Adaptive cross-country optimisation strategies in thermal soaring birds

TL;DR

This study analyzes how different soaring bird species adapt their flight strategies based on morphology, confirming aerodynamic principles and revealing universal behavioral rules that could inspire autonomous glider technology.

Contribution

The paper provides a comprehensive analysis of morphology-related differences in cross-country soaring strategies across 12 bird species, supported by high-frequency tracking data.

Findings

Higher wing loading correlates with faster flight and larger turning radius.

Thermal strength influences the adaptivity of flight strategies.

Universal rules govern cross-country soaring behaviors across species.

Abstract

Thermal soaring enables birds to perform cost-efficient flights during foraging or migration trips. Yet, although all soaring birds exploit vertical winds effectively, this group contains species that vary strongly in their morphologies. Aerodynamic rules dictate the costs and benefits of flight, but, depending on their ecological needs, species may use different behavioural strategies. To quantify these morphology-related differences in behavioural cross-country strategies, we compiled and analysed a large dataset, which includes data from over a hundred individuals from 12 soaring species recorded with high frequency tracking devices. We quantified the performance during thermalling and gliding flights, and the overall cross-country behaviour that is the combination of both. Our results confirmed aerodynamic theory across the 12 species; species with higher wing loading typically flew faster, and consequently turned on a larger radius, than lighter ones. Furthermore, the combination of circling radius and minimum sink speed determines the maximum benefits soaring birds can obtain from thermals. Also, we observed a spectrum of strategies regarding the adaptivity to thermal strength and uncovered a universal rule for cross-country strategies for all analysed species. Finally, our newly described behavioural rules can provide inspirations for technical applications, like the development of autopilot systems for autonomous robotic gliders.