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

Åkesson, S., Bianco, G. & Hedenström, A. (2016). Negotiating an ecological barrier: crossing the Sahara in relation to winds by common swifts. Phil. Trans. R. Soc. B 371: 20150393. doi: 10.1098/rstb.2015.0393

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.

A Common Swift (Apus apus) equipped with a micro data logger that measures light (Susanne Åkesson / Lund University)

A Common Swift (Apus apus) equipped with a micro data logger that measures light (Susanne Åkesson / Lund University).

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.

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 (Åkesson et al. 2016 – DOI: 10.1098/rstb.2015.0393)

 

Wind effects on the migration routes of trans-Saharan soaring raptors

Vidal-Mateo, J., Mellone, U., López-López, P., De La Puente, J., García-Ripollés, C., Bermejo, A. & Urios, V. (2016). Wind effects on the migration routes of trans-Saharan soaring raptors: geographical, seasonal, and interspecific variation. Current Zoology 62: 89–97. doi: 10.1093/cz/zow008 (Open Access)

Abstract:

Wind is among the most important environmental factors shaping birds’ migration patterns. Birds must deal with the displacement caused by crosswinds and their behavior can vary according to different factors such as flight mode, migratory season, experience, and distance to goal areas. Here we analyze the relationship between wind and migratory movements of three raptor species which migrate by soaring–gliding flight: Egyptian vulture Neophron percnopterus, booted eagle Aquila pennata, and short-toed snake eagle Circaetus gallicus. We analyzed daily migratory segments (i.e., the path joining consecutive roosting locations) using data recorded by GPS satellite telemetry. Daily movements of Egyptian vultures and booted eagles were significantly affected by tailwinds during both autumn and spring migrations. In contrast, daily movements of short-toed eagles were only significantly affected by tailwinds during autumn migration. The effect of crosswinds was significant in all cases. Interestingly, Egyptian vultures and booted eagles showed latitudinal differences in their behavior: both species compensated more frequently at the onset of autumn migration and, at the end of the season when reaching their wintering areas, the proportion of drift segments was higher. In contrast, there was a higher drift at the onset of spring migration and a higher compensation at the end. Our results highlight the effect of wind patterns on the migratory routes of soaring raptors, with different outcomes in relation to species, season, and latitude, ultimately shaping the loop migration patterns that current tracking techniques are showing to be widespread in many long distance migrants.

Response of three migratory raptors to crosswinds in spring (upper panel) and autumn (lower panel). Egyptian vulture’s routes are shown in (A) and (D); booted eagle’s routes in (B) and (E); and short-toed snake eagle’s routes in (C) and (F). Colors indicate drift (green), compensation (blue), and overcompensation (orange) in daily segments

Response of three migratory raptors to crosswinds in spring (upper panel) and autumn (lower panel). Egyptian vulture’s routes are shown in (A) and (D); booted eagle’s routes in (B) and (E); and short-toed snake eagle’s routes in (C) and (F). Colors indicate drift (green), compensation (blue), and overcompensation (orange) in daily segments

Consistency in long-distance bird migration: contrasting patterns in time and space for two raptors

Vardanis, Y., Nilsson, J.-Å., Klaassen, R. H. G., Strandberg, R. & Alerstam, T. (2016). Consistency in long-distance bird migration: contrasting patterns in time and space for two raptors. Animal Behaviour 113: 177–187. doi: 10.1016/j.anbehav.2015.12.014
PDF in ResearGate.net

Abstract :

As the evolutionary responses to environmental change depend on selection acting on individual differences, disentangling within- and between-individual variation becomes imperative. In animal migration research, multiyear tracks are thus needed to estimate the individual consistency of phenotypic traits. Avian telemetry studies have recently provided the first evidence of individuality across space and time in animal migration. Here, we compare repeatability patterns of routes and timing between two migratory birds, the marsh harrier, Circus aeruginosus, and the osprey, Pandion haliaetus, as recorded by satellite tracking. We found interspecific contrasts with low repeatability in timing and duration and a high repeatability in routes for ospreys, but the reverse pattern for marsh harriers. This was mainly caused by (1) larger between-individual variation in routes for ospreys (broad-front migration) than for marsh harriers (corridor migration) and a higher degree of repeated use of the same stopover sites among ospreys, and (2) higher within-individual consistency of timing and duration among marsh harriers, while individual ospreys were more flexible. Our findings suggest that individuality in space and time is not a shared trait complex among migrants, but may show adaptive variation depending on the species’ life history and ecology.

Voir aussi:

Trierweiler, C., Klaassen, R. H. G., Drent, R. H., Exo, K.-M., Komdeur, J., Bairlein, F., & Koks, B. J. (2014). Migratory connectivity and population-specific migration routes in a long-distance migratory bird. Proc. R. Soc. B 281: 20132897.

Maps showing the routes of eight adult ospreys (first row) and six adult marsh harriers (second row) that completed at least one round trip between the breeding grounds in Sweden and the wintering quarters in West Africa during 1996–2012

Maps showing the routes of eight adult ospreys (first row) and six adult marsh harriers (second row) that completed at least one round trip between the breeding grounds in Sweden and the wintering quarters in West Africa during 1996–2012. Each panel highlights the three individuals with most repeated journeys of each species (a: OM1; b: OM2; c: OF1, d: MHM1; e: MHF1, f: MHF2; see Table 1 for details) in blue (autumn) and red (spring), as well as the trips of all other individuals of the species in grey

The migration of the Great Snipe (Gallinago media): intriguing variations on a grand theme

Lindström, Å., Alerstam, T., Bahlenberg, P., Ekblom, R., Fox, J. W., Råghall, J., & Klaassen, R. H. G. (2016). The migration of the great snipe Gallinago media: intriguing variations on a grand theme. Journal of Avian Biology 47: 321–334.
Free access (now)

Abstract:

The migration of the great snipe Gallinago media was previously poorly known. Three tracks in 2010 suggested a remarkable migratory behaviour including long and fast overland non-stop flights. Here we present the migration pattern of Swedish male great snipes, based on 19 individuals tracked by light-level geolocators in four different years. About half of the birds made stopover(s) in northern Europe in early autumn. They left the breeding area 15 d earlier than those which flew directly to sub-Sahara, suggesting two distinct autumn migration strategies. The autumn trans-Sahara flights were on average 5500 km long, lasted 64 h, and were flown at ground speeds of 25 m s−1 (90 km h−1). The arrival in the Sahel zone of west Africa coincided with the wet season there, and the birds stayed for on average three weeks. The birds arrived at their wintering grounds around the lower stretches of the Congo River in late September and stayed for seven months. In spring the great snipes made trans-Sahara flights of similar length and speed as in autumn, but the remaining migration through eastern Europe was notably slow. All birds returned to the breeding grounds within one week around mid-May. The annual cycle was characterized by relaxed temporal synchronization between individuals during the autumn–winter period, with maximum variation at the arrival in the wintering area. Synchronization increased in spring, with minimum time variation at arrival in the breeding area. This suggests that arrival date in the breeding area is under strong stabilizing selection, while there is room for more flexibility in autumn and arrival to the wintering area. The details of the fast non-stop flights remain to be elucidated, but the identification of the main stopover and wintering areas is important for future conservation work on this red-listed bird species.

The autumn (a) and spring (b) migration of great snipes travelling between the breeding site in Sweden and their winter quarters in central Africa

The autumn (a) and spring (b) migration of great snipes travelling between the breeding site in Sweden and their winter quarters in central Africa. Green dots show the breeding site according to the light geolocators, orange and yellow dots show stopover sites in Europe/northern Africa and sub-Saharan Africa, respectively, and blue dots the final wintering sites. Red solid lines mark the nonstop flights, and grey thin lines show shorter flights.

Costs of migratory decisions: A comparison across eight white stork populations

Flack, A., Fiedler, W., Blas, J., Pokrovsky, I., Kaatz, M., Mitropolsky, M., Aghababyan, K., Fakriadis, I., Makrigianni, E., Jerzak, L. and Azafzaf, H., Feltrup-Azafzaf, C., Rotics, S., Mokotjomela, T. M., Nathan, R. & Wikelski, M. 2016. Costs of migratory decisions: A comparison across eight white stork populations. Science Advances 2(1): p.e1500931. doi: 10.1126/sciadv.1500931 (Open Access)

Abstract:

Annual migratory movements can range from a few tens to thousands of kilometers, creating unique energetic requirements for each specific species and journey. Even within the same species, migration costs can vary largely because of flexible, opportunistic life history strategies. We uncover the large extent of variation in the lifetime migratory decisions of young white storks originating from eight populations. Not only did juvenile storks differ in their geographically distinct wintering locations, their diverse migration patterns also affected the amount of energy individuals invested for locomotion during the first months of their life. Overwintering in areas with higher human population reduced the stork’s overall energy expenditure because of shorter daily foraging trips, closer wintering grounds, or a complete suppression of migration. Because migrants can change ecological processes in several distinct communities simultaneously, understanding their life history decisions helps not only to protect migratory species but also to conserve stable ecosystems.

Migratory behavior of juveniles from eight different White Stork populations

Fig. 1. Migratory behavior of juveniles from eight different White Stork populations (Flack et al. 2016)