Compass Of Birds Research Articles

Relationship of the magnetic compass and vision in birds: in search of a receptor cell

The existence of the magnetic compass system was first shown in birds. Since then, a large amount of data has been accumulated on the performance of the avian magnetic compass and its relationship with visual reception. The current dominant concept is that the receptor for the magnetic compass in birds is located in the retina. The most popular hypothesis for the mechanism of operation of magnetic field receptors is the radical pair model, and a candidate for the role of the primary magnetoreceptor molecule is cryptochrome, and more specifically, its isoform, cryptochrome 4a. In recent years, data have been published on the interaction of cryptochrome with some proteins involved in the phototransduction cascade, as well as promising data from electrophysiological studies combining light and magnetic stimuli. In addition, a number of morphological studies of the avian retina also allow us to narrow down the range of promising cells for the role of a magnetoreceptor, and the double cone is currently the most likely candidate. In this review, we discuss the latest research data in this area.

Cryptochrome 1a localisation in light- and dark-adapted retinae of several migratory and non-migratory bird species: no signs of light-dependent activation

The magnetic compass of birds seems to be based on light-dependent radical-pair processes in the eyes. Cryptochromes are currently the only candidate proteins known in vertebrates that may serve as the primary radical-pair-based magnetoreceptor molecules. Previous immunohistochemical studies have suggested that cryptochrome 1a (Cry1a) is localised in the photoreceptor outer segments of the ultraviolet/violet (UV/V) cones, and it has been claimed that differences in Cry1a antibody staining intensities show that Cry1a is activated by light and that this should make Cry1a the most likely magnetoreceptive candidate molecule. Here, we present an independent study of Cry1a distribution within retinae of several bird species, ranging from non-migratory domestic chicken and rock pigeon to night-migratory passerines, using both the previously used antibody and two newly generated antibodies, one against the same epitope as the originally used antibody and one against a different epitope of Cry1a. We confirm the UV/V cone outer segment localisation of Cry1a in all the tested bird species. In some stainings, we found Cry1a immunoreactivity as a distinct punctate pattern throughout the whole length of the UV/V cone outer segments. These dots with a diameter of around 170 nm might suggest that many Cry1a molecules accumulate in distinct spots in the UV/V cone outer segments. However, we did not see any notable difference in Cry1a immunoreactivity between light- and dark-adapted retinae. We find no evidence whatsoever that a C-terminal antibody against Cry1a labels only a light-activated form of the Cry1a protein.

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.

Searching for magnetic compass mechanism in pigeon retinal photoreceptors.

Migratory birds can detect the direction of the Earth’s magnetic field using the magnetic compass sense. However, the sensory basis of the magnetic compass still remains a puzzle. A large body of indirect evidence suggests that magnetic compass in birds is localized in the retina. To confirm this point, an evidence of visual signals modulation by magnetic field (MF) should be obtained. In a previous study we showed that MF inclination impacts the amplitude of ex vivo electroretinogram (ERG) recorded from isolated pigeon retina. Here we present the results of an analysis of putative MF effect on one component of ERG, the photoreceptor’s response, isolated from the total ERG by adding sodium aspartate and barium chloride to the perfusion solution. Photoresponses were recorded from isolated retinae of domestic pigeons Columba livia. The retinal samples were placed in MF that was modulated by three pairs of orthogonal Helmholtz coils. Light stimuli (blue and red) were applied under two inclinations of MF, 0° and 90°. In all the experiments, preparations from two parts of retina were used, red field (with dominant red-sensitive cones) and yellow field (with relatively uniform distribution of cone color types). In contrast to the whole retinal ERG, we did not observe any effect of MF inclination on either amplitude or kinetics of pharmacologically isolated photoreceptor responses to blue or red half-saturating flashes. A possible explanations of these results could be that magnetic compass sense is localized in retinal cells other than photoreceptors, or that photoreceptors do participate in magnetoreception, but require some processing of compass information in other retinal layers, so that only whole retina signal can reflect the response to changing MF.

Expression patterns of cryptochrome genes in avian retina suggest involvement of Cry4 in light-dependent magnetoreception.

The light-dependent magnetic compass of birds provides orientation information about the spatial alignment of the geomagnetic field. It is proposed to be located in the avian retina, and be mediated by a light-induced, biochemical radical-pair mechanism involving cryptochromes as putative receptor molecules. At the same time, cryptochromes are known for their role in the negative feedback loop in the circadian clock. We measured gene expression of Cry1, Cry2 and Cry4 in the retina, muscle and brain of zebra finches over the circadian day to assess whether they showed any circadian rhythmicity. We hypothesized that retinal cryptochromes involved in magnetoreception should be expressed at a constant level over the circadian day, because birds use a light-dependent magnetic compass for orientation not only during migration, but also for spatial orientation tasks in their daily life. Cryptochromes serving in circadian tasks, on the other hand, are expected to be expressed in a rhythmic (circadian) pattern. Cry1 and Cry2 displayed a daily variation in the retina as expected for circadian clock genes, while Cry4 expressed at constant levels over time. We conclude that Cry4 is the most likely candidate magnetoreceptor of the light-dependent magnetic compass in birds.

The radical pair mechanism and the avian chemical compass: Quantum coherence and entanglement

We review the spin radical pair mechanism which is a promising explanation of avian navigation. This mechanism is based on the dependence of product yields on (1) the hyperfine interaction involving electron spins and neighboring nuclear spins and (2) the intensity and orientation of the geomagnetic field. This review describes the general scheme of chemical reactions involving radical pairs generated from singlet and triplet precursors; the spin dynamics of the radical pairs; and the magnetic field dependence of product yields caused by the radical pair mechanism. The main part of the review includes a description of the chemical compass in birds. We review: the general properties of the avian compass; the basic scheme of the radical pair mechanism; the reaction kinetics in cryptochrome; quantum coherence and entanglement in the avian compass; and the effects of noise. We believe that the quantum avian compass can play an important role in avian navigation and can also provide the foundation for a new generation of sensitive and selective magnetic‐sensing nano‐devices. © 2015 Wiley Periodicals, Inc.

Iron-mineral-based magnetoreceptor in birds: polarity or inclination compass?

In the present paper we demonstrate that the iron-mineral-based magnetoreceptor model developed in [1] can provide birds with a magnetic compass in addition to the generally believed “magnetic map". We show that the iron-mineral-based magnetoreceptor system possesses all properties of a polarity compass, which is extremely important for avian navigation. We study how parameters of the magnetoreceptor system influence on the properties of the compass and show that at certain conditions it acquires features of an inclination compass. In the present paper we address the question of avian magnetoreception theoretically and suggest several experiments which should be performed for better understanding of the iron-mineral-based compass in birds.

Pigeon orientation: effects of the application of magnets under overcast skies.

To verify the existence of a magnetic compass in birds, researchers have often released homing pigeons under overcast skies that are equipped with bar magnets on various parts of their body. In particular, Keeton was successful in finding disorientation in overcast conditions in a first series of tests, but not in a second series. The experiments reported here attempt to explain this contradiction on the basis of findings obtained by releasing pigeons equipped in a way similar to that reported in Keeton's tests and pigeons equipped in a way similar to that reported by other authors.

Pigeon homing: sun compass use in the southern hemisphere

Although the sun compass of birds is based on learning the sun's arc during development, it was unclear whether birds can use the sun when its apparent movement is reversed, in particular, whether northern birds that have been introduced into the southern hemisphere can use the southern sun. To answer this question, clock-shift experiments were performed with local homing pigeons in Auckland, New Zealand (37°S). In three fast-shift tests and two slow-shift tests, the experimental birds showed deflections from the untreated controls that were the mirror images of those observed in the northern hemisphere. These results clearly show that homing pigeons in New Zealand use a sun compass that is adapted to the situation in the southern hemisphere. The learning processes establishing the compensation mechanisms thus appear to be free of constraints concerning the direction of the sun's movement. Differences from recent findings with migratory birds, where the direction of celestial rotation proved of crucial importance for establishing the migratory direction, are discussed: the differences may arise from the different orientation tasks, in particular, from the involvement of innate information in establishing the migratory direction.