Adaptive drift and barrier-avoidance by a fly-forage migrant along a climate-driven flyway

Abstract

Background Route choice and travel performance of fly-forage migrants are partly driven by large-scale habitat availability, but it remains unclear to what extent wind support through large-scale wind regimes moulds their migratory behaviour. We aimed to determine to what extent a trans-equatorial fly-forage migrant engages in adaptive drift through distinct wind regimes and biomes across Africa. The Inter-tropical Front (ITF) marks a strong and seasonally shifting climatic boundary at the thermal equator, and we assessed whether migratory detours were associated with this climatic feature. Furthermore, we sought to disentangle the influence of wind and biome on daily, regional and seasonal travel performance. Methods We GPS-tracked 19 adult Eleonora’s falcons Falco eleonorae from the westernmost population on the Canary Islands across 39 autumn and 36 spring migrations to and from Madagascar. Tracks were annotated with wind data to assess the falcons’ orientation behaviour and the wind support they achieved in each season and distinct biomes. We further tested whether falcon routes across the Sahel were correlated with the ITF position, and how realized wind support and biome affect daily travel times, distances and speeds. Results Changes in orientation behaviour across Africa’s biomes were associated with changes in prevailing wind fields. Falcons realized higher wind support along their detours than was available along the shortest possible route by drifting through adverse autumn wind fields, but compromised wind support while detouring through supportive spring wind fields. Movements across the Sahel-Sudan zone were strongly associated to the ITF position in autumn, but were more individually variable in spring. Realized wind support was an important driver of daily travel speeds and distances, in conjunction with regional wind-independent variation in daily travel time budgets. Conclusions Although daily travel time budgets of falcons vary independently from wind, their daily travel performance is strongly affected by orientation-dependent wind support. Falcons thereby tend to drift to minimize or avoid headwinds through opposing wind fields and over ecological barriers, while compensating through weak or supportive wind fields and over hospitable biomes. The ITF may offer a climatic leading line to fly-forage migrants in terms of both flight and foraging conditions.

DynamoVis 1.0: an exploratory data visualization software for mapping movement in relation to internal and external factors

“…As turkey vultures often do, traveling over long distances limits time for rest and feeding, highlighting the importance of low-cost soaring. Studying movement patterns helps to reveal how turkey vultures rely on thermal corridors to reduce the energy cost of flying [ 7 , 25 , 26 ], whereas other species may be more dependent on other environmental conditions such as prevailing wind speeds [ 27 ]. For example, Galapagos Albatrosses rely on wind to optimize their return flights to Galapagos after foraging along the Peruvian coast [ 4 ].…”

Wind effects on the long‐distance migration of GPS‐tracked adult ospreys Pandion haliaetus from Germany

“…However, McLaren et al (2012) predicted vector‐oriented migrants will select winds that result in small drift angles. This indicates that ospreys sometimes save energy reserves by allowing for drift at weak sidewinds, which blow a bird off track only slightly, and deal with the resulting small detours, similarly to honey buzzards Pernis apivorus , which drift over the Sahara desert to save energy reserves (Vansteelant et al 2017b), and even to non‐soaring migrants like Eleonora's falcons Falco eleonorae , which allow for detours to utilize supportive winds (Vansteelant et al 2021). By contrast, strong sidewinds were compensated to avoid large detours.…”

Rush or relax: migration tactics of a nocturnal insectivore in response to ecological barriers

“…Two position estimates from the East Asia/East African Flyway, one categorized as “Boreal Forest” and one as “(Sub)Tropical Moist Broadleaf Forest” were excluded from further analyses since these were the only recorded position estimates in these biomes. In further analyses we distinguished between two categories of biomes, namely ecological barriers, and hospitable biomes ( sensu 39 ). The ecological barriers include the “Desert and Xeric scrub” and the “Mediterranean Forest, Woodland and Scrub” biome as well as the “Tropical Moist Broadleaf Forest” biome which is presumed to be a soft ecological barrier for migrating nightjars 34 .…”