Negotiating an ecological barrier: crossing the Sahara in relation to winds by common swifts

Abstract

One contribution of 17 to a theme issue 'Moving in a moving medium: new perspectives on flight'. The Sahara Desert is one of the largest land-based barriers on the Earth, crossed twice each year by billions of birds on migration. Here we investigate how common swifts migrating between breeding sites in Sweden and wintering areas in sub-Saharan Africa perform the desert crossing with respect to route choice, winds, timing and speed of migration by analysing 72 geolocator tracks recording migration. The swifts cross western Sahara on a broad front in autumn, while in spring they seem to use three alternative routes across the Sahara, a western, a central and an eastern route across the Arabian Peninsula, with most birds using the western route. The swifts show slower migration and travel speeds, and make longer detours with more stops in autumn compared with spring. In spring, the stopover period in West Africa coincided with mostly favourable winds, but birds remained in the area, suggesting fuelling. The western route provided more tailwind assistance compared with the central route for our tracked swifts in spring, but not in autumn. The ultimate explanation for the evolution of a preferred western route is presumably a combination of matching rich foraging conditions (swarming insects) and favourable winds enabling fast spring migration.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Wind‐associated detours promote seasonal migratory connectivity in a flapping flying long‐distance avian migrant

“…Similar detours via West Africa have been described in other species (e.g. Åkesson, Bianco, & Hedenström, ; Åkesson, Klaassen, Holmgren, Fox, & Hedenström, ; Trierweiler et al, ; Vansteelant, Shamoun‐Baranes, van Manen, van Diermen, & Bouten, ). Prevailing wind pattern over northern Africa is likely influencing the routes of individual migrants and the convergent evolution into flyways (Kranstauber et al, ; La Sorte et al, ).…”

Route simulations, compass mechanisms and long-distance migration flights in birds

“…In reality, terrestrial bird migrants regularly spend time at stopover for refuelling along the route (e.g., Schmaljohann et al 2012; Willemoes et al 2014), while only in extreme situations, they are forced to engage in long continuous flights (e.g., Gill et al 2009; Klaassen et al 2011; Åkesson et al 2016). In the simulations, birds are, furthermore, assumed to respond only to cues affecting their compass mechanisms (e.g., the geomagnetic field, stars, and the sun), and not to other local topography such as coastlines and mountain ridges that could affect their orientation.…”

“…In the simulations, birds are, furthermore, assumed to respond only to cues affecting their compass mechanisms (e.g., the geomagnetic field, stars, and the sun), and not to other local topography such as coastlines and mountain ridges that could affect their orientation. Coastlines could be used as visual cues for navigation and wind drift compensation at low flight altitudes (e.g., Åkesson 1993b; Bruderer and Liechti 1998; Hedenström and Åkesson 2016), while ecological barriers could require a change of strategy such, as, for example, crossing deserts in the fastest possible way (e.g., Åkesson et al 2016), or by performing detours to avoid large inhospitable regions such as oceans or large mountains (e.g., Gudmundsson et al 1995). …”

“…Most of the migrations, however, occur at much higher altitudes (>1000 m asl; e.g., Able 1970; Bruderer and Liechti 1998; Zehnder et al 2001), where they may be less affected by topography (Zehnder et al 2001; Nilsson et al 2014). Migratory birds may selectively depart on migration flights in tailwind conditions (Åkesson and Hedenström 2000; Green 2004; Sjöberg et al 2015; Åkesson et al 2016), and perform high altitude migration in light tailwind conditions (e.g., Richardson 1978, 1990; Zehnder et al 2001), as tailwinds are expected to reduce the transportation cost substantially depending on level of support from the wind (e.g., Alerstam 1976; Richardson 1978). Tailwinds might even be a prerequisite for successful migration in some bird migration systems (Piersma and Jukema 1990; Butler et al1997, Green et al 2004; Gill et al 2014; but see Hedenström and Weber 1999).…”

Timing avian long-distance migration: from internal clock mechanisms to global flights

“…Birds may, however, respond opportunistically to winds on migration across the Sahara resulting in extended flight periods in tailwind conditions [125][126][127] . Winds and food availability seem to be important for common swifts Apus apus crossing the Sahara 128 . The timing of spring passage for this species is highly synchronous between populations as compared to autumn when populations differ in departure date and time taken for the passage.…”