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

Abstract

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'.

Keywords: barrier crossing; common swift; migration; migration routes; the Sahara; wind assistance.

Figure 1.

Map of stopover areas before initiating migration across the Sahara Desert (triangles), stopover areas on passage (filled yellow circles) and stopover or final wintering areas at arrival after crossing the barrier (squares), for different populations of common swifts breeding in north, central and south Sweden as recorded for spring and autumn by miniature geolocators. Solid lines are connecting routes for birds recorded outside equinox periods, while dashed lines connect starting and endpoints for swifts passing the Sahara during the equinox period. Lines connecting departure, stopover and arrival events simplify the assumed migratory pathway of the birds.

Figure 2.

Wind support on arrival at during stopover (median of the entire duration) and at departure from the stopover site before the Sahara crossing during both autumn and spring migration. (a) The boxplot of the birds' tailwind component for each pressure level (and corresponding altitude) and for the optimal level, that is the resultant tailwind if each bird was able to select the pressure level that provided the best wind condition along its own departure directions (presented in electronic supplementary material, figure S4). (b) The pressure layers that provided the optimal level included in upper panel. When the winds are not favourable, that is no tailwind opportunity available, the birds are forced to depart with the wind that provided with the lowest headwind (negative values in (a); shaded colour in (b)). To exclude birds that started the spring migration crossing from the wintering area, we included in the analysis only birds departing after a stopover duration not longer than 30 days (autumn n = 54, spring n = 33).

Figure 3.

Air-to-ground speed ratio (AGR) for two groups of birds departing from different locations (west and east, respectively) before the Sahara crossing in both the autumn and spring migrations (AGR < 1 represents wind support; AGR > 1 represents wind impediment). (a) As function of latitude the AGR at the pressure level that provided the highest wind assistance (see text; AGR for all pressure levels in electronic supplementary material, figure S7). Dots are AGR for all locations, solid lines are local polynomial regression fitting and the shaded grey area represents the extension of the Sahara Desert (16–35° N). (b) The boxplot of the mean AGR for each individual (autumn n = 21 and n = 23, spring n = 53 and n = 17, for west and east departure locations, respectively) during the Sahara crossing (i.e. while crossing the shaded area in the upper panel). Statistical tests between departing locations has been performed using linear mixed effects models to account for repeated measures of the same individuals and year of sampling (see text for details).