Does migratory distance affect fuelling in a medium-distance passerine migrant?: results from direct and step-wise simulated magnetic displacements

A conceptual framework on the role of magnetic cues in songbird migration ecology

Migrating animals perform astonishing seasonal movements by orienting and navigating over thousands of kilometres with great precision. Many migratory species use cues from the sun, stars, landmarks, olfaction and the Earth's magnetic field for this task. Among vertebrates, songbirds are the most studied taxon in magnetic-cue-related research. Despite multiple studies, we still lack a clear understanding of when, where and how magnetic cues affect the decision-making process of birds and hence, their realised migratory behaviour in the wild. This understanding is especially important to interpret the results of laboratory experiments in an ecologically appropriate way. In this review, we summarise the current findings about the role of magnetic cues for migratory decisions in songbirds. First, we review the methodological principles for orientation and navigation research, specifically by comparing experiments on caged birds with experiments on free-flying birds. While cage experiments can show the sensory abilities of birds, studies with free-flying birds can characterise the ecological roles of magnetic cues. Second, we review the migratory stages, from stopover to endurance flight, in which songbirds use magnetic cues for their migratory decisions and incorporate this into a novel conceptual framework. While we lack studies examining whether and when magnetic cues affect orientation or navigation decisions during flight, the role of magnetic cues during stopover is relatively well studied, but mostly in the laboratory. Notably, many such studies have produced contradictory results so that understanding the biological importance of magnetic cues for decisions in free-flying songbirds is not straightforward. One potential explanation is that reproducibility of magnetic-cue experiments is low, probably because variability in the behavioural responses of birds among experiments is high. We are convinced that parts of this variability can be explained by species-specific and context-dependent reactions of birds to the study conditions and by the bird's high flexibility in whether they include magnetic cues in a decision or not. Ultimately, this review should help researchers in the challenging field of magnetoreception to design experiments meticulously and interpret results of such studies carefully by considering the migration ecology of their focal species.

Sense of doubt: inaccurate and alternate locations of virtual magnetic displacements may give a distorted view of animal magnetoreception ability

Virtual magnetic displacements are used to examine the magnetoreceptive ability of animals by changing the local magnetic field to emulate one that exists elsewhere. This technique can be used to test whether animals use a magnetic map. The viability of a magnetic map is dependant upon which magnetic parameters an animal's coordinate system is composed of, and how sensitive they are to those parameters. Previous research has not considered the degree to which sensitivity can change an animal's impression of where a virtual magnetic displacement is located. We re-assessed all published studies that use virtual magnetic displacements assuming the highest likely level of sensitivity to magnetic parameters in animals. The vast majority are susceptible to the existence of alternate possible virtual locations. In some cases, this can cause results to become ambiguous. We present a tool for visualising all possible virtual magnetic displacement alternative locations (ViMDAL) and propose changes to how further research on animal magnetoreception is conducted and reported.

The importance of time of day for magnetic body alignment in songbirds

Spontaneous magnetic alignment is the simplest known directional response to the geomagnetic field that animals perform. Magnetic alignment is not a goal directed response and its relevance in the context of orientation and navigation has received little attention. Migratory songbirds, long-standing model organisms for studying magnetosensation, have recently been reported to align their body with the geomagnetic field. To explore whether the magnetic alignment behaviour in songbirds is involved in the underlying mechanism for compass calibration, which have been suggested to occur near to sunset, we studied juvenile Eurasian reed warblers (Acrocephalus scirpaceus) captured at stopover during their first autumn migration. We kept one group of birds in local daylight conditions and an experimental group under a 2 h delayed sunset. We used an ad hoc machine learning algorithm to track the birds' body alignment over a 2-week period. Our results show that magnetic body alignment occurs prior to sunset, but shifts to a more northeast-southwest alignment afterwards. Our findings support the hypothesis that body alignment could be associated with how directional celestial and magnetic cues are integrated in the compass of migratory birds.

Flexibility and Control of Circadian Activity, Migratory Restlessness and Fueling in Two Songbird Migrants

Juvenile songbirds rely on an endogenous program, encoding direction, distance, fueling, and timing of migration. Migratory distance is species-specific, expressed as a period of migratory restlessness, for which the length is correlated with distance, while fueling is modified to meet anticipated flight distances controlled by geomagnetic cues and amount of day-light available for foraging. How daylength affect onset and level of migratory activity and fueling decisions in wild birds have so far received limited attention. Here we study how photoperiod controls onset, level and extent of autumn migratory activity and fueling in juvenile diurnally migrating dunnocks, and nocturnally migrating European robins by experimentally increasing daylength. For both species, we kept a control group indoors at the location of capture in southern Sweden exposed to the natural photoperiod, and an experimental group with increased and advanced photoperiod by 2 h in the morning. Dunnocks initiated migratory activity at sunrise (or artificial sunrise) in both groups, demonstrating a highly responsive and flexible component for the onset of migration triggered by light. Experimental robins anticipated the end of nocturnal migratory activity predicting the earlier sunrise immediately after the time-shift and expressed this behavior already under darkness, supporting a fast-resetting mechanism to the new diel period. Timing of end of morning activity was not affected by the earlier sunrise in both species, suggesting a fixed endogenous control that persisted throughout the 13-day study period. Experimental dunnocks expressed higher overall activity and lower fuel loads than controls, while robins did not change their overall activity and fuel load in response to the shifted and increased photoperiod. These results reveal important adaptations for circadian timekeeping including both a flexible onset open to fast modifications and a more rigid end, with differential effects by the treatment on migratory activity and fueling in the two species.

Magnetic storms disrupt nocturnal migratory activity in songbirds

Birds possess a magnetic sense and rely on the Earth's magnetic field for orientation during migration. However, the geomagnetic field can be altered by solar activity at relative unpredictable intervals. How birds cope with the temporal geomagnetic variations caused by solar storms during migration is still unclear. We addressed this question by reproducing the effect of a solar storm on the geomagnetic field and monitoring the activity of three songbird species during autumn migration. We found that only the European robin reduced nocturnal migratory restlessness in response to simulated solar storms. At the same time, robins increased activity during early morning. We suggest that robins reduced activity at night when the perception of magnetic information would be strongly disrupted by temporal variations of the magnetic field, to extend their migration during daytime when several visual cues become available for orientation. The other two species, chiffchaff and dunnock, showing low or no nocturnal migratory activity, did not respond to the solar storm by changing activity.

Magnetic body alignment in migratory songbirds: a computer vision approach

Several invertebrate and vertebrate species have been shown to align their body relative to the geomagnetic field. Many hypotheses have been proposed to explain the adaptive significance of magnetic body alignment outside the context of navigation. However, experimental evidence to investigate alternative hypotheses is still limited. We present a new setup to track the preferential body alignment relative to the geomagnetic field in captive animals using computer vision. We tested our method on three species of migratory songbirds and provide evidence that they align their body with the geomagnetic field. We suggest that this behaviour is involved in the underlying mechanism for compass orientation and calibration, which may occur near to sunrise and sunset periods. Our method could easily be extended to other species and used to test a large set of hypotheses to explain the mechanisms behind the magnetic body alignment and the magnetic sense in general.

Emlen funnel experiments revisited: methods update for studying compass orientation in songbirds

Migratory songbirds carry an inherited capacity to migrate several thousand kilometers each year crossing continental landmasses and barriers between distant breeding sites and wintering areas. How individual songbirds manage with extreme precision to find their way is still largely unknown. The functional characteristics of biological compasses used by songbird migrants has mainly been investigated by recording the birds directed migratory activity in circular cages, so‐called Emlen funnels. This method is 50 years old and has not received major updates over the past decades. The aim of this work was to compare the results from newly developed digital methods with the established manual methods to evaluate songbird migratory activity and orientation in circular cages.

We performed orientation experiments using the European robin (Erithacus rubecula) using modified Emlen funnels equipped with thermal paper and simultaneously recorded the songbird movements from above. We evaluated and compared the results obtained with five different methods. Two methods have been commonly used in songbirds’ orientation experiments; the other three methods were developed for this study and were based either on evaluation of the thermal paper using automated image analysis, or on the analysis of videos recorded during the experiment.

The methods used to evaluate scratches produced by the claws of birds on the thermal papers presented some differences compared with the video analyses. These differences were caused mainly by differences in scatter, as any movement of the bird along the sloping walls of the funnel was recorded on the thermal paper, whereas video evaluations allowed us to detect single takeoff attempts by the birds and to consider only this behavior in the orientation analyses. Using computer vision, we were also able to identify and separately evaluate different behaviors that were impossible to record by the thermal paper.

The traditional Emlen funnel is still the most used method to investigate compass orientation in songbirds under controlled conditions. However, new numerical image analysis techniques provide a much higher level of detail of songbirds’ migratory behavior and will provide an increasing number of possibilities to evaluate and quantify specific behaviors as new algorithms will be developed.