Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions
Abstract Background Each spring, migratory birds converge along the Croatian coast from various directions, creating a dynamic intersection of flight paths. Many birds are thought to cross the Adriatic Sea, while others follow a northward route along the coastline. As most migratory birds initiate f...
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2025-06-01
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| Online Access: | https://doi.org/10.1186/s40462-025-00572-3 |
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| author | Simon Hirschhofer Peter Ranacher Barbara Helm Davor Ćiković Robert Weibel Sanja Barišić Vesna Tutiš Louie Taylor Maja Bjelić Laušić Baptiste Schmid |
| author_facet | Simon Hirschhofer Peter Ranacher Barbara Helm Davor Ćiković Robert Weibel Sanja Barišić Vesna Tutiš Louie Taylor Maja Bjelić Laušić Baptiste Schmid |
| author_sort | Simon Hirschhofer |
| collection | DOAJ |
| description | Abstract Background Each spring, migratory birds converge along the Croatian coast from various directions, creating a dynamic intersection of flight paths. Many birds are thought to cross the Adriatic Sea, while others follow a northward route along the coastline. As most migratory birds initiate flight shortly after sunset, we hypothesized that sea-crossing migrants would arrive with a delay at the Croatian coast, compared to migrants that were following the coast, resulting in potentially intricate spatiotemporal patterns that remain poorly understood. Methods We deployed four ornithological radar devices along the Croatian coastline: two in southwestern Istria and two in northern Dalmatia. These radars tracked migratory bird activity up to 1000 m above ground, recording intensity and flight directions and their variations across sites, seasons, and individual nights. We conducted an exploratory analysis of these variations, applied functional principal component analysis and hierarchical clustering to summarise within-night activity profiles, and compared these profiles between sites and across the migration season, alongside associated flight direction distributions. Results During the early migration season, migration intensity was similar across all technically active sites, but site-to-site variation increased markedly in May. In March, flight directions were predominantly towards NNE, indicating mainly sea-crossing migration throughout the night. In April, NW directions dominated the first half of the night, shifting to scattered N directions later; in Dalmatia, even strong W components were observed early in the night. By May, W to NW movement towards the Italian coast were typical for early-night activity. Later in the night, flight directions shifted towards NE over Istria and N at the Dalmatian site in Zadar, while the site at Vrana exhibited a wide scatter, warranting further discussion. Contrary to our expectations, within-night intensity profiles could not be fully linked to specific directional patterns. Conclusion This study points to the complex interplay between coastal and sea-crossing migration along the Croatian coast. Our results demonstrate significant variability in the timing of migration within single nights in the context of aquatic barriers. Crossing such a barrier results in downstream delays compared to birds migrating along the barrier. Between nights, one behaviour or the other may dominate the overall activity, causing the shifts in within-night timing. In addition, barrier crossing is likely to be strongly influenced by weather conditions, contributing to the variability in the within-night timing of migration. However, measured flight directions were not always consistent with within-night timing, highlighting the complexity of avian migration in the context of aquatic barriers. This highlights the need for further research with high temporal resolution to gain a deeper understanding of migration behaviour in response to such barriers. |
| format | Article |
| id | doaj-art-957e25a6c8a2416b9926fc5708de1812 |
| institution | DOAJ |
| issn | 2051-3933 |
| language | English |
| publishDate | 2025-06-01 |
| publisher | BMC |
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| series | Movement Ecology |
| spelling | doaj-art-957e25a6c8a2416b9926fc5708de18122025-08-20T03:22:57ZengBMCMovement Ecology2051-39332025-06-0113111410.1186/s40462-025-00572-3Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directionsSimon Hirschhofer0Peter Ranacher1Barbara Helm2Davor Ćiković3Robert Weibel4Sanja Barišić5Vesna Tutiš6Louie Taylor7Maja Bjelić Laušić8Baptiste Schmid9Department of Geography, University of ZurichDepartment of Geography, University of ZurichBird Migration Unit, Swiss Ornithological InstituteCroatian Academy of Sciences and Arts, Institute of OrnithologyDepartment of Geography, University of ZurichCroatian Academy of Sciences and Arts, Institute of OrnithologyCroatian Academy of Sciences and Arts, Institute of OrnithologyCroatian Academy of Sciences and Arts, Institute of OrnithologyPublic Institution Nature Park Vransko jezeroBird Migration Unit, Swiss Ornithological InstituteAbstract Background Each spring, migratory birds converge along the Croatian coast from various directions, creating a dynamic intersection of flight paths. Many birds are thought to cross the Adriatic Sea, while others follow a northward route along the coastline. As most migratory birds initiate flight shortly after sunset, we hypothesized that sea-crossing migrants would arrive with a delay at the Croatian coast, compared to migrants that were following the coast, resulting in potentially intricate spatiotemporal patterns that remain poorly understood. Methods We deployed four ornithological radar devices along the Croatian coastline: two in southwestern Istria and two in northern Dalmatia. These radars tracked migratory bird activity up to 1000 m above ground, recording intensity and flight directions and their variations across sites, seasons, and individual nights. We conducted an exploratory analysis of these variations, applied functional principal component analysis and hierarchical clustering to summarise within-night activity profiles, and compared these profiles between sites and across the migration season, alongside associated flight direction distributions. Results During the early migration season, migration intensity was similar across all technically active sites, but site-to-site variation increased markedly in May. In March, flight directions were predominantly towards NNE, indicating mainly sea-crossing migration throughout the night. In April, NW directions dominated the first half of the night, shifting to scattered N directions later; in Dalmatia, even strong W components were observed early in the night. By May, W to NW movement towards the Italian coast were typical for early-night activity. Later in the night, flight directions shifted towards NE over Istria and N at the Dalmatian site in Zadar, while the site at Vrana exhibited a wide scatter, warranting further discussion. Contrary to our expectations, within-night intensity profiles could not be fully linked to specific directional patterns. Conclusion This study points to the complex interplay between coastal and sea-crossing migration along the Croatian coast. Our results demonstrate significant variability in the timing of migration within single nights in the context of aquatic barriers. Crossing such a barrier results in downstream delays compared to birds migrating along the barrier. Between nights, one behaviour or the other may dominate the overall activity, causing the shifts in within-night timing. In addition, barrier crossing is likely to be strongly influenced by weather conditions, contributing to the variability in the within-night timing of migration. However, measured flight directions were not always consistent with within-night timing, highlighting the complexity of avian migration in the context of aquatic barriers. This highlights the need for further research with high temporal resolution to gain a deeper understanding of migration behaviour in response to such barriers.https://doi.org/10.1186/s40462-025-00572-3Ornithological radarAeroecologyBird migrationEast Adriatic CoastFunctional data analysis |
| spellingShingle | Simon Hirschhofer Peter Ranacher Barbara Helm Davor Ćiković Robert Weibel Sanja Barišić Vesna Tutiš Louie Taylor Maja Bjelić Laušić Baptiste Schmid Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions Movement Ecology Ornithological radar Aeroecology Bird migration East Adriatic Coast Functional data analysis |
| title | Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions |
| title_full | Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions |
| title_fullStr | Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions |
| title_full_unstemmed | Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions |
| title_short | Avian spring migration at the east Adriatic coast: coastal and sea-crossing dynamics of intensity, timing, and flight directions |
| title_sort | avian spring migration at the east adriatic coast coastal and sea crossing dynamics of intensity timing and flight directions |
| topic | Ornithological radar Aeroecology Bird migration East Adriatic Coast Functional data analysis |
| url | https://doi.org/10.1186/s40462-025-00572-3 |
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