Abstract
Red-throated divers (Gavia stellata) are among the species most susceptible to anthropogenic disturbance from shipping traffic and offshore wind farms (OWFs). The species’ preferred wintering and staging areas are relatively shallow coastal waters where such activities are widespread. Divers maintain large avoidance distances from OWFs, resulting in substantial habitat loss and conflicts under species protection legislation.
Although existing data from the eastern German Bight do not indicate a recent decline in the local population of red-throated divers, they do suggest a shift in the species’ main distribution area. However, the long-term consequences of this habitat loss and spatial displacement remain unclear so far.
To enable the continued expansion of offshore wind energy in line with the objectives of the German government, it is essential to improve our understanding of how red-throated divers respond to OWFs and associated maintenance vessel traffic. This includes assessing potential population level effects and developing strategies to avoid or mitigate negative impacts while simultaneously
optimizing maintenance logistics.
The current study assesses whether the displacement of divers from areas surrounding OWFs in the German and Danish North Sea has led to changes in foraging behaviour, activity budgets, and energy expenditure. Such behavioural changes may entail fitness costs and could have long-term effects on local populations. The aim of this project was to extend investigations of individual behavioural patterns and to record them with much higher precision using modern GPS loggers. This approach allows for a comprehensive assessment of the impacts of OWFs on red-throated divers, ranging from individual behavioural responses to potential population-level effects. In addition, the study goes beyond quantifying direct habitat loss by focusing on the species’ most important activity— foraging—through detailed analyses of diving behaviour.
Results on habitat quality, daily movements, activity budgets and energy expenditure lay the foundation for individual-based modelling, which can be used to estimate the effects on fitness and potential population level effects. The aim was to compare the current situation in the North Sea (Eastern German Bight) with several operational OWFs to a hypothetical scenario where no OWFs
are present and to estimate the consequences on body condition. This can help to better understand the impacts of OWFs on the individual and population level.
In this study, during three years (2022-2024), 68 red-throated divers were equipped with GPS transmitters (external or implanted). External transmitters also recorded diving activity. All birds were captured and tagged during spring within their main concentration area in the Eastern German Bight of the North Sea. High-resolution GPS and dive data were analysed for a focal area in the
North Sea, while also (long-distance) migration movements to breeding grounds were investigated for birds with implanted transmitters.
In the current study, breeding locations as well as migration routes of divers were very similar to the previous DIVER project. 70% of birds migrated to northern Russia, while the remaining birds migrated to potential breeding grounds in Norway, Sweden, Greenland and Svalbard. Only two birds, one migrating to Sweden and one to southern Norway, could be regarded as short-distance migrants. These results confirm that our sample size was representative of showing the full range of breeding locations of birds staging in the focal area. Little direct overlap with existing or planned OWFs during the non-breeding season was found, however the potentially large disturbance effects from installations and associated vessel traffic have to be taken into account.
A habitat model for the focal area showed that the distribution of divers was affected by water depth, salinity, distance to OWF and ship traffic. The results were similar to previous work, although the OWF effect appeared to be weaker than in previous analyses and was mainly limited to the OWF footprint. The model also showed a significant amount of variance that could not be explained
by the model covariates, suggesting that also other factors play an important role for the distribution of birds, such as prey availability or site fidelity, which could not be measured directly.
This study also provided the first fine-scale assessment of the foraging behaviour of red-throated divers in an important non-breeding offshore habitat. Foraging activity was found to be strictly diurnal, lasting only on average 2.3 h per day. Most dives were short in duration (mostly <30 seconds) and shallow in depth within the water column (depths <10 m). We found no evidence of divers
changing their foraging patterns in the presence of OWFs.
An analysis of time-activity budgets showed that divers spend most of their time resting at the water surface and engaging in low‑energy activities, a pattern that is essential for interpreting results, because it highlights how even small behavioural shifts towards more energetically costly activities (such as flight) can influence overall daily energy expenditure. Flight activity was only recorded during 1.3% of the time. Step-length was significantly longer during daylight hours, suggesting that most locomotion—both active swimming and flight—occurs mainly during the day. As divers approach OWFs, they exhibit a slight but significant increase in flying behaviour, driving modest increases in daily energy expenditure.
The above described analyses provided important estimates for the habitat use, dive behaviour, time-activity budgets as well as energy expenditure of these birds in the focal area, and in a next step these data were integrated into an individual-based model (IBM) in order to determine the energetic consequences of the presence of OWFs on individual body condition.
The IBM was parametrized such that the activity and energy budgets of simulated divers resembled those from our dataset of real divers during the simulation period of four weeks of spring staging phase. The results of the IBM indicate that the presence of OWFs has a slight negative effect on the mean body mass of divers. A lower negative-density dependent effect led to improved foraging efficiency, alleviating the costs of OWF presence on body mass. Because the extent of both negative density dependence and food constraints in overwintering diver populations in nature is currently unknown, it remains speculative to what degree the modest negative effect observed in the IBM translates into long-term consequences for population viability (i.e. over multiple years). Nevertheless, the results suggest that any future potential reduction in foraging efficiency within the focal area (e.g., due to climate-change) is likely to amplify the negative effects of OWFs. Developing methods to study foraging efficiency (e.g., by use of body mounted video loggers or stomach temperature logger) in these birds, particularly in the non-breeding phase is challenging but could provide us with much-needed data on this topic.
Given the observed impacts of OWFs on red-throated divers, the most important measure to avoid such impacts is maritime spatial planning such as choosing development sites away from key areas of these species. In cases when (complete) avoidance of sensitive areas is not an option or when OWFs were built before knowledge of their large-scale effects emerged, direct mitigation measures might be required. Based on the results of this study as well as a review of previous work, the only mitigation measure proven to successfully reduce negative effects of OWFs on red-throated divers is the regulation of OWF related shipping traffic. Other promising measures (e.g. reducing nightlighting of OWFs) will need to be investigated further, and long-term aerial-survey monitoring can help to track any changes in bird behaviour towards OWFs over time.