Importance of the connectivity of Spanish stopovers for Black Storks

Abstract

Capsule Sixteen Black Storks (Ciconia nigra) were tracked by satellite during their autumnal and spring migrations in order to identify their major stopover sites and connections between stopovers in Europe and Africa. Among journeys with stopovers, the longest distance that a stork travelled without stopover was 2433 km (defined here as ‘accessible distance’) meaning that those storks which have stopovers use only a single stopover on average, and this is usually in Spain. We identified nine crucial stopovers (seven in Spain and two in Africa) with high connectivity highlighting the importance of Spanish stopover locations on the flyway of Black Storks.

European monitoring programmes currently indicate that the number of migrant birds in Central Europe has decreased by more than 50% (Marchant et al. 1990). The conversion of natural habitats to agriculture in European breeding areas and the aridity of African wintering areas have long been considered as the main factors responsible for the population decline in long-distance migrants (Johst et al. 2001, Moreno-Opo et al. 2011). For example, the decline of the western population of the White Stork (Ciconia ciconia) has partly been attributed to limited food availability in African wintering areas due to Sahelian droughts (Dallinga & Schoenmakers 1987, Kanyamibwa et al. 1990). Moreover, migratory costs and mortality along migration routes also play a key role (Berthold 1993, Thiollay 2007) and can depend on migratory strategies that differ between species and among populations within species (Piersma 1987, O'Reilly & Wingfield 1995). Some individuals travel for long distances during their migration without using stopovers (Gill et al. 2009, Egevanga et al. 2010), whereas others rest on stopover sites. An optimal stopover site should offer access to water and food so that birds can restore energy reserves (Kanai et al. 2002).

Migration routes used by European Black Storks (C. ciconia) are well known based on visual and ringing observations and further confirmed by satellite tracking (Hancock et al. 1992, Bobek et al. 2008). The western population flies from Europe over the Straits of Gibraltar to West Africa, whereas the eastern population travels from Europe via the Bosphorus to East Africa (Cramp & Kel 1977, Peske et al. 1996, Bobek et al. 2008, Chevallier et al. 2010a, 2010b, 2011). We focused our work on the occidental European Black Stork population. Storks are soaring birds that take advantage of thermal convection for travelling very long distances at low energetic costs (Liechti et al. 1996). Because thermal convection is only present over land, Storks commuting between Europe and Africa follow continental detours to avoid long sea crossings. Soaring flight allows some Black Storks to migrate nonstop (Bobek et al. 2008, Chevallier et al. 2011, Cano et al. 2013), whereas the majority of birds stop at least once during their migration, half of the stopovers being located in Spain (Chevallier et al. 2011). In this regard, a good knowledge of stopover locations, their duration and their connection to other stopover locations may be important to determine crucial sites for migration success. In the present study, we investigated the influence of the connectedness between main stopovers, i.e. the capacity for Black Storks to fly from one stopover to another. These results are important for our understanding of population dynamics, evolution of migration strategies (Salewski & Schaub 2007) and conservation of migrant birds (Berthold & Terrill 1991, Shimazaki et al. 2004).

From 1998 to 2006, 16 Black Storks (11 adults and 5 sub-adults/juveniles) were tracked between Western Europe and the Sahelo-Sudanian zone of West Africa, using satellite platform transmitters (PTT; North Star Science and Technology, King George, USA and Microwave Telemetry, Columbia, USA) fitted as backpacks using a Teflon ribbon harness. Thirteen Black Storks were tagged with conventional transmitters of different weights (45–100 g), while the three remaining Black Storks were tagged with Solar-GPS 45–70 g PTTs.

Travel and stopover days were distinguished using a threshold of 30 km/day (defined as the ratio of distance between two locations and the time elapsed between these two positions; Hourlay 2003, Shimazaki et al. 2004). Among the tracked birds, five individuals did not use any stopover at all. These birds were not considered in the following calculations. The average time spent at a migratory stopover was 14.8 ± 5.1 days per bird (Chevallier et al. 2011). We have shown in another study based on GPS (Global Positioning System)-Argos tracks that Black Storks arrived on their stopovers after 18.00 h, staying on the night roost and, therefore, do not feed (Chevallier et al. 2010a). No Stork travelled during the night, probably because travel costs are too high without diurnal thermal convections, with high altitude flights occurring between 12.00 and 18.00 h (Chevallier et al. 2010a).

Our work in a previous study showed that 29 stopovers were used by the tracked Black Storks (Chevallier et al. 2011). A stopover was considered to be ‘important’ when the bird stopped for >10 days (for adults) and 3–4 days (for sub-adults/juveniles). For all stopovers, the number of neighbouring stopovers within an accessible distance from a given site was calculated, as defined by Shimazaki et al. 2004. The distance between two stopovers was measured using a Geographical Information System (Arcview GIS Version 3.2 1998; Mapinfo Professional Version 8.0 2005). The accessible distance ‘D’ between two sites was defined as the maximum distance that the Storks travelled without making a stopover, following D = v × t, where v is the mean travel speed (in km/day) between stopovers and t the mean maximum duration (in days) Storks travelled without stopping. We calculated this accessible distance from the data obtained in a previous study on stopovers for all Storks irrespective of age (Chevallier et al. 2011). By multiplying the mean of maximal number of travelling days without a stopover (9.2 ± 1.4 days, Chevallier et al. 2011) with the mean travel speed of each individual (total mean travel speed was 263.6 ± 13.3 km/day, Chevallier et al. 2011), we obtained an accessible distance, for those birds that made a stopover, of up to 2433 ± 362 km. After taking into account age, we did not find any significant difference between adults and sub-adults (n = 7 vs. 6, 2145 ± 311 vs. 2779 ± 525 km, respectively, T-test, P < 0.37). Site connection was defined as the number of sites situated within 2433 km ± 10% radius (D = 2190 and 2676 km) from a given stopover. Among the 29 stopovers used by the Black Stork, 10 were classed as important sites and had the highest level of connection. They were situated in Spain (n = 7) and Africa (n = 2) (Fig. 1). The mean level of connection among the 29 stopovers was 24 ± 0.8 (range 11–28). Stopovers 1–4 (in France) and 27 (in Mali) to 29 (in Mauritania) were the least connected among the 29 sites.

Figure 1. Important stopover sites and neighbouring sites located at the accessible distance of 2433 km ± 10% (indicated with the lines). (a): Landes ponds, Natural Park Monfragüe (site 5) and Regional Park of Gredos (site 7); (b): National Hunting Reserve in Cíjara (site 10), Natural Park Hornachuelos (site 11), Sierra de Cardena y Montoso (site 12), Despeñaperros (site 13); (c): Doñana Park (site 16); (d): Aguelmous (site 19); (e): Oasis of Lemghaïti (site 24); and (f): Lake Faguibine (site 27).

Our data demonstrate that the longest average distance a Black Stork covers if it makes a stopover during migration (called accessible distance) is 2433 km for nine travel days. This distance is more than twice the accessible distance calculated for the oriental White Stork (C. ciconia) for the same number of days (1071 km, Shimazaki et al. 2004). However, the connection level (maximum of 24) for 29 sites is slightly lower than that observed in the oriental White Stork (27 for 42 sites). This suggests that this species has fewer stopovers compared with those used by Black Storks along their migration routes.

The majority of time (77%) spent at stopovers involved locations in Spain (Chevallier et al. 2011). Five of these Spanish stopovers (Special Protection Area of Monfragüe, Southern and Northern valleys at the bottom of the Regional Park of Gredos, National Hunting Reserve in Cíjara, Natural Park of the Sierra Hornachuelos, Special Protection Area of Doñana, Fig. 1) are predominantly located along rivers flowing within protected areas. They are important connecting sites because they are located midway between the breeding and wintering areas. Other studies on Black Storks indicate a similar use of these areas as stopovers (Cano Alonso 2012). The management of these sites appears to be one of the most important measures to be undertaken for conservation of long-distance migratory bird species, in accordance with the recommendations of the Conference on the Ramsar Convention (1971) addressed to the Spanish government and regional authorities. In contrast, there are probably fewer available or favourable sites where the birds can replenish their energy reserves in Africa. For example, Faguibine Lake (site 7) is a semi-permanent wetland in Africa, situated just before the wintering area which is the least connected site to other stopovers, therefore likely constituting a strategic stopover (Trolliet et al. 2003).

Furthermore, nine stopovers, based on the time spent by birds and their high connection level, may be considered as crucial sites for migration success. Black Storks that made a stopover used one stopover, usually in Spain, on their journey to breed and on their return to Africa. During travel towards their breeding area, minimizing the number of stopovers allows an early arrival and possibly increases the chance of finding a nest site or initiating early reproduction.

We have shown in this study that while some individuals may travel nonstop, others must make stopovers to achieve full migration. Thus, the accessible distance may vary from one individual to another and must presumably depend on both of their energy expenditure during the flight and body reserves accumulated before departure. In this context, the stopovers highlighted in this study and their connectivity are probably crucial for survival of large part of Black Storks population.

ACKNOWLEDGEMENTS

We thank E. Challet and J. Munro for their valuable comments on the manuscript. We would like to thank the Eaux et Forêts of Burkina Faso, the Office National des Forêts in collaboration with SOLON and SOBA associations, Bure les Templiers School, the Muséum National d'Histoire Naturelle de Paris and the Centre de Recherche Biologique des Populations d'Oiseaux.

FUNDING

The program received financial supports from the IRD, the CNRS, the Zoological Parks of Doué la Fontaine and Amnéville (France), the EU grant GOCE-2003-EDEN, the West African Ornithological Society, the Louis D. Fondation of the Institut de France and the ACOVARENA.