be the main reason why studies that expose birds only once to a cue-conflict situation often seem to indicate a dominance of celestial cues, whereas studies exposing the birds repeatedly usually indicate a dominance of magnetic cues.

The direction of celestial rotation influences the development of stellar orientation in young garden warblers (Sylvia borin)

The study presented here was conducted in order to analyze the role of the direction of celestial rotation in the development of stellar orientation in young migratory birds. The test birds were garden warblers, Sylvla borin, which leave their breeding ground on a southwesterly compass course. The birds were hand-raised and, during the premigratory period, exposed to an artificial 'sky' in the local geomagnetic field. For the control group C, the star pattern was rotating in the natural direction, with the centre of rotation and magnetic North coinciding. For the three experimental groups, the star pattern was rotating in the opposite direction; for group E1, the centre of rotation coincided with magnetic North, for group E2 the centre of rotation was at magnetic West and for group E3 it was at magnetic East. During autumn migration, the birds were tested without magnetic information under the same, now stationary, sky. All four groups were able to use stellar information for orientation, but only the control group preferred the normal southwesterly course. The three experimental groups, in contrast, all oriented towards a significantly different direction, preferring due south. The results for group E1 showed less scatter than those for the other two experimental groups. These results indicate that the direction of celestial rotation is crucial for the development of the normal migratory course with respect to the stars in young garden warblers. Establishing the species-specific southwesterly migratory course requires an interaction between celestial rotation and magnetic cues; this interaction appears to depend on the natural direction of celestial rotation. Rotation in the reverse direction allowed the birds to respond only in a manner that oriented them away from the centre of rotation.

Magnetic orientation of snow buntings (Plectrophenax nivalis), a species breeding in the high Arctic: passage migration through temperate-zone areas

Orientation tests were conducted with snow buntings (Plectrophenax nivalis) exposed to artificially manipulated magnetic fields, during both spring and autumn migration. Experiments were run under clear sunset skies and under simulated complete overcast. The birds closely followed experimental shifts of the magnetic fields during both seasons regardless of whether they had access to celestial cues. Clear-sky tests in vertical magnetic fields resulted in a significant bimodal orientation, the directionality of which was almost identical during spring and autumn. When the snow buntings were deprived of celestial directional information and tested in vertical magnetic fields, they failed to show any statistically significant mean directions in either spring or autumn. The results demonstrate that snow buntings possess a magnetic compass and suggest that magnetic cues are of primary importance for their migratory orientation while on passage through temperate-zone areas. However, the axial orientation in vertical magnetic fields under clear skies may indicate an involvement of celestial cues as an auxiliary source of directional information.

A sensitive optically detected magnetic compass for animals.

Recent experiments have indicated that at least one important magnetic compass used by many animals for navigation may be located in the eye. Here it is shown that a very sensitive magnetic compass is formed by the incorporation of a small quantity of ferrimagnetic single-domain crystals in a droplet of nematic liquid crystal. Optical detection of the compass output is illustrated by experiment and the predicted properties of a biological compass, based upon these principles, are compared with the known properties of the natural compass. Some experiments that could test the model are described.

The geographical scale factor in orientation of migrating birds

Migration routes of birds throw light on orientation performance at different geographic scales, over distances ranging from a few kilometres to more than 104 km. Detailed knowledge about the flight routes may be used to test predictions about optimal orientation according to theoretical principles and about the use of compasses based on celestial or magnetic cues. Ringing recoveries demonstrate that the migratory journey of many species, such as the wheatear and willow warbler, is divided into successive legs with different main orientation. Autumn and spring migration routes are often different, sometimes diverging on a continental scale. Aerial radiotracking of whooping cranes in North America and satellite tracking of brent geese migrating from Iceland across the Greenland ice cap point to the significant role of large-scale topography for the shaping of migration routes. Compass and position control are also required, e.g. during long passages across featureless sea or ice, but how these elements are integrated into the birds' orientation system remains unclear. Radar studies from the Arctic Ocean illustrate the importance of map projections for interpreting flight paths and suggest that birds accomplish approximate great circle orientation. Gradual course changes shown by migrating knots monitored by radar in Scandinavia are at variance with expected changes if the birds were to use a star, sun or magnetic compass over longer distances. Accurate recording of short flight segments shows how flying birds respond to visual, audible and electromagnetic cues, and also documents orientation precision and capacity to integrate rapidly shifting courses into a consistent resulting orientation. Analyses of flight patterns are crucial for understanding how birds find and follow their migration routes over different ranges of geographical scale.

The flexible migratory orientation system of the savannah sparrow (Passerculus sandwichensis)

The orientation system of the Savannah sparrow (Passerculus sandwichensis) is typical of nocturnal migrant passerine birds. It is based on a system of interacting compass senses: magnetic, star, polarized light and, perhaps, sun compasses. The magnetic compass capability develops in birds that have never seen the sky, but the preferred direction of magnetic orientation may be calibrated by celestial rotation (stars at night and polarized skylight patterns during the day). This ability to recalibrate magnetic orientation persists throughout life and enables the bird to compensate for variability in magnetic declination that may be encountered as it migrates. The polarized light compass may be manipulated by exposing young birds to altered patterns of skylight polarization. There is some evidence that the magnetic field may be involved in calibration of the polarized light compass. In short-term orientation decision-making during migration, visual information at sunset overrides both stars and magnetic cues, and polarized skylight is the relevant stimulus in dusk orientation. The star pattern compass seems to be of little importance. This extremely flexible orientation system enables the birds to respond to spatial and temporal variability in the quality and availability of orientation information.

Comparative orientation experiments with different species of passerine long-distance migrants: effect of magnetic field manipulation

Abstract. The orientation of four species of passerine long-distance migrants was studied in spring and autumn by orientation cage experiments during the twilight period after sunset in Sweden. Two groups of migrants from the Palaearctic-African migration system were used: migrants wintering mainly north of the magnetic equator in west Africa (pied flycatcher, Ficedula hypoleuca, and redstart, Phoenicurus phoenicurus) and migrants wintering south of the magnetic equator in southeast Africa (thrush nightingale, Luscinia luscinia, and marsh warbler, Acrocephalus palustris). Orientation experiments were conducted in three magnetic conditions, in the local geomagnetic field and in a deflected and in a vertical magnetic field, under clear and simulated total overcast conditions, respectively. The results did not provide any convincing indications about differences in the orientation system between the two groups of migrants. The responses of all species seemed to be affected in a similarly complex way by celestial as well as magnetic cues, involving conflicting elements of a possible attraction towards the brightest part of the twilight sky as well as orientation in the migratory direction. Shifting the horizontal component of the magnetic field neither shifted nor disrupted orientation. Abolishing the horizontal component of the magnetic field increased the orientational scatter in three species in spring and one in autumn. Simulated total overcast largely abolished orientation, except in two species (in the local geomagnetic field only) that do not migrate across the geomagnetic equator.

ACQUISITION OF MAGNETIC DIRECTIONAL PREFERENCE IN HATCHLING LOGGERHEAD SEA TURTLES

During their natal migration, hatchling loggerhead sea turtles (Caretta caretta L.) establish courses towards the open ocean and maintain them after swimming beyond sight of land. Laboratory experiments have demonstrated that swimming hatchlings can orient using the earth's magnetic field. For the magnetic compass to function in guiding the offshore migration, however, hatchlings must inherit or acquire a magnetic directional preference that reliably leads them towards the open sea. On land, hatchlings find the ocean using light cues associated with the seaward horizon. To determine whether turtles might acquire a preference for a specific magnetic direction on the basis of such cues, we studied the magnetic orientation of turtles initially exposed to light from either magnetic east or west. Hatchlings that had been exposed to light in the east subsequently oriented eastward when tested in darkness, whereas those that had been exposed to light in the west swam westward. Reversing the magnetic field resulted in a corresponding shift in orientation, indicating that the turtles were orienting to the ambient magnetic field. These results demonstrate that light cues can set the preferred direction of magnetic orientation by loggerhead hatchlings. We therefore hypothesize that hatchlings initially establish a seaward course using visual cues available on or near land, then maintain the course using magnetic cues as they migrate into the open sea.

Orientation cues used by migratory birds: A review of cue-conflict experiments

During the late 1960s and early 1970s the accumulating evidence of magnetic orientation forced the conclusion that the orientation of migratory birds and homing pigeons is based upon multiple stimuli. ‘Cue-conflict experiments’ have provided a powerful means of asking how these directional cues relate one to another. The weight of evidence suggests that in short-term orientation decision making, magnetic cues take precedence over stars, and visual information at sunset overrides both these stimuli. Recent experiments point to polarized skylight patterns as the relevant cue in dusk orientation. Although cue-conflict experiments have now been performed on a diversity of species, generalizations are weakened because of differences in experimental design, in the cues examined and in our ability to manipulate those cues. There remains a need for carefully designed comparative studies.

Clock-Shift Experiments with Migratory Yellow-Faced Honeyeaters, Lichenostomus Chrysops (Meliphagidae), an Australian Day-Migrating Bird

ABSTRACT The behaviour of an Australian day migrant, the yellow-faced honeyeater Lichenostomus chrysops, was studied in order to assess the role of the sun in migratory orientation. During autumn migration, all tests took place under a sunny sky; birds were tested while living in the natural photoperiod (control) and with their internal clock shifted 4h fast and 4h slow. In spring, all birds were shifted 3h fast; tests in overcast conditions, with the birds relying on their magnetic compass, served as controls. In control tests in both seasons, the birds preferred directions corresponding to those observed in the wild. When tested under sunny conditions with their internal clock shifted, the birds changed their directional tendencies. However, their preferred directions were different from those expected if a time-compensating sun compass was being used. After about 6 days, the shifted birds’ directions were no longer different from the control direction. This behaviour argues against a major role of the sun compass in the orientation of day migrants. The dramatic changes of the sun’s arc with geographic latitute might cause day-migrating birds to prefer a more constant orientation cue, such as the geomagnetic field. The initial response to the clock-shift might have occurred because the birds were confused by the conflicting information from solar and magnetic cues. This suggests that the sun is usually used as a secondary cue in combination with the magnetic field.

Magnetic compass orientation in the yellow-faced honeyeater, Lichenostomus chrysops, a day migrating bird from Australia

In Australia, the southern populations of the yellow-faced honeyeater, Lichenostomus chrysops (Meliphagidae), perform annual migrations, with routes following the eastern coastline. In order to assess the role of magnetic cues in the migratory orientation of this diurnal migrant, its directional behaviour was recorded in recording cages under natural and experimentally manipulated magnetic-field conditions. During autumn the birds tested indoors in the local geomagnetic field showed a directional change from north initially to northwest later in the season (Fig. 1 a, b), which corresponds well with the general pattern of movement of this species in the field. Deflecting magnetic north to ESE resulted in a clockwise shift of the mean direction by 77° and 71°, respectively (Fig. 1 c, d), while no significant directional tendencies were observed in a magnetic field with a compensated horizontal component (Fig. 1 e, f; see Table 1). In outdoor tests in spring, the birds preferred southerly directions when tested in the local geo-magnetic field. In a magnetic field with a reversed vertical component (i.e. with an inclination pointing down instead of upwards) the birds reversed their directional tendencies and oriented northward (Fig. 2, Table 2). These results clearly show: (1) that yellow-faced honeyeaters can use the magnetic field for direction finding, and (2) that their magnetic compass functions as an “inclination compass”, as has been shown for several holarctic migrants.

Magnetic Orientation Of Migratory Wheatears (Oenanthe Oenanthe) In Sweden And Greenland

ABSTRACT Orientation experiments were performed with wheatears (Oenanthe oenanthe) subjected to artificially manipulated magnetic fields, in Sweden and Western Greenland, during the autumn migration period. The objective was to compare responses by birds exposed to widely different geomagnetic conditions and, specifically, to clarify if birds are able to use magnetic cues for orientation at high geomagnetic latitudes, as in Western Greenland. Orientation experiments were run under clear sunset skies and under simulated total overcast. Clear-sky tests did not reveal any clearcut orientation responses by wheatears in deflected and vertical magnetic fields. There was a tendency, however, for previous experience of the relationship between geomagnetic cues and visual information to affect the birds’ orientation in a vertical magnetic field. Under simulated overcast, the birds closely followed a 90° shift in magnetic direction in both study areas, and both samples failed to exhibit statistically significant mean directions when tested in vertical magnetic fields. The results clearly demonstrate that wheatears possess a magnetic compass. Furthermore, the birds are able to detect and use local geomagnetic information even at high magnetic latitudes in Western Greenland, notwithstanding the steep inclination (+81°) and large declination (—46°). A persistent attraction towards magnetic northwesterly headings, under both clear and overcast skies, is not consistent with migratory directions according to ringing recoveries and warrants further investigation.

Ontogeny of migratory orientation in the savannah sparrow, Passerculus sandwichensis: calibration of the magnetic compass

The ontogeny of magnetic orientation was examined in the savannah sparrow by manipulating the early experience of hand-raised birds with visual and magnetic cues. Tests of magnetic orientation were performed indoors in orientation cages covered by translucent sheets so that the birds could see nothing outside the cages. Control birds, raised entirely indoors, oriented in a magnetic north-northwest to south-southeast axis during autumn migration. Birds raised outdoors, exposed to clear daytime skies in a normal magnetic field, showed north-south magnetic orientation. Three groups were given experience with the natural sky only within a set of coils that shifted magnetic north clockwise to geographical east-southeast: one group saw only the daytime sky, one group saw only the clear night sky, and the third saw both day and night skies. The birds of all three groups oriented magnetic northeast-southwest, significantly different from the controls. Magnetic northeast-southwest corresponded approximately to geographical northwest-southeast within the magnetic coils in which the birds obtained their visual experience. The differences in the orientation directions chosen by the groups show that the primary magnetic compass may be calibrated early in life by some reference to geographical directions. Experience with either the clear daytime or night sky was sufficient to effect this modification. Celestial rotation, which provides a source of geographical directions both day and night, is proposed to be that geographical reference. At night the birds presumably used the stars to assess celestial rotation. In the daytime sky, both the sun itself and patterns of polarized skylight provide a means of determining geographical north. Experiments designed to determine which of these two visual cues was the relevant stimulus in the calibration of the magnetic compass were inconclusive.

Sensory basis of bird orientation

Sensory information which may be essential for the complex process of orientation of birds is described in this article. The use of vibrational, visual, chemical, olfactory, magnetic cues and their receptive mechanisms, as far as they are known, are explained. Special reference is given to the behavioral and physiological aspects of magnetic sensitivity.