due to a direct effect of light on the newts' perception of the magnetic field, we trained newts under long-wavelength light by covering the training tank with a long-wavelength-transmitting gel filter (two layers of Lee #101) 5 . Under long-wavelength light, these newts orientated themselves parallel to the shoreward axis, indicating that they had learned the direction of the shore with respect to the rotated magnetic information under long-wavelength light (Fig. 1c,f).As well as ocular photoreceptors, newts have extraocular photoreceptors in the pineal complex 8 and possibly the hypothalamus 9 . To determine which photoreceptors are involved in the magnetic compass response, we manipulated the wavelength of light reaching the extraocular photorecep- Extraocular magnetic compass in newtsGeomagnetic orientation is widespread among organisms, but the mechanism(s) of magnetoreception has not been identified convincingly in any animal 1 . In agreement with biophysical models proposing that the geomagnetic field interacts with photoreceptors 2-4 , changes in the wavelength of light have been shown to influence magnetic compass orientation in an amphibian, an insect and several species of birds (reviewed in ref. 5). We find that light-dependent magnetic orientation in the eastern red-spotted newt, Notophthalmus viridescens, is mediated by extraocular photoreceptors, probably located in the pineal complex or deeper in the brain (perhaps the hypothalamus).Experiments investigating shoreward magnetic compass orientation have demonstrated that the newt's perception of the direction of the magnetic field is rotated 90 o under long-wavelength (greater than 500 nm) light 5,6 . We recently trained newts under natural skylight to aim for the shore by placing them for 12-16 hours in waterfilled tanks with an artificial shore at one end 5,7 . The magnetic orientation of individual newts was then tested in a circular, visually symmetrical indoor arena under depolarized light. Under full-spectrum light (from a xenon arc source), they exhibited bimodal magnetic orientation parallel to the shoreward axis in the training tank (Fig. 1a,d). In contrast, under long-wavelength light, they orientated themselves perpendicular to the shoreward direction (Fig. 1b,e).To demonstrate that the 90 o shift in orientation under long-wavelength light was tors. Small round 'caps' (5 mm in diameter) were attached to the dorsal surface of the head of each newt using cyanoacrylate glue, and remained in place during both training and testing. Equal numbers of newts were capped with either a clear filter (Lee #130) or a filter that transmitted only long-wavelength light (equivalent to two layers of Lee #101). The caps were positioned to alter the spectral properties of light reaching the pineal and surrounding structures, whereas light reaching the eyes was unaffected.Clear-capped newts were tested to control for any nonspecific effects of the caps on the newts' orientation behaviour.All newts were trained outdoors under natural skylight and tested for m...

Fat reserves influence the orientation of migrating songbirds at ecological barriers, such as expansive water crossings. Upon encountering a body of water, fat migrants usually cross the barrier exhibiting ‘forward’ migration in a seasonally appropriate direction. In contrast, lean birds often exhibit temporary ‘reverse’ orientation away from the water, possibly to lead them to suitable habitats for refueling. Most examples of reverse orientation are restricted to autumn migration and, in North America, are largely limited to transcontinental migrants prior to crossing the Gulf of Mexico. Little is known about the orientation of lean birds after crossing an ecological barrier or on the way to their breeding grounds. We examined the effect of fat stores on migratory orientation of both long‐ and short‐distance migrants before and after a water crossing near their breeding grounds; Catharus thrushes (Swainson's and gray‐cheeked thrushes, C. ustulatus and C. minimus) and white‐throated sparrows Zonotrichiaalbicollis were tested for orientation at the south shore of Lake Ontario during spring and autumn. During both spring and autumn, fat birds oriented in a seasonally appropriate, forward direction. Lean thrushes showed a tendency for reverse orientation upon encountering water in the spring and axial, shoreline orientation after crossing water in the autumn. Lean sparrows were not consistently oriented in any direction during either season. The responses of lean birds may be attributable to their stopover ecology and seasonally‐dependent habitat quality.

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SUMMARYThe interaction and hierarchy of celestial and magnetic compass cues used by migratory songbirds for orientation has long been the topic of an intense debate. We have previously shown that migratory Savannah sparrows, Passerculus sandwichensis, use polarized light cues near the horizon at sunrise and sunset to recalibrate their magnetic compass. Birds exposed to a ±90deg. shifted artificial polarization pattern at sunrise or sunset recalibrated their magnetic compass, but only when given full access to celestial cues, including polarized light cues near the horizon. In the current study, we carried out cue conflict experiments with white-throated sparrows, Zonotrichia albicollis, during both spring and autumn migration in a transition zone between the species' breeding and wintering areas on the south shore of Lake Ontario. We show that white-throated sparrows also recalibrate their magnetic compass by polarized light cues at sunrise and sunset. Sunrise exposure to an artificial polarization pattern shifted relative to the natural magnetic field or exposure to a shift of the magnetic field relative to the natural sky both led to recalibration of the magnetic compass, demonstrating that artificial polarizing filters do not create an anomalous, unnatural orientation response. Our results further indicate that there is no evidence for a difference in compass hierarchy between different phases of migration, confirming previous work showing that polarized light cues near the horizon at sunrise and sunset provide the primary calibration reference both in the beginning and at the end of migration.

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