https://doi.org/10.30456/AVO.2019103 Avocetta 43: 13-21 (2019) Repeated large scale loop migrations of an adult European Honey Buzzard NICOLANTONIO AGOSTINI1,*, MÁTYÁS PROMMER2, MIKLÓS VÁCZI3, MICHELE PANUCCIO1 1 MEDRAPTORS (Mediterranean Raptor Migration Network) - Via Mario Fioretti 18, 00152 Rome, Italy 2 Herman Ottó Institute Nonprofit Ltd. - Park Utca 2, 1223 Budapest, Hungary 3 Fert -Hanság National Park Directorate - Rév-Kócsagvár 4, 9435 Sarród, Hungary * Corresponding author: [email protected] Abstract – During migration birds adopt flight strategies that often differ between spring and autumn. This behaviour can lead to diffe- rences in migratory paths as well as in flight performances between seasons, and may help to explain seasonal differences in the numbers of birds passing through migratory bottlenecks. Visual observations have revealed that larger numbers of adult European Honey Buzzards Pernis apivorus pass through the Central Mediterranean region during spring rather than during autumn migration, while the opposite oc- curs at the Strait of Gibraltar. It suggested that substantial numbers of birds, probably belonging to the population breeding in eastern Eu- rope, use an anticlockwise loop migration performing a risk-minimization strategy during autumn and a time minimization strategy during spring migration to maximize their fitness. In this study we analyze for the first time three autumn and three spring migrations of an adult bird belonging to this population tracked by satellite telemetry, also in relation to wind conditions. Each year the bird used the large-scale loop migration between the breeding site located in northern Hungary and the wintering ground in northern Cameroon, showing a higher overall migration speed, shorter paths and longer water crossings during spring movements. The bird benefited from a more efficient tail wind support during spring, while compensated the effect of lateral winds mostly at the onset of both autumn and spring migration. Fi- nally, the bird initiated the spring sea-crossing at the same location each year, but showed remarkable flexibility in route choice across the sea in response to annual differences in different wind conditions. These results support the idea that the stronger aggregation of adult European Honey Buzzards in Central Mediterranean bottlenecks in spring vs autumn largely consists of adults breeding in central-eastern Europe that return to their nesting sites along a relatively direct route. Key-words: loop migration, navigation, Pernis apivorus, biologging, water crossing, wind drift. INTRODUCTION 2013). In a recent review Schmaljohann (2018) analyzed the results of 64 studies made by satellite telemetry show- During spring birds are expected to minimize the duration ing that in waders, gulls, swifts, and songbirds speeds are of migration to reach their breeding grounds as soon as significantly higher in spring, while the opposite occurs in possible. That is because individuals can increase their re- waterfowl and owls. The author concluded that seasonal productive performance by arriving early and occupying variation in stopover duration is the main biological mech- high-quality breeding sites (van Noordwijk et al. 1995, anism regulating seasonal differences in migration speed. McNamara et al. 1998, Kokko 1999, Moore et al. 2005). In Interestingly, species of raptors analyzed in that study mi- several species the migration speed is higher during spring grated faster during spring than during autumn but the dif- than during autumn, but others show the opposite (Frans- ference was not significant. Besides stopover duration and son 1995, Shamoun-Baranes et al. 2003, Mosbech et al. daily migration speed, seasonal differences in route choice 2006, Yohannes et al. 2009, Karlsson et al. 2012, Bustnes are common among migrant birds and greatly affect the et al. 2013, López-López et al. 2014, Nuijten et al. 2014, total distance, and therefore the duration of migration. In DeLuca et al. 2015, Alves et al. 2016, Kölzsch et al. 2016). populations that perform such loop migrations, one route Migration speed can be affected by stopover duration and/ might be considered the result of the path of colonization, or daily travel speed, while differences between spring the other that evolved to shorten the distance and the dura- and autumn could be affected by less or more favourable tion of migration, or to avoid adverse conditions (Newton weather conditions during the two seasons (Nilsson et al. 2008). Not necessarily all individuals of a population use © 2019 CISO - Centro Italiano Studi Ornitologici 13 Agostini et al. a loop migration, and different individuals may use differ- buzzards (Buteo spp.; Agostini et al. 2015). Visual obser- ent loop migrations. vations, however, showed that larger numbers of adult Eu- The European Honey Buzzard Pernis apivorus breeds ropean Honey Buzzards cross the Central Mediterranean in the Palearctic region and overwinters mostly in central during spring rather than during autumn migration, while West Africa (Ferguson-Lees & Christie 2001). This spe- the opposite occurs at the Strait of Gibraltar (Agostini & cies stores fat before migrating and usually does not feed Panuccio 2005, 2015, De La Cruz et al. 2011, Programa en route (Panuccio et al. 2006). Unlike juveniles, which Migres 2009). It has been suggested that thousands of perform extensive movements within the tropics once birds, mostly breeding in Central-eastern Europe, regularly reached the wintering ground, adults remain stationary use an anticlockwise loop migration on a large scale choos- within restricted territories (Hake et al. 2003, Strandberg ing more direct routes in spring to reach earlier their breed- et al. 2012, Vansteelant et al. 2015). The seasonal route ing sites. They would concentrate at the Strait of Gibraltar choice of European Honey Buzzards has been studied by during post-breeding movements as a result of a risk mini- satellite and GPS-tracking in various breeding populations. mization strategy and would cross the Central Mediterra- Adults breeding in the Netherlands and wintering in Cen- nean during spring performing shorter paths between win- tral West Africa passed via the Strait of Gibraltar during tering and breeding grounds but undertaking longer water both spring and autumn, and were found to achieve higher crossings (Agostini & Panuccio 2005, 2015). In this study travel speed during pre-breeding movements (Vansteelant we analyze for the first time three autumn and three spring et al. 2015). However, the higher hourly and daily spring migrations of an adult male breeding in Central-eastern migration speed was entirely accounted for by higher wind Europe (Hungary) tracked by satellite telemetry, also fo- support in spring (Vansteelant et al. 2015) and fine-scale cusing on how seasonal wind conditions may mediate be- analyses of their thermal-soaring behaviour revealed they tween risk avoiding and time minimizing route choice in alternate between risk averse and time optimal gliding air- both seasons. speeds in both seasons (Vansteelant et al. 2017a). Moreo- ver, the fact that European Honey Buzzards travel faster in spring due to wind assistance is at least partly due to the MATERIAL AND METHODS fact these birds leave West Africa along a westward detour to catch favourable winds for crossing the desert en route Tagging to the Strait of Gibraltar (Vansteelant et al. 2017). In con- The adult male was trapped and tagged first time in 2014, trast to the Dutch adults, anecdotal cases from other popu- however the logger failed without providing a proper data- lations show that at least some European Honey Buzzards set. In 2015 the bird was relocated in the area and suc- take seemingly risky routes directly across the Central cessfully trapped again. The logger was replaced, and Sahara and Mediterranean on spring migration. An adult the new logger was working properly. In both years the male breeding in northern Germany and wintering in Ni- bird was trapped during the breeding season close to his geria, for example, crossed the Mediterranean at the Strait nest in northern Hungary (in 2015: Lat 47.8801 N; Long of Gibraltar in 2004, but in 2005 and 2006 he made longer 17.485116 E; Fig. 1). For trapping a realistic Eagle Owl sea crossings from Algeria to northern Spain via Balearic (Bubo bubo) model and mist net (mesh size 10x10 cm) were Islands (Meyburg et al. 2010). Another bird showed loop used. In both cases, the bird was tagged with a “DUCK-H” migration at a larger scale, undertaking the crossing of the type GPS-GSM logger manufactured by ECOTONE Te- Central Mediterranean during spring movements (Mey- lemetry and it was attached to the bird with a Teflon har- burg et al. 2013). As such it seems European Honey Buz- ness. The harness was tailor-made fitting the bird’s body zards engage in different loop migrations between differ- and it forms a backpack with the logger (Prommer et al. ent breeding populations, and the causes and consequences 2012). The solar-powered logger has an in-built GPS unit of this variation are not well understood. In this picture, it that can locate the tagged bird with a few metre accuracy. is important to map loop migration patterns across multiple After every 4th recorded location the logger transmits the populations to help understand seasonal patterns in migra- data through GSM network to an online database. tion counts at geographical bottlenecks around the Medi- terranean. Tracking data In contrast to large soaring birds like eagles and vul- As mentioned above the dataset included three complete tures, European Honey Buzzards seem capable of long sea migration cycles, three during autumn (2015, 2016, 2017) crossings in both seasons thanks to its size and morphol- and three during spring (2016, 2017, 2018). To calculate ogy, intermediate between harriers (Circus spp.) and true migration distances covered by the bird, in each season 14 Large scale loop migration in a soaring raptor N 0 750 1500 km Figure 1. Migratory routes of the adult European Honey Buzzard in autumn (via Strait of Gibraltar) and spring (via Central Mediterra- nean).