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For studies on magnetic compass orientation and navigation performance in small bird species, controlled experiments with orientation cages inside an electromagnetic coil system are the most prominent methodological paradigm. These are, however, not applicable when studying larger bird species and/or orientation behaviour during free flight. For this, researchers have followed a very different approach, attaching small magnets to birds, with the intention of depriving them of access to meaningful magnetic information. Unfortunately, results from studies using this approach appear rather inconsistent. As these are based on experiments with birds under free-flight conditions, which usually do not allow exclusion of other potential orientation cues, an assessment of the overall efficacy of this approach is difficult to conduct. Here, we directly tested the efficacy of small magnets for temporarily disrupting magnetic compass orientation in small migratory songbirds using orientation cages under controlled experimental conditions. We found that birds which have access to the Earth's magnetic field as their sole orientation cue show a general orientation towards their seasonally appropriate migratory direction. When carrying magnets on their forehead under these conditions, the same birds become disoriented. However, under changed conditions that allow birds access to other (i.e. celestial) orientation cues, any disruptive effect of the magnets they carry appears obscured. Our results provide clear evidence for the efficacy of the magnet approach for temporarily disrupting magnetic compass orientation in birds, but also reveal its limitations for application in experiments under free-flight conditions., Summary: Orientation tests in a small migratory songbird provide clear evidence for the efficacy of the magnet approach for temporarily disrupting magnetic compass orientation, but also reveal its limitations in experiments under free-flight conditions.

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.

Summary 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 , 3 , 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 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 In birds, true navigation appears to require previous migratory experience 5 , 6 , 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 , 19 , 20 , 21 , 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.

How blood parasite infections influence the migration of hosts remains a lively debated issue as past studies found negative, positive, or no response to infections. This particularly applies to small birds, for which monitoring of detailed migration behavior over a whole annual cycle has been technically unachievable so far. Here, we investigate how bird migration is influenced by parasite infections. To this end, we tracked great reed warblers (Acrocephalus arundinaceus) with multisensor loggers, characterized general migration patterns as well as detailed flight bout durations, resting times and flight heights, and related these to the genus and intensity of their avian haemosporidian infections. We found migration distances to be shorter and the onset of autumn migration to be delayed with increasing intensity of blood parasite infection, in particular for birds with Plasmodium and mixed-genus infections. Additionally, the durations of migratory flight bout were prolonged for infected compared to uninfected birds. But since severely infected birds and particularly birds with mixed-genus infections had shorter resting times, initial delays seemed to be compensated for and the timing in other periods of the annual cycle was not compromised by infection. Overall, our multisensor logger approach revealed that avian blood parasites have mostly subtle effects on migratory performance and that effects can occur in specific periods of the year only., Ecology and Evolution, 11 (2), ISSN:2045-7758

Currently, the deployment of tracking devices is one of the most frequently used approaches to study movement ecology of birds. Recent miniaturization of light‐level geolocators enabled studying small bird species whose migratory patterns were widely unknown. However, geolocators may reduce vital rates in tagged birds and may bias obtained movement data. There is a need for a thorough assessment of the potential tag effects on small birds, as previous meta‐analyses did not evaluate unpublished data and impact of multiple life‐history traits, focused mainly on large species and the number of published studies tagging small birds has increased substantially. We quantitatively reviewed 549 records extracted from 74 published and 48 unpublished studies on over 7,800 tagged and 17,800 control individuals to examine the effects of geolocator tagging on small bird species (body mass, + Publisher's Statement: This is the peer reviewed version of the following article: "Brlík, V, Koleček, J, Burgess, M, et al. Weak effects of geolocators on small birds: A meta‐analysis controlled for phylogeny and publication bias. J Anim Ecol. 2020; 89: 207– 220", which has been published in final form at https://doi.org/10.1111/1365-2656.12962. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.; Read Only Lizenz.

Long-distance avian migrants, e.g. Eurasian reed warblers (Acrocephalus scirpaceus), can precisely schedule events of their annual cycle. However, the proximate mechanisms controlling annual cycle and their interplay with environmental factors are poorly understood. We artificially interrupted breeding in reed warblers by bringing them into captivity and recording birds’ locomotor activity for 5–7 days. Over this time, most of the captive birds gradually developed nocturnal locomotor activity not observed in breeding birds. When the birds were later released and radio-tracked, the individuals with highly developed caged activity performed nocturnal flights. We also found that reed warblers kept indoors without access to local cues developed a higher level of nocturnal activity compared to the birds kept outdoors with an access to the familiar environment. Also, birds translocated from a distant site (21 km) had a higher motivation to fly at night-time after release compared to the birds captured within 1 km of a study site. Our study suggests that an interrupted breeding triggers development of nocturnal locomotor activity in cages, and the level of activity is correlated with motivation to perform nocturnal flights in the wild, which can be restrained by familiar environment.

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.

Currently at least two independent systems of magnetoreception are believed to exist in birds, based on different biophysical principles, located in different parts of their bodies, and having different innervation. One magnetoreceptory system is located in the retina and may be based on photo-induced biradical chemical reactions on the basis of cryptochrome. Information from these receptors is processed in a specialized part of visual Wulst, the so-called Cluster N. There are good reasons to believe that this visual magnetoreceptor processes compass magnetic information which is necessary for migratory orientation. The second magnetoreceptory system is probably iron-based (biogenic magnetite), is located somewhere in the upper beak (its exact location and ultrastructure of receptors remain unknown), and is innervated by the ophthalmic branch of trigeminal nerve. It cannot be ruled out that this system participates in spatial representation and helps forming either a kind of map or more primitive signposts, based on regular spatial variation of the geomagnetic field. The magnetic map probably governs navigation of migrating birds across hundreds and thousands of kilometers. Apart from these two systems whose existence may be considered to be convincingly shown (even if their details are not yet fully clear), there are data on the existence of magnetoreceptors based on the vestibular system. It cannot be ruled out that iron-based magnetoreception takes place in lagena (a structure homologous to cochlea of marsupials and eutherians), and the information perceived is processes in vestibular nuclei. The very existence of this magnetoreception system needs verification, and its function remains completely open.

The ability to navigate implies that animals have the capability to compensate for geographical displacement and return to their initial goal or target. Although some species are capable of adjusting their direction after displacement, the environmental cues used to achieve this remain elusive. Two possible cues are geomagnetic parameters (magnetic map hypothesis) or atmospheric odour-forming gradients (olfactory map hypothesis). In this study, we examined both of these hypotheses by surgically deactivating either the magnetic or olfactory sensory systems in experienced white-throated sparrows (Zonotrichia albicollis) captured in southern Ontario, Canada, during spring migration. Treated, sham-treated, and intact birds were then displaced 2,200 km west to Saskatchewan, Canada. Tracking their initial post-displacement migration using an array of automated VHF receiving towers, we found no evidence in any of the groups for compensatory directional response towards their expected breeding grounds. Our results suggest that white-throated sparrows may fall back to a simple constant-vector orientation strategy instead of performing true navigation after they have been geographically displaced to an unfamiliar area during spring migration. Such a basic strategy may be more common than currently thought in experienced migratory birds and its occurrence could be determined by habitat preferences or range size.

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.

SUMMARY Migratory birds are able to use the sun and associated polarised light patterns, stellar cues and the geomagnetic field for orientation. No general agreement has been reached regarding the hierarchy of orientation cues. Recent data from naturally migrating North American Catharus thrushes suggests that they calibrate geomagnetic information daily from twilight cues. Similar results have been shown in caged birds in a few studies but not confirmed in others. We report that free-flying European migrants, song thrushes Turdus philomelos, released after pre-exposure to a horizontally rotated magnetic field, do not recalibrate their magnetic compass from solar cues, but rather show a simple domination of either the magnetic or the stellar compass. We suggest that different songbird species possess different hierarchies of orientation cues, depending on the geographic and ecological challenges met by the migrants.

Migratory birds are known to be able to navigate—to determine their position on the globe and the direction toward their distant goal—even if they cannot perceive any information emanating from the goal. One hypothesis, that of true bicoordinate navigation, claims that birds should be able to sense and use a grid of two natural parameters as coordinates. Some indirect data support the idea that migratory birds can determine their north—south position, and several recent studies have suggested that at least longdistance migrants, including the Eurasian Reed Warbler (Acrocephalus scirpaceus), can determine their east-west position. How they do this remains a mystery. Birds could theoretically detect the magnitude of east-west displacements if they have two clocks, one synchronizing faster to local time than the other. We tested whether this putative “double-clock” mechanism may serve as a navigational tool for detecting east-west position. We captured Eurasian Reed Warblers during spring migration...

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.

Field studies suggest that in autumn, passerine Siberian-African migrants make a detour around Central Asia. We tested whether it results from an innate spatiotemporal programme. We hand-raised juvenile pied flycatchers from Europe and western Siberia in captivity and studied their migratory orientation by testing in Emlen funnels. The birds were kept outdoors in the local natural magnetic field throughout the experiment. Siberian birds showed a purely westerly orientation in mid August–mid September, before changing direction in late September. These data suggest that juvenile Siberian pied flycatchers indeed have an innate spatiotemporal programme that brings them to Europe before migration to West African winter quarters. Siberian pied flycatchers displaced to the Baltic area as nestlings, raised and tested there showed no significant second-order orientation vector in August; in September their mean orientation direction was south-southwestern (202°) and differed significantly from the western direction shown by their conspecifics in Siberia in August–mid September. A possible explanation is that the displaced birds detected displacement on the basis of the innate knowledge of some signposts. They may have ‘skipped’ the section of the route from Siberia to Europe and ‘switched on’ their migratory programme when in Europe, already towards the south-southwest.

During migration, birds must fly over suboptimal habitats differing from those selected during breeding and wintering. Nocturnally migrating passerines need to assess the suitability of potential stopover habitats during landfall. Before actual landfall, distant cues may play a significant role in habitat selection. In this paper, we studied the possibility that migrant songbirds use acoustic information as distant cues for habitat selection when ceasing flight. We also investigated differences between habitat generalists and specialists in their use of acoustic cues by comparing the proportion of species killed at lighthouses with those captured in mist nets using song playback in both suitable habitats and suboptimal stopover habitats during spring and fall migratory seasons. Our observations showed that during twilight landfall, birds may respond to acoustic cues, especially when visual cues are reduced or absent. This was true for habitat specialists (Eurasian reed warblers and sedge warblers) whose songs are also more attractive to conspecifics and other birds of wetland habitats than to habitat generalists (pied flycatchers and redwings). Adult Eurasian reed warblers had a tendency to be more attracted by acoustic cues than juveniles. This finding suggests that previous experience may play a role in habitat recognition using acoustic stimuli. Key words: heterospecific attraction, lighthouses, song playback. [Behav Ecol]

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.

Arising from W. Wiltschko et al. 419, 467-470 (2002); Wiltschko et al. replyThe magnetic compass of migratory birds is embedded in the visual system and it has been reported by Wiltschko et al. that European Robins, Erithacus rubecula, cannot show magnetic compass orientation using their left eye only. This has led to the notion that the magnetic compass should be located only in the right eye of birds. However, a complete right lateralization of the magnetic compass would be very surprising, and functional neuroanatomical data have questioned this notion. Here we show that the results of Wiltschko et al. could not be independently confirmed using double-blind protocols. European Robins can perform magnetic compass orientation with both eyes open, with the left eye open only, and with the right eye open only. No clear lateralization is observed.

Magnetic compass information has a key role in bird orientation, but the physiological mechanisms enabling birds to sense the Earth's magnetic field remain one of the unresolved mysteries in biology. Two biophysical mechanisms have become established as the most promising magnetodetection candidates. The iron-mineral-based hypothesis suggests that magnetic information is detected by magnetoreceptors in the upper beak and transmitted through the ophthalmic branch of the trigeminal nerve to the brain. The light-dependent hypothesis suggests that magnetic field direction is sensed by radical pair-forming photopigments in the eyes and that this visual signal is processed in cluster N, a specialized, night-time active, light-processing forebrain region. Here we report that European robins with bilateral lesions of cluster N are unable to show oriented magnetic-compass-guided behaviour but are able to perform sun compass and star compass orientation behaviour. In contrast, bilateral section of the ophthalmic branch of the trigeminal nerve in European robins did not influence the birds' ability to use their magnetic compass for orientation. These data show that cluster N is required for magnetic compass orientation in this species and indicate that it may be specifically involved in processing of magnetic compass information. Furthermore, the data strongly suggest that a vision-mediated mechanism underlies the magnetic compass in this migratory songbird, and that the putative iron-mineral-based receptors in the upper beak connected to the brain by the trigeminal nerve are neither necessary nor sufficient for magnetic compass orientation in European robins.

SummaryIn order to perform true bicoordinate navigation, migratory birds need to be able to determine geographic latitude and longitude. The determination of latitude is relatively easy from either stellar or magnetic cues [1–3], but the determination of longitude seems challenging [4, 5]. It has therefore been suggested that migrating birds are unable to perform bicoordinate navigation and that they probably only determine latitude during their return migration [5]. However, proper testing of this hypothesis requires displacement experiments with night-migratory songbirds in spring that have not been performed. We therefore displaced migrating Eurasian reed warblers (Acrocephalus scirpaceus) during spring migration about 1000 km toward the east and found that they were correcting for displacements by shifting their orientation from the northeast at the capture site to the northwest after the displacement. This new direction would lead them to their expected breeding areas. Our results suggest that Eurasian reed warblers are able to determine longitude and perform bicoordinate navigation. This finding is surprising and presents a new intellectual challenge to bird migration researchers, namely, which cues enable birds to determine their east-west position.

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.

Recent research into the navigational strategies of homing pigeons (Columba livia) inthe familiar area has highlighted the phenomenon of route fidelity – birds formingidiosyncratic flight paths to which they are loyal over multiple releases from the samesite, and even returning to this path when released from a near-by unfamiliar location.Such results highlight the potential importance of visual landmark cues in the homingprocess. However, not all birds have been shown to produce idiosyncratic routes orshow this route-joining behaviour. Here we use birds with and without flight experienceto study the formation of idiosyncratic routes when released repeatedly from a singlelocation, followed by two off-route releases with differing topography to see how flightexperience and local landmark features can influence navigational strategy in thefamiliar area. We found that, over the course of 20 sequential releases, birds withgreater flight experience tended to form idiosyncratic routes whereas less experiencedbirds did not show this tendency. When released from near-by sites (from which thebirds had not previously been released), a range of navigational strategies were seen,including flying parallel to the learned route (suggestive of a learned compassdirection), a direct flight path towards home (again indicative of compass use), re-joining the learned route, and following the coastline. These latter strategies aresuggestive of landmark usage. Analysis using time lag embedding was also used toassess the off-route releases, and the short-term correlation dimension valuesproduced (ranging from 1.5-2.5) were also indicative of strategies using one or twofactors (landmarks, compass, or a combination of these two). Individual birds oftenshowed different strategies at different sites, suggesting that the use of differentnavigational cues is highly flexible and situationally dependent.

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.