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Night-migratory songbirds have a light-dependent magnetic compass sense, the mecha- nism of which is thought to depend on the photochemical formation of radical pairs in cryptochrome (Cry) proteins located in the retina. The finding that weak radiofrequency (RF) electromagnetic fields can prevent birds from orienting in the Earth’s magnetic field has been regarded as a diagnostic test for this mechanism and as a potential source of information on the identities of the radicals. The maximum frequency that could cause such disorientation has been predicted to lie between 120 and 220 MHz for a flavin–tryptophan radical pair in Cry. Here we show that the magnetic orientation capabilities of Eurasian blackcaps (Sylvia atricapilla) are not affected by RF noise in the frequency bands 140 to 150 MHz and 235 to 245 MHz. From a consideration of its internal magnetic interactions, we argue that RF field effects on a flavin-containing radical- pair sensor should be approximately independent of frequency up to 116 MHz and that birds’ sensitivity to RF disorientation should fall by about two orders of mag- nitude when the frequency exceeds 116 MHz. Taken together with our earlier finding that 75 to 85 MHz RF fields disrupt the magnetic orientation of blackcaps, these results provide compelling evidence that the magnetic compass of migratory birds operates by a radical pair mechanism. [ABSTRACT FROM AUTHOR]

Geomagnetic cues have been shown to influence migratory orientation and migratory fuelling in night-migratory songbird species. Here, we used captive-bred northern wheatears Oenanthe oenanthe from the southern Norwegian population to show that other aspects of the birds' migratory program can be influenced by magnetic cues as well. We observed that the amount of migratory restlessness increased strongly with progression of the migratory season when the birds were kept constantly in the magnetic field of northern Germany, but the amount of migratory restlessness decreased when the magnetic field changed along the birds' natural flyway are simulated. Thus, the Earth's magnetic field can also act as a 'signpost' cue for fine-tuning the spatio-temporal course of migration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Previous studies on European robins, Erithacus rubecula, and Australian silvereyes, Zosterops lateralis, had suggested that magnetic compass information is being processed only in the right eye and left brain hemisphere of migratory birds. However, recently it was demonstrated that both garden warblers, Sylvia borin, and European robins have a magnetic compass in both eyes. These results raise the question if the strong lateralization effect observed in earlier experiments might have arisen from artifacts or from differences in experimental conditions rather than reflecting a true all-or-none lateralization of the magnetic compass in European robins. Here we show that (1) European robins having only their left eye open can orient in their seasonally appropriate direction both during autumn and spring, i.e. there are no strong lateralization differences between the outward journey and the way home, that (2) their directional choices are based on the standard inclination compass as they are turned 180u when the inclination is reversed, and that (3) the capability to use the magnetic compass does not depend on monocular learning or intraocular transfer as it is already present in the first tests of the birds with only one eye open. [ABSTRACT FROM AUTHOR]

Birds have remained the dominant model for studying the mechanisms of animal navigation for decades, with much of what has been discovered coming from laboratory studies or model systems. The miniaturisation of tracking technology in recent years now promises opportunities for studying navigation during migration itself (migratory navigation) on an unprecedented scale. Even if migration tracking studies are principally being designed for other purposes, we argue that attention to salient environmental variables during the design or analysis of a study may enable a host of navigational questions to be addressed, greatly enriching the field. We explore candidate variables in the form of a series of contrasts (e.g. land vs ocean or night vs day migration), which may vary naturally between migratory species, populations or even within the life span of a migrating individual. We discuss how these contrasts might help address questions of sensory mechanisms, spatiotemporal representational strategies and adaptive variation in navigational ability. We suggest that this comparative approach may help enrich our knowledge about the natural history of migratory navigation in birds. [ABSTRACT FROM AUTHOR]

This article discusses a study which tested whether the putative double-clock mechanism may serve as a navigational tool for detecting east-west position in long-distance migrants, including the Eurasian Reed Warbler (Acrocephalus scirpaceus). Researchers captured Eurasian Reed Warblers during spring migration and tested their orientation in Emlen funnels under capture-site photoperiodic conditions. They found that light-dark regime effects alone are unlikely to trigger compensation for the longitudinal displacement.

Magnetic compass orientation in a night-migratory songbird requires that Cluster N, a cluster of forebrain regions, is functional. Cluster N, which receives input from the eyes via the thalamofugal pathway, shows high neuronal activity in night-migrants performing magnetic compass-guided behaviour at night, whereas no activation is observed during the day, and covering up the birds’ eyes strongly reduces neuronal activation. These findings suggest that Cluster N processes light-dependent magnetic compass information in night-migrating songbirds. The aim of this study was to test if Cluster N is active during daytime migration. We used behavioural molecular mapping based on ZENK activation to investigate if Cluster N is active in the meadow pipit ( Anthus pratensis), a day- and night-migratory species. We found that Cluster N of meadow pipits shows high neuronal activity under dim-light at night, but not under full room-light conditions during the day. These data suggest that, in day- and night-migratory meadow pipits, the light-dependent magnetic compass, which requires an active Cluster N, may only be used during night-time, whereas another magnetosensory mechanism and/or other reference system(s), like the sun or polarized light, may be used as primary orientation cues during the day. [ABSTRACT FROM AUTHOR]

Cluster N is a cluster of forebrain regions found in night-migratory songbirds that shows high activation of activity-dependent gene expression during night-time vision. We have suggested that Cluster N may function as a specialized night-vision area in night-migratory birds and that it may be involved in processing light-mediated magnetic compass information. Here, we investigated these ideas. We found a significant lateralized dominance of Cluster N activation in the right hemisphere of European robins ( Erithacus rubecula). Activation predominantly originated from the contralateral (left) eye. Garden warblers ( Sylvia borin) tested under different magnetic field conditions and under monochromatic red light did not show significant differences in Cluster N activation. In the fairly sedentary Sardinian warbler ( Sylvia melanocephala), which belongs to the same phyolgenetic clade, Cluster N showed prominent activation levels, similar to that observed in garden warblers and European robins. Thus, it seems that Cluster N activation occurs at night in all species within predominantly migratory groups of birds, probably because such birds have the capability of switching between migratory and sedentary life styles. The activation studies suggest that although Cluster N is lateralized, as is the dependence on magnetic compass orientation, either Cluster N is not involved in magnetic processing or the magnetic modulations of the primary visual signal, forming the basis for the currently supported light-dependent magnetic compass mechanism, are relatively small such that activity-dependent gene expression changes are not sensitive enough to pick them up. [ABSTRACT FROM AUTHOR]

Bird migration has been assumed, mostly implicitly, to represent a distinct class of animal behavior, with deep and strong homologies in the various phenotypic expressions of migratory behavior between different taxa. Here the evidence for the existence of what could be called a "migratory syndrome," a tightly integrated, old group of adaptive traits that enables birds to commit themselves to highly organized seasonal migrations, is assessed. A list of problems faced by migratory birds is listed first and the traits that migratory birds have evolved to deal with these problems are discussed. The usefulness of comparative approaches to investigate which traits are unique to migrants is then discussed. A provisional conclusion that, perhaps apart from a capacity for night-time compass orientation, there is little evidence for deeply rooted coadapted trait complexes that could make up such a migratory syndrome, is suggested. Detailed analyses of the genetic and physiological architecture of potential adaptations to migration, combined with a comparative approach to further identify the phylogenetic levels at which different adaptive traits for migration have evolved, are recommended. [ABSTRACT FROM AUTHOR]

Presents a study which looked the migration of pied flycatchers, Ficedula hypoleuca, in relation to a clock-and-compass model which is used to explain the distribution of ringing recoveries. Methodology used to conduct the study; Investigation of ringing recoveries; Discussion based on the results of the study.

Although it is known that birds can return to their breeding grounds with exceptional precision, it has remained a mystery how they know when and where to stop migrating. Using nearly a century’s worth of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether fluctuations in Earth’s magnetic field predict variation in the sites to which birds return. Ringing recoveries suggest that magnetic inclination is learned before departure and is subsequently used as a uni-coordinate “stop sign” when relocating the natal or breeding site. However, many locations have the same inclination angle. Data from populations with different migratory directions indicate that birds solve this ambiguity by stopping at the first place where the right inclination is encountered on an inherited return vector. [ABSTRACT FROM AUTHOR]

Migratory birds can use a variety of environmental cues for orientation. A primary calibration between the celestial and magnetic compasses seems to be fundamental prior to a bird's first autumn migration. Releasing hand-raised or rescued young birds back into the wild might therefore be a problem because they might not have established a functional orientation system during their first calendar year. Here, we test whether hand-raised European robins that did not develop any functional compass before or during their first autumn migration could relearn to orient if they were exposed to natural celestial cues during the subsequent winter and spring. When tested in the geomagnetic field without access to celestial cues, these birds could orient in their species-specific spring migratory direction. In contrast, control birds that were deprived of any natural celestial cues throughout remained unable to orient. Our experiments suggest that European robins are still capable of establishing a functional orientation system after their first autumn. Although the external reference remains speculative, most likely, natural celestial cues enabled our birds to calibrate their magnetic compass. Our data suggest that avian compass systems are more flexible than previously believed and have implications for the release of hand-reared migratory birds. [ABSTRACT FROM AUTHOR]

Several studies have shown that experienced night-migratory songbirds can determine their position, but it has remained a mystery which cues and sensory mechanisms they use, in particular, those used to determine longitude (east–west position). One potential solution would be to use a magnetic map or signpost mechanism like the one documented in sea turtles. Night-migratory songbirds have a magnetic compass in their eyes and a second magnetic sense with unknown biological function involving the ophthalmic branch of the trigeminal nerve (V1). Could V1 be involved in determining east–west position? We displaced 57 Eurasian reed warblers (Acrocephalus scirpaceus) with or without sectioned V1. Sham operated birds corrected their orientation towards the breeding area after displacement like the untreated controls did. In contrast, V1-sectioned birds did not correct for the displacement. They oriented in the same direction after the displacement as they had done at the capture site. Thus, an intact ophthalmic branch of the trigeminal nerve is necessary for detecting the 1,000 km eastward displacement in this night-migratory songbird. Our results suggest that V1 carries map-related information used in a large-scale map or signpost sense that the reed warblers needed to determine their approximate geographical position and/or an east–west coordinate. [ABSTRACT FROM AUTHOR]

Displacement experiments have demonstrated that experienced migratory birds translocated thousands of kilometers away from their migratory corridor can orient toward and ultimately reach their intended destinations. 1 This implies that they are capable of "true navigation," commonly defined 2–4 as the ability to return to a known destination after displacement to an unknown location without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey. 5–13 In birds, true navigation appears to require previous migratory experience 5–7,14,15 (but see Kishkinev et al. 16 and Piersma et al. 17). It is generally assumed that, to correct for displacements outside the familiar area, birds initially gather information within their year-round distribution range, learn predictable spatial gradients of environmental cues within it, and extrapolate from those to unfamiliar magnitudes—the gradient hypothesis. 6,9,18–22 However, the nature of the cues and evidence for actual extrapolation remain elusive. Geomagnetic cues (inclination, declination, and total intensity) provide predictable spatial gradients across large parts of the globe and could serve for navigation. We tested the orientation of long-distance migrants, Eurasian reed warblers, exposing them to geomagnetic cues of unfamiliar magnitude encountered beyond their natural distribution range. The birds demonstrated re-orientation toward their migratory corridor as if they were translocated to the corresponding location but only when all naturally occurring magnetic cues were presented, not when declination was changed alone. This result represents direct evidence for migratory birds' ability to navigate using geomagnetic cues extrapolated beyond their previous experience. • Birds respond to novel magnetic fields as if displaced to the equivalent location • Changing one magnetic cue only (declination) does not result in re-orientation • The "virtual displacement" only works when all magnetic cues match a real place • This strongly suggests birds can extrapolate beyond previous knowledge to new places It is still unclear how migratory birds navigate from outside their familiar range. By testing their orientation in a changed magnetic field at the capture site, Kishkinev et al. show that birds respond to these changes as if displaced to the simulated location, suggesting they can extrapolate beyond their previous experience of the magnetic field. [ABSTRACT FROM AUTHOR]

Summary Birds seem to use a light-dependent, radical-pair-based magnetic compass. In vertebrates, cryptochromes are the only class of proteins that form radical pairs upon photo-excitation. Therefore, they are currently the only candidate proteins for light-dependent magnetoreception. Cryptochrome 4 (Cry4) is particularly interesting because it has only been found in vertebrates that use a magnetic compass. However, its structure and localization within the retina has remained unknown. Here, we sequenced night-migratory European robin ( Erithacus rubecula ) Cry4 from the retina and predicted the currently unresolved structure of the erCry4 protein, which suggests that erCry4 should bind Flavin. We also found that Cry1a , Cry1b , and Cry2 mRNA display robust circadian oscillation patterns, whereas Cry4 shows only a weak circadian oscillation. When we compared the relative mRNA expression levels of the cryptochromes during the spring and autumn migratory seasons relative to the non-migratory seasons in European robins and domestic chickens ( Gallus gallus ), the Cry4 mRNA expression level in European robin retinae, but not in chicken retinae, is significantly higher during the migratory season compared to the non-migratory seasons. Cry4 protein is specifically expressed in the outer segments of the double cones and long-wavelength single cones in European robins and chickens. A localization of Cry4 in double cones seems to be ideal for light-dependent magnetoreception. Considering all of the data presented here, especially including its localization within the European robin retina, its likely binding of Flavin, and its increased expression during the migratory season in the migratory bird but not in chicken, Cry4 could be the magnetoreceptive protein. [ABSTRACT FROM AUTHOR]

Summary The longitude problem (determining east-west position) is a classical problem in human sea navigation. Prior to the use of GPS satellites, extraordinarily accurate clocks measuring the difference between local time and a fixed reference (e.g., GMT) [ 1 ] were needed to determine longitude. Birds do not appear to possess a time-difference clock sense [ 2 ]. Nevertheless, experienced night-migratory songbirds can correct for east-west displacements to unknown locations [ 3–9 ]. Consequently, migratory birds must solve the longitude problem in a different way, but how they do so has remained a scientific mystery [ 10 ]. We suggest that experienced adult Eurasian reed warblers ( Acrocephalus scirpaceus ) can use magnetic declination to solve the longitude problem at least under some circumstances under clear skies. Experienced migrants tested during autumn migration in Rybachy, Russia, were exposed to an 8.5° change in declination while all other cues remained unchanged. This corresponds to a virtual magnetic displacement to Scotland if and only if magnetic declination is a part of their map. The adult migrants responded by changing their heading by 151° from WSW to ESE, consistent with compensation for the virtual magnetic displacement. Juvenile migrants that had not yet established a navigational map also oriented WSW at the capture site but became randomly oriented when the magnetic declination was shifted 8.5°. In combination with latitudinal cues, which birds are known to detect and use [ 10–12 ], magnetic declination could provide the mostly east-west component for a true bi-coordinate navigation system under clear skies for experienced migratory birds in some areas of the globe. [ABSTRACT FROM AUTHOR]

Summary Displacement studies have shown that long-distance, night-migrating songbirds are able to perform true navigation from their first spring migration onwards [1,2] . True navigation requires both a map and a compass. Whereas birds are known to have sun, star, and magnetic compasses, the nature of the map cues used has remained highly controversial. There is quite strong experimental evidence for the involvement of olfactory map cues in pigeon and seabird homing [3] . In contrast, the evidence for the use of magnetic map cues has remained weak and very little is known about the map cues used by long-distance migratory songbirds. In earlier experiments [2,4] , we have shown that Eurasian reed warblers physically displaced 1,000 km eastward from Rybachy to Zvenigorod ( Figure 1 ) re-orient towards their breeding destinations by changing their orientation in Emlen funnels from the NE to the NW. We have also previously shown that this re-orientation cannot be explained by a ‘jetlag effect’ [5] . We have now used this model system to show that Eurasian reed warblers use geomagnetic map cues to determine their position. [ABSTRACT FROM AUTHOR]