Kralj et al. Avian Res (2020) 11:6 https://doi.org/10.1186/s40657-020-00191-z Avian Research RESEARCH Open Access Geolocator study reveals east African migration route of Central European Common Terns Jelena Kralj1* , Miloš Martinović1, Luka Jurinović2, Péter Szinai3, Szandra Sütő4 and Bálint Preiszner5 Abstract Background: The majority of European Common Terns (Sterna hirundo) migrate south along the western coast of Europe and Africa, while birds from eastern regions are known to cross the Mediterranean Sea from east to west or migrate along the eastern African coast. The migration route of north European terns wintering along the coast of western Africa was already described using geolocator data, while knowledge about movements of the European inland populations is based only on relatively scarce recoveries of ringed birds. Methods: We used light-level geolocators in inland Common Tern colonies in Hungary and Croatia to study their migration route and to identify wintering areas along with stopover sites. Results revealed by geolocators were com- pared with recoveries of ringed birds. Results: All tracked birds used the east African migration route with autumn stopovers at Lower Nile and in the southern part of the Red Sea, and short spring stopover in Israel. Terns wintered along Kenyan coasts and in the southern Mozambique Channel. Autumn migration lasted four times longer than spring migration. Conclusions: This is the frst geolocator study that describes the east African migration route of the Common Tern. Important stopover sites were identifed. More studies of inland populations are needed to better elucidate tern winter movements. Keywords: Inland population, Light-level geolocators, Mozambique Channel, Red Sea, Sterna hirundo, Stopover sites, Wintering area Background geolocators produce less precise location data compared Tracking devices are increasingly used in the study of to GPS tracking, but precision is adequate enough for the seabird migration, providing detailed information about objectives of large-scale tracking, and were already used long-distance journeys which was impossible to gain for studying tern migration (Egevang et al. 2010; Nisbet using other methods such as observations or ringing et al. 2011; McKnight et al. 2013). (Brooke 2018). Many seabird species spend their entire Te Common Tern (Sterna hirundo) breeds in both non-breeding season far from land, therefore migration coastal and inland habitats of the Holarctic. It is a long- strategies and non-breeding areas were largely unknown distance migrant, wintering along sea coasts in the south- prior to the development of tracking devices. Light-level ern hemisphere. Two main migration routes of European breeders were revealed by recoveries of ringed birds. Based on these recoveries, most Common Terns appear *Correspondence: [email protected] to migrate south along the western coast of Europe and 1 Institute of Ornithology, Croatian Academy of Sciences and Arts, Gundulićeva 24, 10000 Zagreb, Croatia Africa. Populations from central and eastern Europe Full list of author information is available at the end of the article either cross the Mediterranean Sea from east to west © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. 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Avian Res (2020) 11:6 Page 2 of 11 and join the west African route, or migrate across the 2012), such knowledge is crucial in developing strategies eastern Mediterranean, the Red Sea, and along eastern for conservation actions at sites used during the non- African coast (Cramp and Simmons 2006). Recoveries breeding season. of inland Common Terns breeding in Central Europe Te aims of this study were (1) to elucidate migration are scarce. Recoveries of Hungarian terns showed pre- routes used by Common Terns belonging to European dominantly the use of the southeastern route, following inland populations, and (2) to identify main stopover the Danube to the western coast of the Black Sea, and sites and wintering areas. Based on previous ring recov- southward along the eastern coast of the Mediterranean ery data, we expected Common Terns from Hungarian Sea, with wintering areas along the coast of Mozambique colonies to use the eastern fyway and birds from Croa- and Republic of South Africa (Szinai 2009). Te limited tian colonies to use the western one. Apart from Israel, number of recoveries of Croatian birds indicate the use of no other stopover site was known for inland populations the western route with wintering areas in the Republic of (Cramp and Simmons 2006). In long-lived species as South Africa (Kralj 2013). terns, population trends are more likely to be infuenced Previous geolocator studies on Common Terns were by survival rates than annual changes in productivity conducted on populations breeding along marine coasts (Bracey et al. 2018), therefore identifcation of stopover of North America (Nisbet et al. 2011) and Europe and wintering sites is an important step in threat assess- (Becker et al. 2016), on the Azores (Neves et al. 2015) and ment. Here we present the results of two geolocator on North American lakes (Bracey et al. 2018). Geoloca- tracking projects of Central European inland Common tor tracking of German North Sea populations already Tern colonies; to our knowledge, this is the frst geolo- helped to describe the migration route, stopover loca- cator study of an inland population of the species in tions and wintering sites along the coast of western Europe. Africa (Becker et al. 2016). However, movements of Euro- pean inland populations breeding along middle reaches Methods of rivers are less known. In Europe, rivers are among the We studied the migration of two inland Common Tern scarcest and the most threatened habitats (European colonies: one breeding on lakes of the lagooning facil- Environment Agency 2015). Inland populations of Com- ity near the town of Várpalota (Hungary), and the other mon Terns are threatened by changes of river hydrol- along the river Sava and surrounding gravel pits in the ogy: river management causes frequent breeding failures vicinity of Zagreb (Croatia). Te Hungarian study colony (Atamas and Tomchenko 2015) or complete destruction of 60 pairs nests on three artifcial breeding platforms on of breeding habitats. In many countries terns are there- two waterbodies: Nagybivalyos and Inota30 fshponds fore largely dependent on artifcial habitats (Scharf 1981; (47°11ʹ N, 18°10ʹ E). Platforms hold a gravel bed for nest- Bogliani et al. 1982; Becker and Sudmann 1998). Readi- ing and artifcial shelters are provided. Although these ness to accept artifcial habitats has helped recovery of are non-natural waterbodies with regular human pres- inland populations in Europe after the collapses dur- ence, the characteristics and diversity of bird taxa are ing the 19th and 20th centuries (Glutz von Blotzheim of good quality habitat. Croatian breeding ground com- and Bauer 1999). However, to provide long-term nest- prises the area of 250 km2 that holds about 120 breeding ing opportunities, man-made breeding habitats need pairs. We studied the colony on a gravel island at Rakitje regular maintenance. Terns can therefore only be pre- gravel-pit (45°48ʹ N, 15°50ʹ E), the biggest and the most served by re-establishing and conserving suitable habi- stable colony in that area. tats (Becker and Sudmann 1998). Climate change is also A total of 91 adult Common Terns (18 in Hungary expected to afect Common Tern breeding grounds: the and 73 in Croatia) were captured on their nests during Common Tern’s simulated future potential distribution, the incubation stage between late May and early July in based on the relationship between its breeding distribu- 2014‒2018. Walk-in traps and target activated drop traps tion and local climate, forecasts loss of inland habitats were used to capture breeding adults. After ringing the and a patchier distribution in continental Europe (Hunt- birds with metal and plastic rings, mass was measured for ley et al. 2007). Efective conservation of long-distance pre-deployment and recovery comparison. In Hungary, migrants requires the understanding of their full annual we ftted 5 and 2 birds in 2014 and 2016, respectively, cycle (Marra et al. 2015). Tracking of individual birds is with archival light level recorders (MK5090, Biotrack Ltd) of great importance in identifying important stopover mounted on plastic leg rings. In Croatia, similar loggers locations during migration and wintering. As Common (Intigeo-W65A9-SEA, Migrate Technology Ltd) were Terns face potential risks, such as decline in fsh popula- deployed on 10 birds per year in 2016 and 2017. Mass of tions, adverse weather, sea pollution or outbreaks of dis- the ring, logger, tie, and glue was 1.35 g (MK5090) and eases (Feare et al. 2007; Croxall et al. 2012; Le Corre et al. 0.95 g (Intigeo-W65A9-SEA), representing 1.2% and Kralj et al. Avian Res (2020) 11:6 Page 3 of 11 0.8% of the mean body mass of adult birds in these pop- Biotrack loggers, as they had stopped working before the ulations, which corresponds with international stand- terns returned to the breeding area.