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A number of studies have shown that migrating birds can navigate to their destinations even when displaced to unfamiliar territory. It has been demonstrated that adult Eurasian Reed Warblers (Acrocephalus scirpaceus) captured in spring in the Eastern Baltic, displaced 1000 km eastward to the Moscow region and tested in orientation cages, show a clear orientation tendency towards their breeding grounds. This response requires the ability to determine a new geographic position relative to the goal. The natural cues that are used as coordinates for this behaviour remain controversial. Among other natural cues, both magnetic and olfactory sources of information have received the most experimental attention. More recently, virtual displacement experiments have shown that the geomagnetic information alone is sufficient for Reed Warblers to find their geographic position. However, the role of olfaction was not explicitly examined. In the present study, we displaced anosmic Reed Warblers together with untreated controls between the same capture and displacement sites where the Emlen funnel tests were previously performed. Following release, we radio-tracked birds for the first few kilometres using an array of automated radio-tracking towers. The results strongly suggest a navigational response of both anosmic and intact birds (anticlockwise re-orientation), unlike some other experiments showing impaired navigational abilities of anosmic migrating birds. This data supports the hypothesis that, at least in this songbird species, the olfactory system is not crucial for determining geographic position, and that the zinc sulfate anosmia treatment is unlikely to have any non-specific effects on navigational abilities.

Displacement studies have clearly shown that experienced avian migrants are able to perform true navigation, i.e., they can find the correct direction leading to a target destination from unfamiliar sites. The sensory mechanisms of true navigation remain poorly understood, though some remarkable progress has been made in the last 10–15 years. There are two primary hypotheses explaining the sensory nature of navigation: (1) a magnetic map hypothesis proposes that birds use parameters of the geomagnetic field that are predictably distributed on the globe. As for the sensory nature of this hypothesis, it has been assumed by some researchers that the magnetic receptor cells reside in the upper beak (the so-called “beak organ”), and transmit information via the trigeminal nerve to the brain; (2) an olfactory map hypothesis assumes that birds can smell their position by taking advantage of odours distributed in the atmosphere. There are a growing number of studies supporting both of the hypotheses mentioned though in different avian model species. In this review, an attempt is made to provide an overview of the evidence for different navigational cues proposed thus far, with the main focus on the recent studies addressing the magnetic and olfactory navigation hypotheses. Also, a list of key open questions, together with possible experimental approaches, is proposed.

Birds are thought to possess two magnetosensory systems: (1) a chemical sensor in the bird’s eye, and (2) a magnetoreceptor innervated by the ophthalmic branch of the trigeminal nerve (V1) and presumably located in the upper beak. It has been recently demonstrated that the visually mediated magnetosensory system is crucial to the magnetic compass of the European Robin (Erithacus rubecula). In contrast, the trigeminal nerve system, despite neuronal responses to magnetic stimuli, is neither necessary nor sufficient for magnetic compass orientation in this species. Unfortunately, the potential role of the trigeminal organ is unlikely to be elucidated by the classical behavioral paradigm of the orientation cage, because it tests only for spontaneous compass responses. An operant conditioning study by Mora et al. (Nature 432:508–511, 2004) demonstrated that Homing Pigeons can be conditioned to discriminate the presence and absence of a strong gradient magnetic field stimulus. This discrimination depended on intact ophthalmic branches of the trigeminal nerves. Here, we report detailed attempts aimed at adapting the behavioral paradigm used by the above study for a model migratory bird, the European Robin. We tested three variants of a conditioning procedure very similar to that of Mora et al. (Nature 432:508–511, 2004). Despite extensive training, we were not able to demonstrate that our experimental birds were able to discriminate the magnetic stimuli presented to them. This was, however, not due to a general unsuitability of the conditioning setup for this species, because the robins were able to successfully discriminate the presence and absence of an auditory stimulus in the same setup.

We studied the effects of weather and the lunar cycle on long-distance nocturnal pre-migratory flights of Reed Warblers (Acrocephalus scirpaceus). Noturnal tape luring was used to capture the birds, and the study was carried out in a habitat atypical of this species on the Courish Spit (southeastern Baltic) between1999 and 2002. A total of 443 juvenile Reed Warblers were captured during 120 nights of trapping. Based on data on the moult and body condition of the birds, it was possible to identify 163 individuals as being on post-fledging movements. More than half of the birds (54.0%) were captured at the end of night during the nautical and civil twilight period; the remaining individuals were caught during the astronomical twilight period and darkest periods of night. Many birds performed pre-migratory flights in the middle of the night, the period during which a large part of the moon was visible. We suggest that the increased visibility under the full moon may provide conditions in which Reed Warblers increase the distance or intensity of nocturnal post-fledging movements. During the entire course of the night, the birds generally preferred to fly under rainless conditions, limited cloud cover and/or in still air. The birds performed flights under winds stronger than 2 m s−1 when the wind was blowing along the axis of the spit. We also suggest that the birds generally fly along the spit. We found a weak but significant relationship between the numbers of Reed Warblers captured during post-fledging movements and the individual weather parameters and their interaction. Our data suggest that endogenous stimuli rather than weather parameters or lunar cycle phase determine the decision of Reed Warblers to undertake pre-migratory long-distance flights at night.