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.

Ghrelin, not corticosterone, is associated with transitioning of phenotypic states in a migratory Galliform

In both captive and free-living birds, the emergence of the migratory phenotype is signalled by rapid and marked increases in food intake and fuelling, as well as changes in amount of nocturnality or migratory restlessness. The metabolic hormone corticosterone and, as more recently suggested, the gut-derived hormone ghrelin have been suggested to play a role in mediating such phenomenal phenotypic flexibility given that they both regulate fuel metabolism and locomotion across vertebrate taxa. Here, using the Common quail (Coturnix coturnix) as our study species, we induced autumn migration followed by a non-migratory wintering phase through controlled changes in daylight. We thus compared plasma corticosterone and ghrelin concentrations between the two sampling phases and assessed whether these hormones might reflect the migratory state. While we found no differences in plasma corticosterone between the two sampling phases and no link of this hormone with changes in body mass, levels of food intake or migratory restlessness, the migratory birds had substantially higher levels of plasma ghrelin relative to the non-migratory birds. Furthermore, while ghrelin did not correlate with the gain in body mass over the entire pre-migratory fuelling phase (over an average of nine weeks preceding blood sampling), plasma ghrelin did positively correlate with the gain in body mass observed during the final fattening stages (over an average of three weeks preceding blood sampling). Again, variation in plasma ghrelin also reflected the amount of body mass depleted over both the long- and short-time frame as birds returned to their non-migratory baseline - lower levels of plasma ghrelin consistently correlated with larger losses in body mass. Thus, while our data do not highlight a role of the hormone corticosterone in sustaining pre-migratory fattening as shown in other bird species, they do add evidence for a potential role of ghrelin in mediating migratory behaviour and further suggest that this hormone might be important in regulating the transitioning of migratory states, possibly by promoting fuel mobilisation and usage.

Corticosterone's roles in avian migration: Assessment of three hypotheses

While corticosterone (CORT) is often suggested to be an important hormone regulating processes necessary for avian migration, there has been no systematic assessment of CORT's role in migration. Prior to migration, birds increase fat stores and flight muscle size to prepare for the high energetic costs associated with long-distance flight. After attaining sufficient energetic stores, birds then make the actual decision to depart from their origin site. Once en route birds alternate between periods of flight and stopovers, during which they rest and refuel for their next bouts of endurance flight. Here, we evaluate three non-mutually exclusive hypotheses that have been proposed in the literature for CORT's role in migration. (1) CORT facilitates physiological preparations for migration [e.g. hyperphagia, fattening, and flight muscle hypertrophy]. (2) CORT stimulates departure from origin or stopover sites. (3) CORT supports sustained migratory travel. After examining the literature to test predictions stemming from each of these three hypotheses, we found weak support for a role of CORT in physiological preparation for migration. However, we found moderate support for a role of CORT in stimulating departures, as CORT increases immediately prior to departure and is higher when migratory restlessness is displayed. We also found moderate support for the hypothesis that CORT helps maintain sustained travel, as CORT is generally higher during periods of flight, though few studies have tested this hypothesis. We provide recommendations for future studies that would help to further resolve the role of CORT in migration.

Geomagnetic field absence reduces adult body weight of a migratory insect by disrupting feeding behavior and appetite regulation

The geomagnetic field (GMF) is well documented for its essential role as a cue used in animal orientation or navigation. Recent evidence indicates that the absence of GMF (mimicked by the near-zero magnetic field, NZMF) can trigger stress-like responses such as reduced body weight, as we have previously shown in the brown planthopper, Nilaparvata lugens. In this study, we found that consistent with the significantly decreased body weight of newly emerged female (-14.67%) and male (-13.17%) adult N. lugens, the duration of the phloem ingestion feeding waveform was significantly reduced by 32.02% in 5th instar nymphs reared under the NZMF versus GMF. Interestingly, 5th instar nymphs that exhibited reduced feeding had significantly higher glucose levels (+16.98% and +20.05%; 24 h and 48 h after molting), which are associated with food aversion, and expression patterns of their appetite-related neuropeptide genes (neuropeptide F, down-regulated overall; short neuropeptide F, down-regulated overall; adipokinetic hormone, up-regulated overall; and adipokinetic hormone receptor, down-regulated overall) were also altered under the absence of GMF in a manner consistent with diminishing appetite. Moreover, the expressions of the potential magnetosensor cryptochromes (Crys) were found significantly altered under the absence of GMF, indicating the likely upstream signaling of the Cry-mediated magnetoreception mechanisms. These findings support the hypothesis that strong changes in GMF intensity can reduce adult body weight through affecting insect feeding behavior and underlying regulatory processes including appetite regulation. Our results highlight that GMF could be necessary for the maintenance of energy homeostasis in insects.

The physiology of movement

Movement, from foraging to migration, is known to be under the influence of the environment. The translation of environmental cues to individual movement decision making is determined by an individual's internal state and anticipated to balance costs and benefits. General body condition, metabolic and hormonal physiology mechanistically underpin this internal state. These physiological determinants are tightly, and often genetically linked with each other and hence central to a mechanistic understanding of movement. We here synthesise the available evidence of the physiological drivers and signatures of movement and review (1) how physiological state as measured in its most coarse way by body condition correlates with movement decisions during foraging, migration and dispersal, (2) how hormonal changes underlie changes in these movement strategies and (3) how these can be linked to molecular pathways. We reveale that a high body condition facilitates the efficiency of routine foraging, dispersal and migration. Dispersal decision making is, however, in some cases stimulated by a decreased individual condition. Many of the biotic and abiotic stressors that induce movement initiate a physiological cascade in vertebrates through the production of stress hormones. Movement is therefore associated with hormone levels in vertebrates but also insects, often in interaction with factors related to body or social condition. The underlying molecular and physiological mechanisms are currently studied in few model species, and show -in congruence with our insights on the role of body condition- a central role of energy metabolism during glycolysis, and the coupling with timing processes during migration. Molecular insights into the physiological basis of movement remain, however, highly refractory. We finalise this review with a critical reflection on the importance of these physiological feedbacks for a better mechanistic understanding of movement and its effects on ecological dynamics at all levels of biological organization.

Corticosterone, food intake and refueling in a long-distance migrant

Elevated baseline corticosterone levels function to mobilize energy in predictable life-history stages, such as bird migration. At the same time, baseline corticosterone has a permissive effect on the accumulation of fat stores (fueling) needed for migratory flight. Most migrants alternate flight bouts with stopovers, during which they replenish the fuel used during the preceding flight (refueling). The role of corticosterone in refueling is currently unclear. In a fasting-re-feeding experiment on northern wheatears (Oenanthe oenanthe) in autumn we found that baseline total and free corticosterone levels were negatively related with both food intake and the rate of fuel deposition after fasting. This confirms our earlier findings in wild conspecifics in spring and indicates that corticosterone does not stimulate stopover refueling. Whether the negative relationship between baseline corticosterone level and fuel deposition rate is causal is questionable, because within-individual comparison of corticosterone metabolite levels in droppings did not reveal differences between refueling and control periods. In other words, corticosterone does not appear to be down-regulated during refueling, which would be expected if it directly hampers refueling. We discuss possible correlates of corticosterone level that may explain the negative association between corticosterone and stopover refueling. Additionally, we found that fasting decreases total corticosterone level, which contrasts with previous studies. We propose that the difference is due to the other studies being conducted outside of the migration life-history stage, and provide a possible explanation for the decrease in corticosterone during fasting in migrating birds.

Wind selectivity and partial compensation for wind drift among nocturnally migrating passerines

A migrating bird's response to wind can impact its timing, energy expenditure, and path taken. The extent to which nocturnal migrants select departure nights based on wind (wind selectivity) and compensate for wind drift remains unclear. In this paper, we determine the effect of wind selectivity and partial drift compensation on the probability of successfully arriving at a destination area and on overall migration speed. To do so, we developed an individual-based model (IBM) to simulate full drift and partial compensation migration of juvenile Willow Warblers (Phylloscopus trochilus) along the southwesterly (SW) European migration corridor to the Iberian coast. Various degrees of wind selectivity were tested according to how large a drift angle and transport cost (mechanical energy per unit distance) individuals were willing to tolerate on departure after dusk. In order to assess model results, we used radar measurements of nocturnal migration to estimate the wind selectivity and proportional drift among passerines flying in SW directions. Migration speeds in the IBM were highest for partial compensation populations tolerating at least 25% extra transport cost compared to windless conditions, which allowed more frequent departure opportunities. Drift tolerance affected migration speeds only weakly, whereas arrival probabilities were highest with drift tolerances below 20°. The radar measurements were indicative of low drift tolerance, 25% extra transport cost tolerance and partial compensation. We conclude that along migration corridors with generally nonsupportive winds, juvenile passerines should not strictly select supportive winds but partially compensate for drift to increase their chances for timely and accurate arrival.

Are stress responses to geomagnetic storms mediated by the cryptochrome compass system?

A controversial body of literature demonstrates associations of geomagnetic storms (GMS) with numerous cardiovascular, psychiatric and behavioural outcomes. Various melatonin hypotheses of GMS have suggested that temporal variation in the geomagnetic field (GMF) may be acting as an additional zeitgeber (a temporal synchronizer) for circadian rhythms, with GMS somehow interfering with the hypothesized system. The cryptochrome genes are known primarily as key components of the circadian pacemaker, ultimately involved in controlling the expression of the hormone melatonin. Cryptochrome is identified as a clear candidate for mediating the effect of GMS on humans, demonstrating the prior existence of several crucial pieces of evidence. A distinct scientific literature demonstrates the widespread use of geomagnetic information for navigation across a range of taxa. One mechanism of magnetoreception is thought to involve a light-dependent retinal molecular system mediated by cryptochrome, acting in a distinct functionality to its established role as a circadian oscillator. There is evidence suggesting that such a magnetosense--or at least the vestiges of it--may exist in humans. This paper argues that cryptochrome is not acting as secondary geomagnetic zeitgeber to influence melatonin synthesis. Instead, it is hypothesized that the cryptochrome compass system is mediating stress responses more broadly across the hypothalamic-pituitary-adrenal (HPA) axis (including alterations to circadian behaviour) in response to changes in the GMF. Two conceptual models are outlined for the existence of such responses--the first as a generalized migrational/dispersal strategy, the second as a stress response to unexpected signals to the magnetosense. It is therefore proposed that GMS lead to disorientation of hormonal systems in animals and humans, thus explaining the effects of GMS on human health and behaviour.