Orientation Cage and Release Experiments with Migratory Wheatears (Oenanthe oenanthe) in Scandinavia and Greenland: The Importance of Visual Cues

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 non-breeding areas independently using one or more inherited compass headings and, potentially, magnetic signposts to gauge where to switch between compass headings. Inherited magnetic-based migration has not yet been assessed as a population-level process, particularly across strong geomagnetic gradients or where long-term geomagnetic shifts (hereafter, secular variation) could create mismatches with magnetic headings. Therefore, it remains unclear whether inherited magnetic headings and signposts could potentially adapt to secular variation under natural selection.

METHODS

To address these unknowns, we modelled migratory orientation programs 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, including intrinsic (stochastic) variability in inheritance. Using the model, we assessed robustness of trans-hemispheric migration of a migratory songbird whose Nearctic breeding grounds have undergone rapid secular variation (mean 34° clockwise drift in declination, 1900-2023), and which travels across strong geomagnetic gradients via Europe to Africa.

RESULTS

Model-evolved magnetic-signposted migration was overall successful throughout the 124-year period, with 60-90% mean successful arrival across a broad range in plausible precision in compass headings and gauging signposts. Signposted migration reduced trans-Atlantic flight distances and was up to twice as successful compared with non-signposted migration. Magnetic headings shifted plastically in response to the secular variation (mean 16°-17° among orientation programs), whereas signpost latitudes were more constrained (3°-5° mean shifts). This plasticity required intrinsic variability in inheritance (model-evolved σ ≈ 2.6° standard error), preventing clockwise secular drift from causing unsustainable open-ocean flights.

CONCLUSIONS

Our study supports the potential long-term viability of inherited magnetic migratory headings and signposts, and illustrates more generally how inherited migratory orientation programs can both mediate and constrain evolution of routes, in response to global environmental change.

Quantitative species-level ecology of reef fish larvae via metabarcoding

The larval pool of coral reef fish has a crucial role in the dynamics of adult fish populations. However, large-scale species-level monitoring of species-rich larval pools has been technically impractical. Here, we use high-throughput metabarcoding to study larval ecology in the Gulf of Aqaba, a region that is inhabited by >500 reef fish species. We analysed 9,933 larvae from 383 samples that were stratified over sites, depth and time. Metagenomic DNA extracted from pooled larvae was matched to a mitochondrial cytochrome c oxidase subunit I barcode database compiled for 77% of known fish species within this region. This yielded species-level reconstruction of the larval community, allowing robust estimation of larval spatio-temporal distributions. We found significant correlations between species abundance in the larval pool and in local adult assemblages, suggesting a major role for larval supply in determining local adult densities. We documented larval flux of species whose adults were never documented in the region, suggesting environmental filtering as the reason for the absence of these species. Larvae of several deep-sea fishes were found in shallow waters, supporting their dispersal over shallow bathymetries, potentially allowing Lessepsian migration into the Mediterranean Sea. Our method is applicable to any larval community and could assist coral reef conservation and fishery management efforts.

Route simulations, compass mechanisms and long-distance migration flights in birds

Bird migration has fascinated humans for centuries and routes crossing the globe are now starting to be revealed by advanced tracking technology. A central question is what compass mechanism, celestial or geomagnetic, is activated during these long flights. Different approaches based on the geometry of flight routes across the globe and route simulations based on predictions from compass mechanisms with or without including the effect of winds have been used to try to answer this question with varying results. A major focus has been use of orthodromic (great circle) and loxodromic (rhumbline) routes using celestial information, while geomagnetic information has been proposed for both a magnetic loxodromic route and a magnetoclinic route. Here, we review previous results and evaluate if one or several alternative compass mechanisms can explain migration routes in birds. We found that most cases could be explained by magnetoclinic routes (up to 73% of the cases), while the sun compas s could explain only 50%. Both magnetic and geographic loxodromes could explain <25% of the routes. The magnetoclinic route functioned across latitudes (1°S-74°N), while the sun compass only worked in the high Arctic (61-69°N). We discuss the results with respect to orientation challenges and availability of orientation cues.

Modelling migration: the clock-and-compass model can explain the distribution of ringing recoveries

The aim of the present study was to test whether the directional distributions found in ringing recoveries could be explained if night-migrating passerines use only a simple clock-and-compass strategy during autumn migration. I developed a mathematical model that predicts the expected directional distributions as a function of distance covered by birds using a clock-and-compass strategy. The predictions were compared with data from natural migration: all ringing recoveries of pied flycatchers, Ficedula hypoleuca, ringed in Scandinavia and recovered elsewhere in Europe and North Africa within the same autumn. The predictions of the model correlated strongly with the distribution of ringing recoveries. This suggests that at least young pied flycatchers, and perhaps night-migrating passerines in general, use a simple clock-and-compass strategy to reach their wintering area. If so, all the prerequisites (a compass and an internal clock) that they need to orient during migration seem to be known at present, at least at the behavioural level, and navigational abilities or any elusive map-sense are not needed for migration. Copyright 1998 The Association for the Study of Animal Behaviour.

Experimental and analytical techniques used in bird orientation research

Our present understanding of orientation behaviour in birds is based on a broad array of observational, experimental and analytical (statistical) techniques, which are briefly reviewed here. As an extremely productive model the homing behaviour of pigeons has allowed especially diverse experimental manipulations documenting the involvement of magnetic, visual and olfactory cues in orientation. Work with migratory birds has profited greatly from the design of several kinds of orientation cages, now commonly used, and from hand-rearing test birds under controlled conditions. Free-flying birds, especially on long-distance migration, are still least amenable to study, but radio transmitter technology is providing important new opportunities in this respect. In general, the most valuable studies have been those involving the ontogenetic development of orientation, and those combining several methods of investigation. Some suggestions for further experiments are made.