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Avian long distance navigation, the mechanisms and factors involved have always been the main topic of our research. Our considerations are based on Kramer's map and compass model that describes avian navigation as a two-step process: in a first step, birds determine the compass course to the goal; in a second step, they locate this course with the help of a compass. In homing pigeons, the first step involves a navigational process, while in migratory birds, it is represented by the innate information on the course of migration. The geomagnetic field provides animals with reliable navigational information that can be used in both steps.

Navigation and homing:
The two compass mechanisms used by pigeons, magnetic compass and sun compass, and their interactions are already known in considerable detail. Hence we focus on the navigational strategies and the cues of the navigational 'map' that enables pigeons to determine their home course at unfamiliar sites, with special interest in the role of magnetic factors.

Traditionally, after release, the birds are observed with binoculars until they vanished from sight. Their vanishing bearings are taken with a compass. Today, we are also able to record the flight paths of pigeons very accurately with the help of miniaturized GPS-based recorders. This allows us to characterize the paths of individual birds at different sites, where the routes chosen reflect the local distribution of the navigational factors used. At the moment, we analyze the influence of the local factors within a magnetic anomaly and, within the home region, study the size of the home zone where pigeon switch to landmark use, as well as in the influence of prominent landmarks such as e.g. high buildings on the routes chosen.

The results of these analyses are to be used to model pigeon navigation in simulations to visualize the distribution of the navigational factors. The simulations will then be compared with results of the behavioral studies to make the models even more realistic.

Publications

Magnetoreception:
The direction of the magnetic field can be used as the basis of a compass; total intensity and inclination (dip), showing gradients between the magnetic poles and the magnetic equator could indicate the 'magnetic latitude' of a location and thus its position. These types of information from the magnetic field are indeed used by many animals: a magnetic compass is widespread, and several examples show the use of magnetic parameter as components of the navigational 'map'.

Two mechanisms of magnetoreception are currently discussed:
    (1)The radical pair model proposes that spin-chemical reactions in specialized photopigments are modulated by the direction of the external magnetic field and could indicate magnetic directions. The right eye has been suggested as the site of magnetoreception.

    (2) The magnetite-hypothesis assumes that magnetoreception is mediated by particles of magnetite, a special form of Fe3O4. These crystals could provide information on magnetic intensity, but also on magnetic directions.
Both types of mechanisms are present in birds; they are normally used for different tasks that compliment each other.

Our behavioral experiments on magnetoreception make use of the fact that during migration season, migratory bird spontaneously head into their innate migratory direction: orientation in migratory direction serves as criterion whether or not birds can derive directional information from the magnetic field. As test birds, we use European robins and Australian silvereyes. We were able to identify the avian magnetic compass as an inclination compass that does not rely on the polarity of the field lines, but on the axial course of the field lines. Spontaneously, it works only in a narrow functional window around the intensity of the local magnetic field. In domestic chicken, we could demonstrate a magnetic compass with the help of directional training; it shows the same characteristics as the magnetic compass of migratory birds. Ongoing studies with young domestic chickens are focussing on the ontogenetic development of the magnetic compass in that species.

Our experiments demonstrated that directional information from the magnetic field is indeed mediated by the eyes, with a strong lateralization in favor of the right eye. The disorienting effect of oscillating fields in the MHz range identified the mechanisms underlying magnetoreception as radical pair processes. By 'behavioral spectroscopy' we could determine the live span of the crucial radical pairs as well as characteristic features of the receptor molecule, namely that one of the radicals is devoid of nuclei with strong magnetic moments. Such a radical pair is ideal to detect the direction of weak magnetic fields, as calculations of our colleagues from biophysics showed.

Cryptochrome, a photopigment with flavin as chromophore, has been suggested as receptor molecule for the avian magnetic compass. Cryptochrome has been found in the retina of birds by molecular and immunhistological methods. The respective studies are continued and expanded to include TEM microscopy to identify the cell types in the retina in which Cryptochrome is located.

The magnetite-based receptors in the upper beak are not involved in the avian magnetic compass, as indicated by experiments with birds whose upper beak was locally anesthetized. These receptors seem to mainly mediate information on magnetic intensity as a component of the navigational 'map'. However, they are also involved in so-called 'fixed direction'-responses observed in migratory birds under certain light conditions. Here, the birds prefer directions that differ from their normal migratory direction and do not show the seasonal shift between spring and autumn. The respective experiments suggest complex interactions between the photoreceptors and the magnetoreceptors in the eyes as well as in the upper beak. Further studies are in progress to better understand these interactions.

Publications

 

geändert am 19. Februar 2009  E-Mail: Webmasterschiffner@bio.uni-frankfurt.de

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