𝗜𝗻𝗳𝗹𝘂𝗲𝗻𝗰𝗲 𝗼𝗳 𝗮𝗴𝗲 𝗼𝗻 𝘀𝗽𝗮𝘁𝗶𝗮𝗹 𝗮𝗻𝗱 𝘁𝗲𝗺𝗽𝗼𝗿𝗮𝗹 𝗺𝗶𝗴𝗿𝗮𝘁𝗼𝗿𝘆 𝗽𝗮𝘁𝘁𝗲𝗿𝗻𝘀 𝗼𝗳 𝗕𝗹𝗮𝗰𝗸 𝗦𝘁𝗼𝗿𝗸𝘀 𝗳𝗿𝗼𝗺 𝗚𝗲𝗿𝗺𝗮𝗻𝘆
𝙁𝙚𝙡𝙞𝙭 𝙁𝙞𝙨𝙚𝙡, 𝙂𝙚𝙤𝙧𝙜 𝙃𝙚𝙞𝙣𝙚, 𝘾𝙖𝙧𝙨𝙩𝙚𝙣 𝙍𝙤𝙝𝙙𝙚, 𝙈𝙖𝙧𝙩𝙞𝙣 𝙒𝙞𝙠𝙚𝙡𝙨𝙠𝙞 & 𝘼𝙣𝙙𝙧𝙚𝙖 𝙁𝙡𝙖𝙘𝙠
𝗔𝗯𝘀𝘁𝗿𝗮𝗰𝘁
How the migration behavior of long-lived birds develops throughout their life is still a research question in ornithology that is largely unanswered. Here, we analyzed GPS data of 70 juvenile Black Storks during the first 3 years of their migration to determine the influence of age and experience on migration patterns. We focused on the choice of the migration flyway, as well as the movements in the wintering area, and the changes in departure times with increasing age. Migratory routes of German Black Storks were divided into the western (82% of all tracked individuals) and eastern flyways (16%). The central migratory flyway through Italy or Greece was taken only by two individuals and never repeatedly. In addition, we found that the wintering area decreased by almost 60% during their first 3 years of life. In addition, spring departure dates from the wintering areas also became earlier with increasing age. However, we did not find changes in departures from the breeding area in autumn. Thus, our study shows that age and migration experiences have a considerable influence on essential migratory decisions in Black Storks. Further, it also demonstrates that older storks rely strongly on previous experiences to perform their migrations efficiently.
𝗜𝗻𝘁𝗿𝗼𝗱𝘂𝗰𝘁𝗶𝗼𝗻
The German population of Black Storks (Ciconia nigra) was nearly driven to extinction due to persecution and intensified agricultural and forestry practices at the beginning of the last century, especially in the southern and western regions. However, from the 1930s onwards, there has been a gradual recovery, with the species re-establishing from Eastern Europe (Janssen et al. 2004). The German Black Stork population has now increased to between 800 and 900 pairs and the population trend remains positive (Gerlach et al. 2019; Bundesamt für Naturschutz 2020). To maintain this positive population trend and support the Black Stork population, it is essential to study not only local breeding behavior but also explore migratory patterns, like routes, destinations, and timing. Knowledge of these critical parameters in migration behavior will enable successful conservation decisions in the future (Flack et al. 2022).
Black Storks are long-distance migrants that, in the western Palaearctic, show a migratory divide (Bobek et al. 2008). Birds from the western flyway cross Europe and northern Africa to overwinter in the west and central Sahel region (Cano and Tellería, 2013; Chevallier et al. 2011; Jadoul et al. 2003). Over the past 20 years, there has been an increase in telemetry studies, providing new insights into individual migration behavior, especially regarding routes, wintering, and stopover sites (Chevallier et al. 2010a, 2011). However, these telemetry projects often had relatively small sample sizes (Bobek et al. 2008; Larue et al. 2016), or the storks were captured as adults (Chevallier et al. 2011; Jadoul et al. 2003) which did not allow the study of the development of the migratory behavior of juvenile Black Storks.
The ontogeny of migration behavior is still an area in ornithology that is largely characterized by knowledge gaps (Flack et al. 2022). While the migration behavior of short‐lived migrants may be rather fixed and stable, long‐lived migrants can progressively adjust their migration behavior to the needs of their ongoing life stage. We know that migration strategies and performances can vary enormously between individual Black Storks (Cano and Tellería 2013), but do these strategies also change throughout the life of an individual? It has been shown that selective mortality can shape lifelong migration patterns at the population level, selecting for individuals with efficient routes (Sergio et al. 2014). In addition, at the individual level, individuals can improve their migration behavior during their lifetime through learning (Aikens et al. 2024). However, due to a lack of long-term tracking data from early life onward, it is for most species unclear whether individuals change their migration patterns with age.
In this paper, we examine a dataset of 70 juvenile Black Storks that were monitored with GPS devices between 2017 and 2022. Despite high juvenile mortality and some technical failures of the transmitters, this project provides a large data, tracking juvenile Black Storks over a maximum of 3 consecutive years. Thus, we examine how the spatial and temporal patterns of the migration behavior of juvenile Black Storks change over the first 3 years of their life. To achieve this goal, we first examine the temporal features of the migration behavior and explore how they change with increasing age. Since Black Storks reach sexual maturity between the ages of 3 and 4 (Janssen et al. 2004), we expect that especially, in their third year of life, a clear temporal shift in departure date becomes visible. We predict that spring departure dates (i.e., departures from the wintering area) take place significantly earlier in older birds, because an earlier arrival at the breeding area may be essential to finding and defending a breeding territory. Second, we also examine the spatial features of the storks’ migration behavior. We assume that age influences the area used during the wintering period. Learning and experiences may allow birds to spend less time exploring and visiting areas they have encountered in previous years (Lewis et al. 2021). There is evidence that juvenile birds show lower fidelity, especially in the wintering area, as they explore the region that is new to them (Fayet 2020). Thus, we expect to see a decrease in the distance traveled during winter with increasing age of the birds.
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https://link.springer.com/article/10.10 ... 24-02170-3
