Abstract
Soaring landbirds typically exploit atmospheric uplift as they fly overland, displaying a highly effective energy-saving locomotion. However, large water bodies lack thermal updrafts, potentially becoming ecological barriers that hamper migration. Here we assessed the effects of a sea surface on the migratory performance of GPS-tagged white storks (Ciconia ciconia) before, during and after they crossed the straits of Gibraltar. Oversea movements involved only flapping and gliding and were faster, traversed in straighter, descending trajectories and resulted in higher movement-related energy expenditure levels than overland, supporting the water barrier hypothesis. Overland movements at both sides of the sea straits resulted in tortuous routes and ascending trajectories with pre-crossing flights showing higher elevations and more tortuous routes than post-crossing, thus supporting the barrier negotiation hypothesis. Individual positions at both ends of the sea narrow were predicted by zonal winds and storks´ location at entry in the European hinterland, and birds did not show compensational movements overland in anticipation to subsequent wind displacements oversea. The length of the water narrow at departure shore, the elevation therein and the winds on route affected major components of sea crossing performance (such as distances and times overwater, minimum elevations, climb angles, speeds and energy expenditure), supporting the departure position and oversea winds hypotheses. In summary, our study provides a prime example at high temporal resolution of how birds adjust their behavior and physiology as they interact with the changing conditions of the travelling medium, reallocating resources and modifying their movement to overcome an ecological barrier.
Highlights
The evolution of terrestrial soaring has allowed birds to maximize their movement capabilities overland while minimizing energy costs[1,2], with large, broad-winged landbirds such as storks and raptors mastering this type of flight[3,4]
Storks travelled at higher speeds and displayed higher mean ODBA levels across the sea straits compared to overland (Fig. 2m–o, Table S2)
Some birds gained elevation at the early stages of the sea crossing, storks displayed an overall descending trajectory across the sea channel (Figs. 2j–k and S1), loosing on average more than 400 m and up to 1300 m overwater and displaying elevations as low as 22 m.a.s.l. upon reaching African shores
Summary
The evolution of terrestrial soaring has allowed birds to maximize their movement capabilities overland while minimizing energy costs[1,2], with large, broad-winged landbirds such as storks and raptors mastering this type of flight[3,4]. Since the atmosphere yields most of the energy required for this type of flight, the movement ecology of soaring landbirds is largely constrained by geography, sun and weather conditions determining uplift (e.g., thermals are generally lacking overwater, overnight and under inclement weather). The migratory movements of white storks (Ciconia ciconia) between Europe and Africa are largely constrained by the Mediterranean sea, which diverts migrations into Western and Eastern routes, respectively funneled through the sea straits of Gibraltar and Bosporus[10,11] where birds can be hesitant to initiate sea crossings, frequently wandering near the coast and often showing failed crossing attempts[3,12,13]. Overwater movements will involve (i) reduced or suppressed soaring, (ii) faster speeds, (iii) straighter routes, (iv) descent trajectories and (v) higher ODBA levels compared to overland movements
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