Introduction:

The amazing navigational capability of animals that facilitates several species to carry out extremely precise long-distance migrations and homing behavior, has fascinated the natural historians for as long as animal behavior has been of interest.

The study of an arctic tern (that is, Sterna paradisaea) carrying out a yearly migration among the arctic areas of the northern and southern hemisphere, a gray whale (that is, Eschrichtius robustus) migrating between cold water feeding regions close to Alaska and birthing sites around the Baja peninsula, a loggerhead sea turtle (that is, Caretta caretta) migrating from feeding regions in the north Atlantic to egg deposition sites on the coastal beaches in the tropical and sub-tropical North America. But how do animals navigate or find their way?

Compass Mechanisms:

The capability to polarize space in certain directional framework is necessary when animals are to sustain movement in a constant direction with respect to the environment. Metaphorically, the challenge is alike to a human navigator requiring employing a compass to recognize directions in space and maintain a constant directional bearing at the same time of moving. Animal navigators have biological compasses based on their sensitivity to the position of the sun projected on the horizon or azimuth, stars and the magnetic field of earth. Such compass methods, however providing just directional information, form the basis from which richer, map like representations of space can come out.

1) Sun compass:  

 In the year 1951 Gustav Kramer introduced the sun compass. He carries out his experiments by placing European Starlings in the orientation cages and then employed mirrors to shift the apparent position of the sun. In reaction, the birds changed their migratory restlessness to match the compass direction pointed out by the apparent new location of the sun.

Advance research disclosed that the sun compass of birds is tied to its circadian rhythm. It looks as birds contain a time compensation capability to make grants for changes in the position of sun over the course of the day. This concept is supported by the other experiment in which pigeons were positioned in a closed room having an altered cycle of light and dark. Over a period of some days their circadian rhythm was rearrange. The birds were then free on a sunny day. As their 'internal clock' had been reset, they misconstrue the position of the sun and made an unsurprising error in their homing direction. The pigeons really overlook the position of the sun relative to its position in the sky, depends on its azimuth direction, that is, the compass direction at which a vertical line from the sun intersects the horizon.

Advance study has as well revealed that pigeons have to learn the path of sun to use it in the navigation. Young pigeons let to see the sun just in the morning require the capability to employ the sun for navigation in the afternoon.

2) Star compass: 

The sun is not the mere celestial body which can be utilized to define directions in space. However nocturnal migrant birds can and do utilize the position of the setting sun to orient their night-time migrations (Moore, 1987), they can as well rely on the stars. However it is not just any star or cluster of stars which can be employed to guide the migration. This is the stars around the axis point of the night's sky apparent rotation which are preferentially dependent on (Emlen, 1967). In the Northern Hemisphere, such would be circumpolar stars similar to those found in the constellations of the Big Dipper and Cassiopeia. Though, this star compass consists of properties dissimilar from the sun compass. For illustration, orientation to the stars is not time compensated; phase shifting migrant birds don't modify their migratory orientation to the stars as they would sun compass orientation. This is as well notable that as birds can be trained to utilize the sun compass to orient to a food source or other goal not related to migration or homing, orientation by the stars has merely been explained in the context of the migration.

3) Geomagnetic compass:  

The other German team did study with the European Robin in the year 1960. In their tests, robins in a migratory mood were positioned in covered cages to get rid of star, sun and other light clues. In spite of the lack of visual clues, the robins were noticed hopping in the accurate migratory direction.

As an extra refinement to the test, a Helmholtz coil was positioned around the covered cages. The coil let the researchers to shift the direction of the earth's magnetic field. If the direction of the magnetic field was modified, the robins changed their direction of hopping.

Advance research points out that as birds can sense the south and north ends of a compass, they can't tell the difference among the two. To decide which direction is north, the birds in fact encompass the ability to sense that the magnetic lines of force line up toward the poles of the earth. They can as well detect the dip in the lines of force as they move toward the earth and, via some presently unknown process, seem to be capable to detect and make navigational judgments based on the dip angle.

Compass mechanisms - Interactions among the different cues:

A few have explained the orientation methods of birds as 'redundant'. Though, the word redundant, proposing that the various sources of compass information give similar information, is clearly unsuitable. There is nothing redundant regarding the magnetic field of earth if the sun or stars are vague by clouds. Likewise, there is nothing redundant regarding the stars or sun for birds close to the magnetic equator where the inclination of the magnetic field of earth would render geomagnetic orientation ambiguous. Numerous sources of compass information are obviously adaptive. However multiple sources as well elevate the question of whether orientation methods are organized hierarchically; is one source of information preferentially employed over the others and may orientation to one cue is standardized against the other?

The explanation to this question is not simple. For young birds learning regarding environmental orientation cues throughout their first summer, both North American and European species look like to preferentially rely on celestial cues, in specific the sun and patterns of skylight polarization, as a geographic reference to define the north. Young birds will however use celestial cues to find out their migratory orientation with respect to the ambient magnetic field (Bingman, 1983).

The utilization of celestial cues to calibrate orientation to the magnetic field of earth is adaptive because as the point of celestial rotation gives a temporally and spatially stable reference to define the geographic compass directions, variation in the magnetic field of earth in space and time render it less reliable.

In the experienced adult migrants, the relationship among geomagnetic and celestial orientation methods is based on geographic position. In Europe, magnetic field information is preferentially employed to calibrate the orientation to celestial cues pointing out an ontogenetic shift in the hierarchy among the orientation methods (Wiltschko & Wiltschko, 1975). By contrary, in North America, at least at more northern latitudes, celestial information carries on to be preferentially employed to calibrate orientation to the ambient magnetic field (Able & Able, 1990; Cochran, Mouritsen, & Wikelski, 2004). Such findings mount the question of why North American and European experienced migrants must behave differently. A possible answer is associated to the relative stability of the sun and geomagnetic information as birds migrate in space and time (Bingman, Budzynski, & Voggenhuber, 2003).

As the bird migrates south in North America, modifies in the angular distance among geomagnetic north and geographic north (declination) and modifies in the compass direction of the setting sun are alike. There would be no benefit to shift away from the developmental pattern of preferentially dependent on celestial cues. By contrary, as a bird migrates south in Europe, the angular distance among geomagnetic north and geographic north remains fundamentally constant whereas the direction of the setting sun changes as the migratory season progresses. Thus, for the European migrants, it would be adaptive to adopt the earths magnetic as the preferential orientation cue once migration starts due to its stability as a directional reference.

Map-like or Navigational Mechanisms:

Compass processes facilitate birds to define directions in space to guide oriented movement. Though, a compass doesn't inform an organism of where it is in space. That birds encompass a map sense of where, in addition to a sense of direction, is eagerly attested to by their remarkable capability to return to the similar breeding and wintering sites year after year, and their capability to do so even after dramatic experimental dislocations; the most notable illustration of which is the homing capability of pigeons. Though, not all goal navigation essentially needs a map sense of where.

Getting there without knowing where:

A young bird on its first migration succeeds in navigating to its population particular over-wintering site devoid of a map sense of where. A genetic program which defines which direction and how long to fly seems adequate to get them close, and in the literature this kind of navigation is frequently termed to as "vector navigation".

 Getting there and knowing where:

As programmed as a young bird's first migration might be, experience gives them by means of opportunities for a far richer representation of space which facilitates a map-like sense of roughly global proportions. This map sense can be employed by birds to navigate to particular goal locations following displacements to unfamiliar places at times thousands of kms away. Layson albatrosses (that is, Phoebastria immutabilis), white-crowned sparrows (that is, Zonotrichia leucophrys), European starlings (that is, Sturnus vulgaris) and routinely homing pigeons (that is, Columba livia) are illustrations of species which have been employed in displacement experiments, successfully explaining the capability to goal navigate over unknown terrain.

The Neural Representation of Space in Birds: The Avian Hippocampus

Beneath natural conditions, birds represent an enormous range of spatial behavior mechanisms comprising various compass mechanisms, vector navigation and navigation through familiar landmarks, and mosaic and gradient maps of atmospheric odors. However there is no reason to suppose we have completely uncovered all the ways birds represent space or their sensory basis. The various behavioral methods would be supported through various neural representational processes, which would to a greater or lesser extent be supported by diverse brain areas. To date, it is the hippocampal formation (HF) which has been most lengthily studied in the context of avian spatial behavior, and not shockingly, its significance appears limited to just a subset of the behavioral methods. However playing certain role in navigational map learning beneath conditions of confinement in homing pigeons, the available data point out that the prevailing role of HF in the spatial behavior of birds is in the map-like representation of familiar landmarks employed to guide goal navigation over the well-known terrain.  

Lesion and immediate early gene studies:

The primary study observing the effects of HF lesions on the homing behavior of experienced pigeons was achieved by the disappointment of beautiful homeward orientation from the distant, unfamiliar position and the mystery that the lesioned birds never showed up at the loft (Bingman, Bagnoli, Ioale, & Casini, 1984). How could one describe an intact navigational map however failed homing? The proposition put forth was that as a pigeon approaches its home loft it becomes rousingly dependent on familiar landmarks to guide the final stages of the homing flight, and it is navigation through familiar landmarks which engages HF. The significance of HF for familiar landmark navigation has afterward been illustrated in many field and laboratory studies.

Intact and HF lesioned homing pigeons were trained from the two familiar positions and then tested to disclose the type of landmark-based strategy they learned to return from the familiar sites (Gagliardo, Ioale and Bingman, 1999). Whenever tested, the pigeons were rendered anosmic. Blocking the capability to smell would eradicate the capability of the birds to rely on their olfactory navigational map to return home, therefore forcing them to rely wholly on their representation of the familiar landmarks.

They as well had their internal clocks phase-shifted. Theoretically, homing pigeons could use memorable landmarks as an independent map and guidance system, by employing the landmarks to guide their flight home by serially positioning their position in space and noting their movement with respect to the landmarks. On the other hand, they could simply utilize the landmarks at the well-known release site to remember the compass direction flown from that site throughout training, and then utilize their sun compass to take up the homeward bearing. Phase-shifting would separate such two strategies.

Navigating home by means of gauging movement with respect to the familiar landmarks alone wouldn't be affected through the phase-shift manipulation. By contrary, recalling the compass direction home and then orienting through the sun would outcome in a shift in orientation away from the home-ward direction.

The outcomes of this study explain how subtle the differences can be in the navigational strategies employed by control and HF lesioned pigeons. Control pigeons oriented in the direction approximating the true direction home, and thus, were for the most part uninfluenced through the phase-shift manipulation. They employed the unspecified array of well-known landmarks in a map and guidance-like fashion. By contrary, the HF lesioned pigeons displayed a shift in orientation away from the home direction pointing out that they relied on their sun compass once fining out their location relative to home, presumably through recognizing landmarks at the training site and then recalling the compass direction home flown throughout training.

It is obvious that the map-like spatial memory representation learned through the control pigeons was much richer in terms of the spatial information available and the potential for inferring route corrections in the event of the displacement. This capability needs recruitment of the HF. Merely learning to relate a compass direction by a cluster of familiar landmarks, instructed through the olfactory navigational map available throughout the training stage of the study, doesn't need an intact HF.

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