Gauge-and-compass migration: inherited magnetic headings and signposts can adapt to changing geomagnetic landscapes

Background For many migratory species, inexperienced (naïve) individuals reach remote nonbreeding areas independently using one or more inherited compass headings and, potentially, magnetic signposts to gauge where to switch between compass headings. Inherited magnetic-based migratory orientation programs have not yet been assessed as a population-level process, particularly where strong geomagnetic spatial gradients or long-term shifts could create mismatches with inherited magnetic headings. In particular, it remains unstudied whether and how, under natural selection, inherited headings and signposts could potentially adapt to long-term geomagnetic shifts. Methods To address these unknowns, we modelled bird migration using an evolutionary algorithm incorporating global geomagnetic data (1900-2023). Modelled population mixing incorporated both natal dispersal and trans-generational inheritance of magnetic headings and signposts, the latter including intrinsic (stochastic) variability. Using the model, we assessed robustness of signposted and non-signposted trans-hemispheric songbird migration across a rapidly magnetically-shifting Nearctic breeding region (mean 34° declination shift) via Europe to Africa. Results Model-evolved magnetic-signposted migration was (i) overall successful throughout the 124-year period, with 60-90% mean successful arrival across a broad range in plausible compass precision, (ii) through reduced trans-Atlantic flight distances, up to twice as successful compared with non-signposted migration, but (iii) to avoid evolving unsustainable open-ocean flights, intrinsic variability in inheritance of magnetic headings was required (model-evolved σ ≈ 2.6° standard error in inherited headings). Conclusions Our study supports the potential long-term viability of inherited magnetic migratory headings and signposts, and illustrates more generally how inherited migratory programs can both mediate and constrain evolution of routes, in response to global environmental change.