Overland and oversea migration of white storks through the water barriers of the straits of Gibraltar

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.

Risk-sensitive response of soaring birds to crosswind over dangerous sea highlights age-specific differences in migratory performance

“…Both age classes increased airspeed for increasing easterly crosswind speed (figure 6), which is in accordance with theoretical predictions for optimal flight [17] and empirical results from other bird species and bats [13,24,33,34]. However, this similar pattern may have resulted from two different sea-crossing strategies.…”

“…Once crossing was initiated, adults compensated for winds with an east speed component but drifted under winds with a west speed component (figure 3). This contrasting behaviour seems to indicate that adult birds perceive easterly crosswinds as a higher risk for their survival owing to their high speed and steady direction (figure 1), having the potential to drag them out over the open Atlantic Ocean or to force long crossings dependent on powered flight [11][12][13]. One should note, however, that sideways speed was ubiquitously lower than crosswind speed (i.e.…”

Drastic reduction of the population distribution of White Storks predicted in absence of landfills

“…Even when White Storks show some tolerance to human presence, as demonstrated by their habit of constructing nests on buildings, this species is not typically found in densely populated urban areas (Hmamouchi et al 2020). High human densities induce physiological stress in White Storks, and the high percentage of impervious cover in massive cities reduces their access to food resources (Garroway and Sheldon 2013, Lowry et al 2013, Blas et al 2018.…”

White stork movements reveal the ecological connectivity between landfills and different habitats

“…In order to standardize sampling effort, we filtered positions separated by more than four minutes and less than 61 min. We also filtered the dataset by deleting position fixes with speeds higher than 100 km/h, because we assume that it is unrealistic that a given stork can fly so fast [ 11 ]. After applying these filters, we re-calculated distance, time difference and speed between consecutive fixes.…”

“…We defined a direct flight from one node to another as the subset of consecutive positions during flight (fixes with speed > 10 km/h), beginning in the first node with speed < 10 km/h and ending in a different node with speed < 10 km/h. To filter a non-stop flight, we used 10 km/h as the cut-off criterion above which we assume that a given white stork is flying [ 11 , 38 ]. Direct flights with any time difference between fixes higher than one hour were removed, to exclude trajectories with long data gaps.…”