title: The challenges of the first migration: movement and behaviour of juvenile vs. adult white storks with insights regarding juvenile mortality.
abstract: Migration conveys an immense challenge, especially for juvenile birds coping with enduring and risky journeys shortly after fledging. Accordingly, juveniles exhibit considerably lower survival rates compared to adults, particularly during migration. Juvenile white storks (Ciconia ciconia), which are known to rely on adults during their first fall migration presumably for navigational purposes, also display much lower annual survival than adults. Using detailed GPS and body acceleration data, we examined the patterns and potential causes of age-related differences in fall migration properties of white storks by comparing first-year juveniles and adults. We compared juvenile and adult parameters of movement, behaviour and energy expenditure (estimated from overall dynamic body acceleration) and placed this in the context of the juveniles' lower survival rate. Juveniles used flapping flight vs. soaring flight 23% more than adults and were estimated to expend 14% more energy during flight. Juveniles did not compensate for their higher flight costs by increased refuelling or resting during migration. When juveniles and adults migrated together in the same flock, the juvenile flew mostly behind the adult and was left behind when they separated. Juveniles showed greater improvement in flight efficiency throughout migration compared to adults which appears crucial because juveniles exhibiting higher flight costs suffered increased mortality. Our findings demonstrate the conflict between the juveniles' inferior flight skills and their urge to keep up with mixed adult-juvenile flocks. We suggest that increased flight costs are an important proximate cause of juvenile mortality in white storks and likely in other soaring migrants and that natural selection is operating on juvenile variation in flight efficiency.
authors.affiliation: Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: Shay
authors.initials: S
authors.lastname: Rotics
authors.affiliation: Vogelschutzwarte Storchenhof Loburg e.V., Chausseestr. 18, D-39279, Loburg, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Kaatz
authors.affiliation: Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.|Edmond and Lily Safra Center for Brain Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: Yehezkel S
authors.initials: YS
authors.lastname: Resheff
authors.affiliation: Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: Sondra Feldman
authors.initials: SF
authors.lastname: Turjeman
authors.affiliation: Dynamic Macroecology, Department Landscape Dynamics, Swiss Federal Research Institute WSL, Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland.
authors.forename: Damaris
authors.initials: D
authors.lastname: Zurell
authors.affiliation: Department of Evolutionary and Environmental Biology, The University of Haifa, 3498838, Haifa, Israel.
authors.forename: Nir
authors.initials: N
authors.lastname: Sapir
authors.affiliation: Plant Ecology and Conservation Biology, Institute for Biochemistry and Biology, University of Potsdam, Maulbeerallee 2, 14469, Potsdam, Germany.
authors.forename: Ute
authors.initials: U
authors.lastname: Eggers
authors.affiliation: Department of Migration and Immuno-Ecology, Max-Planck-Institute for Ornithology, D-78315, Radolfzell, Germany.|Department of Biology, University of Konstanz, D-78468, Konstanz, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Flack
authors.affiliation: Department of Migration and Immuno-Ecology, Max-Planck-Institute for Ornithology, D-78315, Radolfzell, Germany.|Department of Biology, University of Konstanz, D-78468, Konstanz, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Fiedler
authors.affiliation: Plant Ecology and Conservation Biology, Institute for Biochemistry and Biology, University of Potsdam, Maulbeerallee 2, 14469, Potsdam, Germany.|Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), D-14195, Berlin, Germany.|ZALF, Leibniz-Centre for Agricultural Landscape Research, Eberswalder Str. 84, D-15374, Müncheberg, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Jeltsch
authors.affiliation: Department of Migration and Immuno-Ecology, Max-Planck-Institute for Ornithology, D-78315, Radolfzell, Germany.|Department of Biology, University of Konstanz, D-78468, Konstanz, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Wikelski
authors.affiliation: Department of Ecology, Evolution and Behavior, Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel.
authors.forename: Ran
authors.initials: R
authors.lastname: Nathan
mesh_terms: D000367:Age Factors; D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D001522:Behavior, Animal; D001717:Birds / Q000502:physiology*; D004734:Energy Metabolism; D005426:Flight, Animal / Q000502:physiology*; D009026:Mortality; D058998:Remote Sensing Technology; D012919:Social Behavior
keywords: flight; flight efficiency; juvenile mortality; migration; white stork