Star Compass Research Articles

Predicting performance of naïve migratory animals, from many wrongs to self-correction

Migratory orientation of many animals is inheritable, enabling inexperienced (naïve) individuals to migrate independently using a geomagnetic or celestial compass. It remains unresolved how naïve migrants reliably reach remote destinations, sometimes correcting for orientation error or displacement. To assess naïve migratory performance (successful arrival), we simulate and assess proposed compass courses for diverse airborne migratory populations, accounting for spherical-geometry effects, compass precision, cue transfers (e.g., sun to star compass), and geomagnetic variability. We formulate how time-compensated sun-compass headings partially self-correct, according to how inner-clocks are updated. For the longest-distance migrations simulated, time-compensated sun-compass courses are most robust to error, and most closely resemble known routes. For shorter-distance nocturnal migrations, geomagnetic or star-compass courses are most robust, due to not requiring nightly cue-transfers. Our predictive study provides a basis for assessment of compass-based naïve migration and mechanisms of self-correction, and supports twilight sun-compass orientation being key to many long-distance inaugural migrations.

Ontogeny of the star compass in birds: pied flycatchers (Ficedula hypoleuca) can establish the star compass in spring

The star compass of birds, as well as the sun compass, is not innate. To possess either of them, the birds have to observe the rotating sky and determine its' center of rotation (in the star compass case) or the sun's movement (in the sun compass). Young birds are believed to learn how to use the star compass before their first migration, even though the evidence of this is lacking. Here, we test whether hand-raised Pied flycatchers (Ficedula hypoleuca) which had not established the star compass prior to their first autumn migration can gain it later in their ontogeny, in spring. We also attempted to examine if the observation of diurnal celestial cues (the sun and the polarized light) prior to autumn migration would affect the process of the star compass learning in spring. When tested in the vertical magnetic field under the natural starry sky, the group of birds which observed the stars in spring as the first celestial cues could choose the migratory direction. In contrast, the birds, which had never seen the stars, were not able to use the nightly celestial cues in the vertical magnetic field. However, the group of birds which had seen the daytime celestial cues till autumn and the stars at spring was disoriented, although this might be due to the small sample size. Our data suggest the possibility that the star compass may be learned in spring and emphasize the necessity for the further research in the interaction of celestial compasses.

Discrimination of artificial starry sky by pigeons.

Night migratory birds use the star compass for their navigation. Here, I examined discrimination of a starry sky by a nonmigratory bird, the pigeon. Four pigeons were trained in an operant chamber to discriminate between an artificial starry sky created using software (Stella Theater Pro) and a black sky without stars. After they learned the task, they received three generalization tests. Test 1 presented a starry sky with different classes of stars (i.e., the number of stars was changed). Test 2 presented a starry sky at different times (i.e., a horizontal shift of the sky). Test 3 presented a starry sky at different latitudes (i.e., a vertical shift of the sky). The pigeons displayed a clear generalization gradient with a peak shift to the sky with more stars in Test 1 and a peak close to the original sky in Test 3, but almost a flat gradient in Test 2. Therefore, pigeons demonstrated stimulus control using the pattern of the stars, and they were more sensitive to the vertical shift than to the horizontal shift.

An exploratory survey method for archaeoastronomy, applied to standing stones at the hauviri and taputapuātea maraes, Ra'iātea

Polynesia has a vast area, and surveys are few and unsystematic. Rapid, low-cost and personnel surveys are needed to identify cases that merit an in-depth treatment. However, surveying options are limited, especially where relative positions of objects and the azimuths between them are important. In this article, a survey method is tested on standing stones at the Taputāpuatea and Hauviri maraes, Ra ‘iātea. The resulting data set was used to test three hypotheses, namely: the stones line up with voyage destinations, or commemorate significant voyages; stones form an analogue “star compass” of directions where significant navigational stars rise and set; and stones line up with important stars at a significant epoch of the year. The surveys yielded a data set that benchmarks stone positions at the time of survey, is capable of fairly accurate azimuth comparisons, and is sufficiently rich to be mined in different ways in subsequent research. One situation at the season of Matariki in AD1250 is identified for further investigation.

Further evidence of a time-independent stellar compass in a night-migrating songbird

We tested whether European robins Erithacus rubecula have a time-independent stellar compass, i.e. whether they can use the stellar pattern for finding the seasonally appropriate migratory direction without reference to their internal clock. It has been claimed earlier on the basis of experiments with other nocturnal migrants, indigo buntings Passerina cyanea, pied flycatchers Ficedula hypoleuca and blackcaps Sylvia atricapilla, but none of the previous experiments was performed in the vertical magnetic field, which rules out the use of the magnetic compass, and under the natural nocturnal sky. We argue that pervious experiments allow alternative interpretations, albeit unlikely. Our data independently confirm that European robins, like other nocturnal passerine migrants hitherto studied, use their stellar compass independently of their internal clock. It is generally assumed since the classic experiments of Stephen Emlen performed in the 1960s that star compass of migrating songbirds is time-independent, i.e. that the birds use the pattern of constellations and not the angle of rotation of the starry sky. However, it has not been shown unequivocally, because early experiments did not consider the possibility that the birds relied on magnetic compass rather than on time-independent stellar compass system. Here, we show that stellar compass of European robins tested in the vertical magnetic field that did not provide compass information is indeed independent of the birds’ internal clock and cannot be manipulated by resetting this clock. Cognitive challenges of learning the pattern of constellations, which is necessary to use time-independent star compass, are most fascinating and deserve detailed behavioural and neurobiological study.

Compass systems.

Three compass systems based on global cues known to exist in migrating birds are reviewed. Two of these systems are based on celestial cues, a time-dependent sun compass and time-independent, i.e. not involving the internal clock, star compass. The third system is the magnetic compass, based on a separate sensory modality, which currently attracts much attention from behavioural ecologists, physiologists and physicists. The complex pattern of hierarchy and interactions between these compass systems is briefly discussed. It is argued that rules of integration of information from different compass cues are likely dependent on ecological and geographic conditions the birds are facing during their journey, so it is likely that no single set of rules is shared by all migrating birds.

A New View on an Old Debate: Type of Cue-Conflict Manipulation and Availability of Stars Can Explain the Discrepancies between Cue-Calibration Experiments with Migratory Songbirds.

Migratory birds use multiple compass systems for orientation, including a magnetic, star and sun/polarized light compass. To keep these compasses in register, birds have to regularly update them with respect to a common reference. However, cue-conflict studies have revealed contradictory results on the compass hierarchy, favoring either celestial or magnetic compass cues as the primary calibration reference. Both the geomagnetic field and polarized light cues present at sunrise and sunset have been shown to play a role in compass cue integration, and evidence suggests that polarized light cues at sunrise and sunset may provide the primary calibration reference for the other compass systems. We tested whether migratory garden warblers recalibrated their compasses when they were exposed to the natural celestial cues at sunset in a shifted magnetic field, which are conditions that have been shown to be necessary for the use of a compass reference based on polarized light cues. We released the birds on the same evening under a starry sky and followed them by radio tracking. We found no evidence of compass recalibration, even though the birds had a full view of polarized light cues near the horizon at sunset during the cue-conflict exposure. Based on a meta-analysis of the available literature, we propose an extended unifying theory on compass cue hierarchy used by migratory birds to calibrate the different compasses. According to this scheme, birds recalibrate their magnetic compass by sunrise/sunset polarized light cues, provided they have access to the vertically aligned band of maximum polarization near the horizon and a view of landmarks. Once the stars appear in the sky, the birds then recalibrate the star compass with respect of the recalibrated magnetic compass. If sunrise and sunset information can be viewed from the same location, the birds average the information to get a true geographic reference. If polarized light information is not available near the horizon at sunrise or sunset, the birds temporarily transfer the previously calibrated magnetic compass information to the available celestial compasses. We conclude that the type of cue-conflict manipulation and the availability of stars can explain the discrepancies between studies.

THE VAEAKAU-TAUMAKO WIND COMPASS AS PART OF A “NAVIGATIONAL TOOLKIT”

Different voyaging communities in the Pacific use a variety of non-instrument navigational techniques for way-finding across long distances. The use of a star compass has been well documented for several groups. Wind compasses are less well documented and appear to be less utilized throughout the region. This article seeks to understand the Vaeakau-Taumako wind compass as it compares to other wind compasses, as well as how wind and star compasses and other navigational techniques compare to each other as cognitive frameworks for navigation.

Perceptual strategies of pigeons to detect a rotational centre--a hint for star compass learning?

Birds can rely on a variety of cues for orientation during migration and homing. Celestial rotation provides the key information for the development of a functioning star and/or sun compass. This celestial compass seems to be the primary reference for calibrating the other orientation systems including the magnetic compass. Thus, detection of the celestial rotational axis is crucial for bird orientation. Here, we use operant conditioning to demonstrate that homing pigeons can principally learn to detect a rotational centre in a rotating dot pattern and we examine their behavioural response strategies in a series of experiments. Initially, most pigeons applied a strategy based on local stimulus information such as movement characteristics of single dots. One pigeon seemed to immediately ignore eccentric stationary dots. After special training, all pigeons could shift their attention to more global cues, which implies that pigeons can learn the concept of a rotational axis. In our experiments, the ability to precisely locate the rotational centre was strongly dependent on the rotational velocity of the dot pattern and it crashed at velocities that were still much faster than natural celestial rotation. We therefore suggest that the axis of the very slow, natural, celestial rotation could be perceived by birds through the movement itself, but that a time-delayed pattern comparison should also be considered as a very likely alternative strategy.

Star compass learning: how long does it take?

Young night-migratory birds establish a functional star compass during ontogeny by searching the starry night sky for its centre of rotation and interpreting this as “north”. The birds then use the learned location of the rotational centre to calibrate their magnetic compass. Here, we examine the required duration of this learning process. We exposed three groups of hand-raised inexperienced European Robins Erithacus rubecula to a rotating artificial star pattern for 3 weeks, 1 week, or 1 night, respectively, during ontogeny. A control group was exposed to the same, but stationary, artificial star pattern for 3 weeks. During their first autumn migration, we tested the birds’ orientation under the stationary star pattern as well as in the local geomagnetic field with no stars visible. Only the group that had the longest exposure time to celestial rotation during ontogeny was able to show orientation in the appropriate direction during autumn migration in the local geomagnetic field in relation to the memorized former centre of celestial rotation. This suggests that these birds had calibrated their magnetic compass relative to the star pattern seen during ontogeny. All other groups showed inappropriate or random orientation both under the stationary star sky and in the local geomagnetic field, suggesting that 7 nights of observing celestial rotation are not sufficient for young European Robins to establish a star compass and to calibrate their magnetic compass accordingly.

Visual but not trigeminal mediation of magnetic compass information in a migratory bird

Magnetic compass information has a key role in bird orientation, but the physiological mechanisms enabling birds to sense the Earth's magnetic field remain one of the unresolved mysteries in biology. Two biophysical mechanisms have become established as the most promising magnetodetection candidates. The iron-mineral-based hypothesis suggests that magnetic information is detected by magnetoreceptors in the upper beak and transmitted through the ophthalmic branch of the trigeminal nerve to the brain. The light-dependent hypothesis suggests that magnetic field direction is sensed by radical pair-forming photopigments in the eyes and that this visual signal is processed in cluster N, a specialized, night-time active, light-processing forebrain region. Here we report that European robins with bilateral lesions of cluster N are unable to show oriented magnetic-compass-guided behaviour but are able to perform sun compass and star compass orientation behaviour. In contrast, bilateral section of the ophthalmic branch of the trigeminal nerve in European robins did not influence the birds' ability to use their magnetic compass for orientation. These data show that cluster N is required for magnetic compass orientation in this species and indicate that it may be specifically involved in processing of magnetic compass information. Furthermore, the data strongly suggest that a vision-mediated mechanism underlies the magnetic compass in this migratory songbird, and that the putative iron-mineral-based receptors in the upper beak connected to the brain by the trigeminal nerve are neither necessary nor sufficient for magnetic compass orientation in European robins.

Harbour seals (Phoca vitulina) can steer by the stars

Offshore orientation in marine mammals is still a mystery. For visual orientation during night-time foraging and travelling in the open seas, seals cannot rely on distant terrestrial landmarks, and thus might use celestial cues as repeatedly shown for nocturnally migrating birds. Although seals detect enough stars to probably allow for astronavigation, it was unclear whether they can orient by the night sky. The widely accepted cognitive mechanism for bird night-time orientation by celestial cues is a time-independent star compass with learned geometrical star configurations used to pinpoint north as the rotational centre of the starry sky while there is no conclusive evidence for a time-compensated star compass or true star navigation. Here, we present results for two harbour seals orienting in a custom made swimming planetarium. Both seals learned to highly accurately identify a lodestar out of a pseudo-randomly oriented, realistic projection of the northern hemisphere night sky. Providing the first evidence for star orientation capability in a marine mammal, our seals' outstanding directional precision would allow them to steer by following lodestars of learned star courses, a celestial orientation mechanism that has been known to be used by Polynesian navigators but has not been considered for animals yet.

Magnetic compass of migratory Savannah sparrows is calibrated by skylight polarization at sunrise and sunset

Migratory birds use compass systems derived from the geomagnetic field, the stars, the sun and polarized light patterns. We tested whether birds use a single underlying reference system for calibration of these compasses and, specifically, whether sunset and sunrise polarized light cues from the region of the sky near the horizon are used to calibrate the magnetic compass. We carried out orientation experiments with Savannah sparrows, Passerculus sandwichensis, in Alaska during autumn migration 2005, and compared the magnetic orientations of individual birds before and after exposure to conflicting information between magnetic and celestial cues. Birds exposed to an artificially shifted polarization pattern (±90° shift relative to the natural condition) for 1 h at local sunrise or sunset recalibrated their magnetic compass, but only when given access to the artificial polarization pattern near the horizon. Birds exposed to a 90° clockwise-shifted magnetic field for 1 h at solar noon did not recalibrate their magnetic compass. These results indicate that migratory birds calibrate their magnetic compass using the skylight polarization pattern vertically intersecting the horizon at sunrise and sunset. In conjunction with earlier work showing that sun and star compass calibrations are secondarily derived from magnetic and polarized light cues, our findings suggest that polarized light cues near the horizon at sunrise and sunset provide the primary calibration reference for the compass systems of migratory songbirds.

Night-vision brain area in migratory songbirds

Twice each year, millions of night-migratory songbirds migrate thousands of kilometers. To find their way, they must process and integrate spatiotemporal information from a variety of cues including the Earth's magnetic field and the night-time starry sky. By using sensory-driven gene expression, we discovered that night-migratory songbirds possess a tight cluster of brain regions highly active only during night vision. This cluster, here named "cluster N," is located at the dorsal surface of the brain and is adjacent to a known visual pathway. In contrast, neuronal activation of cluster N was not increased in nonmigratory birds during the night, and it disappeared in migrants when both eyes were covered. We suggest that in night-migratory songbirds cluster N is involved in enhanced night vision, and that it could be integrating vision-mediated magnetic and/or star compass information for night-time navigation. Our findings thus represent an anatomical and functional demonstration of a specific night-vision brain area.

Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass.

This paper investigates how young pied flycatchers, Ficedula hypoleuca, and blackcaps, Sylvia atricapilla, interpret and use celestial cues. In order to record these data, we developed a computer-controlled version of the Emlen funnel, which enabled us to make detailed temporal analyses. First, we showed that the birds use a star compass. Then, we tested the birds under a stationary planetarium sky, which simulated the star pattern of the local sky at 02:35 h for 11 consecutive hours of the night, and compared the birds' directional choices as a function of time with the predictions from five alternative stellar orientation hypotheses. The results supported the hypothesis suggesting that birds use a time-independent star compass based on learned geometrical star configurations to pinpoint the rotational point of the starry sky (north). In contrast, neither hypotheses suggesting that birds use the stars for establishing their global position and then perform true star navigation nor those suggesting the use of a time-compensated star compass were supported.

Magnetic information calibrates celestial cues during migration

Migratory birds use celestial and geomagnetic directional information to orient on their way between breeding and wintering areas. Cue-conflict experiments involving these two orientation cue systems have shown that directional information can be transferred from one system to the other by calibration. We designed experiments with four species of North American songbirds to: (1) examine whether these species calibrate orientation information from one system to the other; and (2) determine whether there are species-specific differences in calibration. Migratory orientation was recorded with two different techniques, cage tests and free-flight release tests, during autumn migration. Cage tests at dusk in the local geomagnetic field revealed species-specific differences: red-eyed vireo, Vireo olivaceus, and northern waterthrush, Seiurus noveboracensis, selected seasonally appropriate southerly directions whereas indigo bunting, Passerina cyanea, and grey catbird, Dumetella carolinensis, oriented towards the sunset direction. When tested in deflected magnetic fields, vireos and waterthrushes responded by shifting their orientation according to the deflection of the magnetic field, but buntings and catbirds failed to show any response to the treatment. In release tests, all four species showed that they had recalibrated their star compass on the basis of the magnetic field they had just experienced in the cage tests. Since release tests were done in the local geomagnetic field it seems clear that once the migratory direction is determined, most likely during the twilight period, the birds use their recalibrated star compass for orientation at departure.

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.

Migratory orientation of pied flycatchers: interaction of stellar and magnetic information during ontogeny

To study the interaction between magnetic compass and star compass during ontogeny, young pied flycatchers, Ficedula hypoleuca, were hand-reared, and after becoming self-sufficient were transferred into outdoor aviaries with full view of the sky. They were divided into three groups: controls experienced the sky while sitting in the local magnetic field; for group WSW, magnetic north was turned by 120 ° anticlockwise to geographical west-southwest; and for group ESE it was turned 120 ° clockwise to eastsoutheast. The birds remained in the aviary until the onset of the migratory season. They were then transferred indoors and tested in the local geomagnetic field without access to celestial cues. The controls showed a significant preference for the seasonally appropriate migratory direction. The group WSW was significantly oriented with a mean 119 ° clockwise from that of the controls. This is in agreement with the hypothesis that celestial cues during the pre-migratory period override any innate magnetic information and establish a new compass course with respect to the magnetic field. The orientation of the group ESE was significantly more scattered, and the mean does not fit the hypothesis mentioned above. Possible reasons for this difference in behaviour are discussed. If it reflects a true asymmetry of the orientation system, the sense of rotation of the sky, being an asymmetric factor, might be involved in the sense that a transfer of information from one system to the other might occur more easily in the direction of rotation than against it.

Orientation along great circles by migrating birds using a sun compass

The time-compensated sun compass furnishes migrating birds with a possibility of orientating along great circle routes. When moving across longitudes, birds are exposed to a time shift and, if sun compass orientation is used, the course is expected to change along the migration route. These expected course changes are compared with the changing courses along great circle routes. At high latitudes, sun compass courses are expected to change by approximately 1° for each degree of longitudinal displacement, with little daily and seasonal variation. The resulting routes accord well with great circle routes as far away from the poles as lat. 50°–60°. At lower latitudes, birds may travel along approximate great circle routes if they let themselves be guided by sun compass orientation around the time of sunset/sunrise. Generally, sun compass routes at sunset/sunrise are exactly equal to great circles. This also applies to routes where birds, travelling during the night, are guided by stars close to the horizon using a time-compensated star compass. It remains to be elucidated if migrating birds make use of the possibilities, offered by their sun compass, for great circle orientation.

Experimental studies of the development of migratory orientation mechanisms

Recent experimental studies (since ca 1985) on the ontogeny of orientation mechanisms in migratory birds are reviewed. The processes and interactions are synthesized into a framework that may help identify critical research questions. Birds that grow up in the earth's magnetic field develop the ability to perform appropriate migratory orientation even in the absence of any experience with relevant visual cues. In two species, large changes in direction during the course of migration seem to be controlled by an endogenous time program. In one of these, the pied flycatcher (Ficedula hypoleuca), the correct magnetic orientation seems to occur only when the magnetic fields appropriate to the latitudes encountered en route were experienced at the proper seasonal time. The magnetic compass may be modified by visual experience with either the daytime or night sky. Celestial rotation may be the calibrating reference in this case, as it is in the development of the star compass. Young Savannah sparrows (Passerculus sandwichensis) learn to perform compass orientation at sunset based on polarized skylight. This compass capability seems to be calibrated by magnetic directions. Some problems of experimental design and the interpretation of results from experiments on development are discussed.