How might magnetic secular variation impact avian philopatry?

Abstract

A tendency to return to the natal/breeding site, ‘philopatry’, is widespread amongst migratory birds. It has been suggested that a magnetic ‘map’ could underpin such movements, though it is unclear how a magnetic map might be impacted by gradual drift in the Earth’s magnetic field (‘secular variation’). Here, using the International Geomagnetic Reference Field, we quantified how secular variation translates to movement in the implied positions at which combinations of different magnetic cues (inclination, declination and intensity) intersect, noting that the magnitude of such movements is determined by the magnitude of the movements of each of the two isolines, and the angle between their movement vectors. We propose that magnetic parameters varying in a near-parallel arrangement are unlikely to be used as a bi-coordinate map during philopatry, but that birds could use near-orthogonal magnetic gradient cues as a bi-coordinate map if augmented with navigation using more local cues. We further suggest that uni-coordinate magnetic information could also provide a philopatry mechanism that is substantially less impacted by secular variation than a bi-coordinate ‘map’. We propose that between-year shifts in the position of magnetic coordinates might provide a priori predictions for changes in the breeding sites of migratory birds.

Collective movement as a solution to noisy navigation and its vulnerability to population loss

“…Since there are no known sensory modalities that can provide this level of precision over such a large geographical scale, our model suggests that agents would still benefit from collective navigation strategies. Nevertheless, it would be difficult for an animal to rely exclusively on the magnetic field for a multi-year migration since the geomagnetic field drifts over time [29]. This would indicate that either an alternative sensory cue is still necessary, or that the high variance in individual's responses to magnetic cues is an evolutionary adaptation to ensure the success of some individuals despite secular drift [30].…”

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

“…Furthermore, viewing magnetic parameters as context-specific indicators 43 , or step-wise cues 44 , rather than coordinates on a map, may yield further breakthroughs in understanding how animals sense and use magnetic fields. It is also paramount to keep in mind that magnetic parameter values, and the relationships between them, fluctuate over time 45 . Finally, we also want to highlight that there may be a need for further investigation not only in the sensitivity of animals to magnetic fields, but in also which components of the field are detectable, and to what extent a detector may be sensitive to discrete x , y and z components.…”

Naive songbirds show seasonally appropriate spring orientation in the laboratory despite having never completed first migration

“…a latitudinal cue without a longitudinal counterpart) would be adequate for philopatry, with magnetic inclination specifically postulated as a possible candidate for such a uni-coordinate ‘stop sign’ mechanism [ 3 , 4 , 11 ]. The use of uni-coordinate information, magnetic or otherwise, is substantially less susceptible to year-on-year (secular) variation in the position denoted by cue values, owing to the potentially multiplicative effects of secular variation in multiple cues [ 16 , 17 ]. However, such a mechanism relies on the presence of an inherited vector to reduce ambiguity caused by multiple sites sharing the same cue value.…”

“…While a highly resolute positioning system would negate any requirement for an inherited programme, birds could in principle use learnt landmarks and spatial gradients to refine a course loosely determined by genetically inherited information [ 3 , 4 ]. In turn, this would reduce any reliance on the resolution at which learnt spatial information is remembered, which could be useful given that inter-annual variations in positions associated with spatial gradient cues can be substantial [ 16 , 17 ].…”