Artificial light at night confounds broad-scale habitat use by migrating birds.

With many of the world's migratory bird populations in alarming decline, broad-scale assessments of responses to migratory hazards may prove crucial to successful conservation efforts. Most birds migrate at night through increasingly light-polluted skies. Bright light sources can attract airborne migrants and lead to collisions with structures, but might also influence selection of migratory stopover habitat and thereby acquisition of food resources. We demonstrate, using multi-year weather radar measurements of nocturnal migrants across the northeastern U.S., that autumnal migrant stopover density increased at regional scales with proximity to the brightest areas, but decreased within a few kilometers of brightly-lit sources. This finding implies broad-scale attraction to artificial light while airborne, impeding selection for extensive forest habitat. Given that high-quality stopover habitat is critical to successful migration, and hindrances during migration can decrease fitness, artificial lights present a potentially heightened conservation concern for migratory bird populations.

Stop early to travel fast: modelling risk-averse scheduling among nocturnally migrating birds.

Many migrating birds divide their journeys into nocturnal flights interspersed by stopovers where they build up energy reserves (fuel) for subsequent flights. Given the difficulty in monitoring fuel loads of individual migrants over long distances, theoretical models are often used to interpret observed relations between departure fuel loads (DFLs) and fuel deposition rates (FDRs) in the context of time-minimised migration. Models applicable to nocturnal migration have hitherto considered only the departure decision, i.e. ignored interim stopover possibilities before fuel loads are depleted. This results in 'risk-prone' migratory schedules in terms of choice of stopover. In this study we assess 'risk-averse' behaviour, whereby nocturnal migrants minimise migration time by adjusting not only DFLs to experienced and expected FDRs, but also stopping fuel loads (SFLs), below which they stop to refuel at high-quality sites. We developed analytical formulae to solve for maximal risk-prone and risk-averse migration speeds in modelled environments comprised of two stopover qualities (high- and low-quality), and a constant probability of encountering a high-quality site (encounter probability). Risk-aversion was beneficial to migration speeds in over 99% of the modelled environments, with median ratios of risk-averse to risk-prone migration speeds ranging from 1.5 to 2.8. Among modelled environments, this benefit increased with increasing FDRs at high-quality sites, and was highest with low probabilities of encountering high-quality sites. Time-minimising risk-averse DFLs at low-quality sites were minimal in nearly all modelled environments, and whenever encounter probabilities were low, risk-averse SFLs typically indicated that migrants should stop immediately on encountering high-quality sites. Modelled fuel loads in environments with high and low encounter probabilities resembled observed fuel loads of migratory populations with ubiquitous and scarce food availability, respectively. This study demonstrates the benefits of risk-averse exploitation of variable resources, presents a new approach to model time-minimised migration in heterogeneous environments and emphasises the importance of premium stopover sites to nocturnally migrating birds.

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.

Across atoms to crossing continents: Application of similarity measures to biological location data.

Biological processes involve movements across all measurable scales. Similarity measures can be applied to compare and analyze these movements but differ in how differences in movement are aggregated across space and time. The present study reviews frequently-used similarity measures, such as the Hausdorff distance, Fréchet distance, Dynamic Time Warping, and Longest Common Subsequence, jointly with several measures less used in biological applications (Wasserstein distance, weak Fréchet distance, and Kullback-Leibler divergence), and provides computational tools for each of them that may be used in computational biology. We illustrate the use of the selected similarity measures in diagnosing differences within two extremely contrasting sets of biological data, which, remarkably, may both be relevant for magnetic field perception by migratory birds. Specifically, we assess and discuss cryptochrome protein conformational dynamics and extreme migratory trajectories of songbirds between Alaska and Africa. We highlight how similarity measures contrast regarding computational complexity and discuss those which can be useful in noise elimination or, conversely, are sensitive to spatiotemporal scales.

Quantifying flow-assistance and implications for movement research.

The impact that flows of air and water have on organisms moving through these environments has received a great deal of attention in theoretical and empirical studies. There are many behavioral strategies that animals can adopt to interact with these flows, and by assuming one of these strategies a researcher can quantify the instantaneous assistance an animal derives from a particular flow. Calculating flow-assistance in this way can provide an elegant simplification of a multivariate problem to a univariate one and has many potential uses; however, the resultant flow-assistance values are inseparably linked to the specific behavioral strategy assumed. We expect that flow-assistance may differ considerably depending on the behavioral strategy assumed and the accuracy of the assumptions associated with that strategy. Further, we expect that the magnitude of these differences may depend on the specific flow conditions. We describe equations to quantify flow-assistance of increasing complexity (i.e. more assumptions), focusing on the behavioral strategies assumed by each. We illustrate differences in suggested flow-assistance between these equations and calculate the sensitivity of each equation to uncertainty in its particular assumptions for a range of theoretical flow conditions. We then simulate trajectories that occur if an animal behaves according to the assumptions inherent in these equations. We find large differences in flow-assistance between the equations, particularly with increasing lateral flow and increasingly supportive axial flow. We find that the behavioral strategy assumed is generally more influential on the perception of flow-assistance than a small amount of uncertainty in the specification of an animal's speed (i.e. <5 ms(-1)) or preferred direction of movement (i.e. <10°). Using simulated trajectories, we show that differences between flow-assistance equations can accumulate over time and distance. The appropriateness and potential biases of an equation to quantify flow-assistance, and the behavioral assumptions the equation implies, must be considered in the context of the system being studied, particularly when interpreting results. Thus, we offer this framework for researchers to evaluate the suitability of a particular flow-assistance equation and assess the implications of its use.

Predicting performance of naïve migratory animals, from many wrongs to self-correction.

Migratory orientation of many animals is inheritable, enabling inexperienced (naïve) individuals to migrate independently using a geomagnetic or celestial compass. It remains unresolved how naïve migrants reliably reach remote destinations, sometimes correcting for orientation error or displacement. To assess naïve migratory performance (successful arrival), we simulate and assess proposed compass courses for diverse airborne migratory populations, accounting for spherical-geometry effects, compass precision, cue transfers (e.g., sun to star compass), and geomagnetic variability. We formulate how time-compensated sun-compass headings partially self-correct, according to how inner-clocks are updated. For the longest-distance migrations simulated, time-compensated sun-compass courses are most robust to error, and most closely resemble known routes. For shorter-distance nocturnal migrations, geomagnetic or star-compass courses are most robust, due to not requiring nightly cue-transfers. Our predictive study provides a basis for assessment of compass-based naïve migration and mechanisms of self-correction, and supports twilight sun-compass orientation being key to many long-distance inaugural migrations.

Optimal orientation in flows: providing a benchmark for animal movement strategies.

Animal movements in air and water can be strongly affected by experienced flow. While various flow-orientation strategies have been proposed and observed, their performance in variable flow conditions remains unclear. We apply control theory to establish a benchmark for time-minimizing (optimal) orientation. We then define optimal orientation for movement in steady flow patterns and, using dynamic wind data, for short-distance mass movements of thrushes (Turdus sp.) and 6000 km non-stop migratory flights by great snipes, Gallinago media. Relative to the optimal benchmark, we assess the efficiency (travel speed) and reliability (success rate) of three generic orientation strategies: full compensation for lateral drift, vector orientation (single-heading movement) and goal orientation (continually heading towards the goal). Optimal orientation is characterized by detours to regions of high flow support, especially when flow speeds approach and exceed the animal's self-propelled speed. In strong predictable flow (short distance thrush flights), vector orientation adjusted to flow on departure is nearly optimal, whereas for unpredictable flow (inter-continental snipe flights), only goal orientation was near-optimally reliable and efficient. Optimal orientation provides a benchmark for assessing efficiency of responses to complex flow conditions, thereby offering insight into adaptive flow-orientation across taxa in the light of flow strength, predictability and navigation capacity.

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.