Variable detours in long‐distance migration across ecological barriers and their relation to habitat availability at ground

Abstract

Migration detours, the spatial deviation from the shortest route, are a widespread phenomenon in migratory species, especially if barriers must be crossed. Moving longer distances causes additional efforts in energy and time, and to be adaptive, this should be counterbalanced by favorable condition en route. We compared migration patterns of nightingales that travelled along different flyways from their European breeding sites to the African nonbreeding sites. We tested for deviations from shortest routes and related the observed and expected routes to the habitat availability at ground during autumn and spring migration. All individuals flew detours of varying extent. Detours were largest and seasonally consistent in western flyway birds, whereas birds on the central and eastern flyways showed less detours during autumn migration, but large detours during spring migration (eastern flyway birds). Neither migration durations nor the time of arrival at destination were related to the lengths of detours. Arrival at the breeding site was nearly synchronous in birds flying different detours. Flying detours increased the potential availability of suitable broad-scale habitats en route only along the western flyway. Habitat availability on observed routes remained similar or even decreased for individuals flying detours on the central or the eastern flyway as compared to shortest routes. Thus, broad-scale habitat distribution may partially explain detour performance, but the weak detour-habitat association along central and eastern flyways suggests that other factors shape detour extent regionally. Prime candidate factors are the distribution of small suitable habitat patches at local scale as well as winds specific for the region and altitude.

Seasonal detours by soaring migrants shaped by wind regimes along the East Atlantic Flyway

“…However, field observations indicate the westward detour is used by many other migrant birds (Moreau ; Gatter ), at least partly because there are more feeding opportunities in the western Sahel, compared to the central Sahelian zone (Hahn et al . ; Åkesson, Bianco & Hedenström ). Ultimately, comparative tracking research is needed to understand what environmental trade‐offs shape complex migration routes in species that differ in their foraging habits, movement capacity and navigational abilities (Nathan et al .…”

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

“…Minimizing migration distance should result in strong migratory connectivity with separate populations using non-overlapping, parallel routes. En route migratory connectivity strength could be weakened when populations encounter variable environmental conditions and strive to maximize arrival condition or minimize time, as opposed to simply minimizing migration distance (Alerstam 2001, Hahn et al 2014, Bayly et al 2017, Shamoun-Baranes et al 2017. suitable habitat; Buler et al 2007, Cohen et al 2017.…”

No apparent gain from continuing migration for more than 3000 kilometres: willow warblers breeding in Denmark winter across the entire northern Savannah as revealed by geolocators

“…Itinerancy is thought to be a response to changing food conditions though direct links such as in raptors [23] are rarely documented. Tracking has revealed both sedentary behaviour [11, 15, 24, 25] and itinerant behaviour [26, 27] in species wintering in West and Central Africa.…”