Curtailment of wind farms effectively reduces collision mortality in bats. Implementing this measure in offshore wind farms requires knowledge on the spatiotemporal occurrence and environmental predictors of migration over sea. In bats, such information can be obtained through acoustic monitoring and individual tracking. However, these techniques provide seemingly contradictory insights into migration patterns. We used a Bayesian capture-recapture state-space model to investigate how environmental predictors influence spring departure decisions of Nathusius' pipistrelle Pipistrellus nathusii migrating over the North Sea. The model was applied to both acoustic and tracking data, enabling comparable analyses across methods and incorporating uncertainty in migration dates of tracked bats. Additionally, we examined nightly offshore bat occurrence to further explore differences in movement patterns detected by the two techniques. Wind conditions at 200 m above sea level were identified as key driver of Nathusius' pipistrelle spring migration. In May-June, most bats migrated from the United Kingdom under westerly and northwesterly tailwinds. Tracked individuals flew in stronger supportive winds than acoustically recorded bats, which were also detected under crosswinds and headwinds. In March-April, acoustic detections occurred mainly during strong southerly winds, suggesting that early-season migrants largely consisted of individuals migrating over the European mainland and drifted northwards onto the North Sea by strong crosswinds. Acoustic detectors primarily recorded bats that landed on offshore platforms, likely because they were unable to cross the North Sea in a single flight due to less favorable wind conditions, or because they departed from more inland locations. In contrast, tracking data mainly represented bats that successfully crossed the North Sea in a non-stop flight under moderate supportive tailwinds. Synthesis and applications: Combining observation techniques improves our understanding of bat migration patterns. Additionally, acoustic monitoring can capture migration from different geographic origins. Current mitigation measures for offshore wind farms at the North Sea rely solely on acoustic data, likely overlooking the part of the population that crosses over sea with optimal wind support. Acoustic and tracking data are therefore complementary rather than contradictory, and both methods should be used together when developing mitigation measures. ### Competing Interest Statement The authors have declared no competing interest. Ministry of Infrastructure and Water Management (Offshore wind ecological program) Ministry of Agriculture, Nature and Food Quality (Nature inclusive energy transition program) Kepwick Ecological Services Norfolk & Norwich Bat Group

Abstract Migrating bats alternate between stopover periods and directed flights. When departing from a stopover site, bats select the night, the specific time within the night, and the flight direction to resume migration. Despite their ecological importance, the factors shaping these stopover departure decisions remain poorly understood. To identify the intrinsic and environmental factors driving departure decisions and movement patterns, we tagged Nathusius’ pipistrelles Pipistrellus nathusii at three coastal locations in the Netherlands and tracked 178 individuals during autumn migration, using the MOTUS Wildlife Tracking System. We examined movement patterns and analysed departure probability in relation to a set of individual and environmental covariates in a Bayesian capture-recapture model in state-space formulation. Additionally, we modelled within-night variation in departure timing. Seasonal patterns were strongly influenced by reproductive behaviour, with decreased migration probability during the mating period. Regardless of their seasonal timing, bats departed under moderate tailwinds and dry conditions, optimizing energy efficiency, while avoiding crosswinds and cloud cover, enhancing navigational safety. Most individuals departed shortly after sunset, whereas headwinds delayed nocturnal departure. Movement patterns were diverse, including migration towards lower latitudes, coastal barrier movements, and long-distance roundtrips, suggesting the use of multiple movement strategies. Our study demonstrates that migration patterns in bats emerge from the interaction between intrinsic factors and external conditions, and highlights the importance of both energy efficiency and safety in shaping stopover departure decisions. The presence of multiple movement strategies complicates predictions of spatiotemporal occurrence, emphasising the need to account for behavioural variability in conservation planning, for example in the context of wind energy developments.

Accelerated biodiversity loss has destabilized functional links within and between ecosystems. Species that cross different ecosystems during migration between breeding and nonbreeding sites are particularly sensitive to global change because they are exposed to various, often ecosystem-specific, threats. Because these threats have lethal and nonlethal effects on populations, many migratory species are declining, making this group especially vulnerable to global change. To mitigate their decline, research at a continental and flyway scale is required to adequately monitor changes in the migratory and demographic processes of populations during all parts of the annual cycle. The Motus Wildlife Tracking System (Motus) could provide a solution to data gaps that exist for small, migratory species. Motus is an automated telemetry system for animal tracking that uses a single very-high-frequency radio signal to track tagged individuals. Motus can provide information on movements made by individuals of small migrant species, thereby aiding the understanding of aspects of their migration that could affect demographic parameters. Conservation-focused research opportunities related to Motus include identification of critical stopover sites that support and connect multiple species and insight into migratory decisions in small migrant birds related to environmental stressors, such as artificial light at night. Examples of stopover studies from the existing network that demonstrate its utility include identification of a high-conservation-value stopover area for the blackpoll warbler (Setophaga striata) in the eastern United States. Geographical gaps in the network across the Mediterranean region and across eastern Europe need to be filled to track continent-wide movements. Motus can provide individual-level migration information for a variety of small-bodied taxa, and a drive to expand the network will improve its ability to direct conservation plans for such species.

Migratory bats are experiencing substantial increases in mortality risk from wind energy developments, but data on their migratory behavior and population dynamics are often lacking. Here, we develop a novel microsatellite panel for one such migratory bat species, the Nathusius’ pipistrelle (Pipistrellus nathusii), and apply it to 448 samples collected at stopover sites along the Dutch coast during autumn migration over four consecutive years. With this dataset, we assessed whether the population is genetically sub-structured, characterize its current genetic diversity, and evaluate whether mothers guide their offspring during migration. We found that the population is panmictic and diverse, with an effective population size estimate that cannot be distinguished from infinite. However, we also observed a consistent decline in allelic richness across the sampling period, as well as a heterozygote excess in individuals sampled as juveniles, both suggesting an ongoing population decline. We did not find any parent-offspring pairs in our dataset, which included 30 box captures where adult female and juvenile bats were found roosting together, suggesting that juvenile bats do not follow their mothers during their first migration. Our findings provide an initial characterization and baseline measure of genetic diversity for the Nathusius’ pipistrelle that can be used as a reference for subsequent studies and systematic efforts to monitor the genetic diversity of the species. Given that monitoring population trends of migratory bat species with traditional methods remains challenging, such tracking of genetic diversity may offer a valuable proxy by which to observe substantial population declines if they occur.

The timing of animal migrations is an interplay between migration programmes and physiological and environmental conditions. The rate of body store deposition underlies many timing options, with a key unresolved question being whether the rate of body store deposition and resource constraints limit advancements in migration departure. To study limits of deposition on departure timing, we captured red knots ( Calidris canutus islandica ) wintering in the Wadden Sea and manipulated hours of food access per day during spring body store deposition and prenuptial moulting phases. These birds were then released and tracked until spring migration departure from the Wadden Sea. When birds were allowed to feed more hours per day in captivity, they did not start body store deposition and plumage moult earlier, but gained stores faster, were heavier and had further progressed in prenuptial feather moult at the moment of release. After release, birds that had ad libitum access to food in captivity departed earliest from the Wadden Sea, and departure timing was explained by the rate of mass deposition in captivity. We experimentally demonstrate flexibility in the timing of annual‐cycle processes during spring migration. Red knots are flexible to adjust the rate of mass gain and moult to food availability, in turn allowing them flexibility in departure timing. This indicates that improvements in foraging conditions at staging sites will facilitate earlier departures, which will help migratory birds to keep pace with global warming. Read the free Plain Language Summary for this article on the Journal blog.

Migratory birds encounter a large variety of parasites and pathogens en route and invest in immune defences to limit the risk and fitness costs of infection. Since both migration and immune defences carry costs, individuals on tight budgets may face trade‐offs between migratory progress and immune status. Many species alternate legs of strenuous migratory flight with stopovers during which birds refuel, rest, and recover physiologically. Despite this, most time and energy consumed during migration are actually spent on stopovers. As a result, identifying what determines stopover duration is key in understanding how migratory birds balance investments in immune defences and migration. Yet, it is unknown under what conditions an individual's immune status may affect migratory progress through the duration of stopovers. We explored whether immune status at arrival affects stopover duration by radio‐tagging and blood‐sampling common blackbirds Turdus merula during autumn stopovers on the Dutch island of Vlieland. To measure immune status, we quantified levels of bacterial killing ability, natural antibodies, complement, and haptoglobin, as well as heterophil–lymphocyte ratios. We show that stopover departures peaked during periods with low cloud cover and strong tailwinds. While lean birds prolonged stopovers, we only found a weak tendency of prolongation in birds with elevated haptoglobin levels. We conclude that effects of immune status on minimum stopover durations are subordinate to those of condition, cloud cover, and tailwinds in autumn‐migrating common blackbirds. Hence, future studies on the link between immune defences and stopover durations should take weather conditions into account.

Migratory bats perform seasonal movements between their summer and winter areas. When crossing ecological barriers, like the open sea, they are exposed to an increased mortality risk due to energetically demanding long-distance flights and unexpected inclement weather events. How such barriers affect bat migratory movements is still poorly known. To study bat migration patterns in response to an ecological barrier, we tagged 44 Nathusius’ pipistrelles Pipistrellus nathusii with radio-transmitters on the East coast of the United Kingdom (UK) in spring 2021 and 2022. Subsequently, we assessed their movements to continental Europe using the MOTUS Wildlife Tracking System. We investigated route selection, timing of migration, overall migration speed and the influence of wind on airspeed, groundspeed and flight altitude during migratory overseas flights. Barrier effects cause migratory movements along the coast, and crossings over sea are shortened by deviating from the general migration direction. Males depart from the UK later in the season compared to females. The overall migration speed of females was 61 km/day and 88 km/day after their last detection in the UK. Our estimated airspeeds during oversea flights correspond well with airspeeds measured in a wind tunnel. Bats use wind adaptively to reduce airspeed when flying under tailwind and increase airspeed when flying under crosswind conditions. Departures over sea coincidence with tailwinds, enabling bats to more than double their airspeed, reaching ground speeds of up to 16.8 m/s (60.5 km/h). Our analysis suggests that bats select altitudes with favourable wind conditions and that they seek altitudes of several hundred meters, possibly extending up to 2,500 m. Low-altitude migration occurs when wind conditions are less favourable. Our study demonstrates that bat migratory movements are highly influenced by barrier effects, sex-biased timing of migration and the adaptive use of winds. The results of our study contribute to a more comprehensive understanding of the decision-making process and adaptations bats employ during their migration. Elucidating bat migration patterns will enable us to develop effective conservation measures, for example in relation to the development and operation of coastal and offshore wind farms.

Accelerated biodiversity loss during the Anthropocene has destabilised functional links within and between ecosystems. Migratory species that cross different ecosystems on their repeated journeys between breeding and non-breeding sites are particularly sensitive to global change because they are exposed to various, often ecosystem-specific threats. As these bring both lethal and non-lethal population impacts, many migratory species are declining, making this group especially vulnerable to global change. To mitigate their decline, research at a continental and flyway scale is required to adequately monitor changes in the demographic processes of populations and understand the needs of migratory species, during all parts of the annual cycle. The Motus Wildlife Tracking System (Motus) could provide a solution to data gaps that exist particularly for small and migratory species. Motus is an automated telemetry system for animal tracking, which originated in North America. It provides a collaborative network by using the same VHF radio frequency for all tracked individuals, in combination with an individual tag identifier. Motus can provide information on movements made by individuals of the smallest bird and bat, and even larger insect species, thus aiding our understanding of aspects of their migration that could impact demographic parameters. Here we emphasise conservation-focused research opportunities, with a particular lense on European migrant taxa. We highlight examples from the existing network, and identify geographical gaps in the network which need to be filled to track continent-wide movements. We conclude that Motus is a useful tool to produce individual-level migration information for a variety of small-bodied taxa, and that a drive to expand the network will improve its ability to conservation plans for such species.

Offshore wind farms likely cause mortality amongst migratory bats. Yet it remains unknown whether resident coastal bat populations may be affected by offshore wind developments. We performed an analysis to assess the potential risk of offshore wind farms in the Dutch North Sea for local coastal populations of noctule (Nyctalus noctula). First, we assessed the potential overlap between their foraging range and areas with operational and planned offshore wind farms. Subsequently, we tracked 14 noctules from a coastal population during late summer and autumn and analysed their movements. In general, it seems unlikely that offshore wind farms in the Netherlands will significantly affect coastal populations of noctule since offshore wind developments take place beyond their regular foraging range. In some cases however, noctules do perform distant flights (‘swarm flights’), possibly in response to migrating insects. We recorded six distant foraging trips both over land and over sea with a maximum distance of 18.5 km from their roost and 12.7 km from shore. Acoustic records confirm that noctules are occasionally present in offshore wind farms at distances of 15-25 km from shore. During such an event, noctules face the risk of a collision as virtually all their flight activity occurs at heights within the rotor swept area of offshore wind turbines.

Migrating birds flexibly adjust their individual migratory decisions, i.e. departing, routing and landing, based on intrinsic (e.g. energy stores) and extrinsic (e.g. landscape features and weather) factors modulating the endogenous stimuli. So far, these decisions have mostly been studied separately. Notably, we lack information on which factors landing decisions during active flight are based on. Therefore, we simultaneously recorded all three decisions in free-flying long-distance migratory songbirds in a coastal stopover area via regional-scale radio-telemetry and related them to the prevailing weather. Birds departed under favourable weather conditions resulting in specific nights with increased departure probability. Once departed, birds could either fly offshore or take a route along the coast, which was predicted by wind support. Radio-tracking revealed that departed individuals more likely interrupted their migratory endurance flight under overcast or headwind conditions. Studying departure, routing and landing decisions in concert, we highlight the importance of weather as a common driver across all migratory decisions. By radio-tracking individuals between stopovers, we provide evidence that avoidance of adverse weather conditions is an important function of stopover. Understanding how birds adjust migratory decisions and how they affect the timing of migration and survival is key to link migration performance to individual fitness.

Seasonal movements between the summer and winter areas are a widespread phenomenon in bats So far, most information on the migration ecology of bats has been obtained by studies in terrestrial habitats, whereas scientific knowledge on migration over sea is scarce. We performed continuous ultrasonic acoustic monitoring at 13 locations in the southern North Sea during four consecutive years (2017–2020) and analysed the spatiotemporal occurrence of Nathusius’ pipistrelle Pipistrellus nathusii during autumn migration in relation to weather parameters and lunar phase. Our analysis showed that the main autumn migration of Nathusius’ pipistrelle at the southern North Sea occurs from mid-August until late October and most bats within the study area occur off the Noord Holland coast. North Sea crossings frequently last longer than one night; the day is spent roosting at an offshore structure. The strongest migration occurs during nights with tailwinds from the east-northeast, but bats are also recorded offshore with low to moderate headwinds or crosswinds. Bat presence decreased between the full moon and the last quarter and increased just before the new moon. Finally, our observations show that the occurrence of bats at sea was reduced in 2020 in comparison to the previous years. The results of this study show clear spatiotemporal patterns of migratory bat occurrence at the southern North Sea. The spatial distribution can be used in spatial planning of future offshore wind farms, whereas the temporal occurrence and environmental factors that shape offshore migration can be used to develop mitigation measures to reduce the number of bat fatalities.

Why and how new migration routes emerge remain fundamental questions in ecology, particularly in the context of current global changes. In its early stages, when few individuals are involved, the evolution of new migration routes can be easily confused with vagrancy, i.e. the occurrence of individuals outside their regular breeding, non-breeding or migratory distribution ranges. Yet, vagrancy can in theory generate new migration routes if vagrants survive, return to their breeding grounds and transfer their new migration route to their offspring, thus increasing a new migratory phenotype in the population. Here, we review the conceptual framework and empirical challenges of distinguishing regular migration from vagrancy in small obligate migratory passerines and explain how this can inform our understanding of migration evolution. For this purpose, we use the Yellow-browed Warbler (Phylloscopus inornatus) as a case study. This Siberian species normally winters in southern Asia and its recent increase in occurrence in Western Europe has become a prominent evolutionary puzzle. We first review and discuss available evidence suggesting that the species is still mostly a vagrant in Western Europe but might be establishing a new migration route initiated by vagrants. We then list possible empirical approaches to check if some individuals really undertake regular migratory movements between Western Europe and Siberia, which would make this species an ideal model for studying the links between vagrancy and the emergence of new migratory routes.

Abstract The wind energy‐bat conflict is well documented for the onshore sector, with high numbers of casualties, specifically for migratory bat species. Offshore wind turbines might be a threat to bats as well, yet offshore bat migration is poorly documented. Accordingly, potential conflicts between bat conservation and offshore wind energy production are difficult to evaluate. Here, we used automated radio‐telemetry to track 50 km continuous offshore movements of two Nathusius' pipistrelles (Pipistrellus nathusii) within the Motus network. After crossing the marine waterbody, tagged bats traveled over several hundred kilometers along the coastline from Germany towards the Netherlands and Belgium. Our study highlights the possibility for migratory bats to collide with offshore and coastal wind turbines. Therefore, we plead for implementing pre‐ and post‐construction surveys and adequate mitigation schemes at offshore wind turbines in sensitive areas of the North and Baltic Sea if not already practised.

Bats regularly migrate over the North Sea, but information on the environmental conditions when this occurs is scarce. Detailed information is urgently needed on the conditions under which bats can be expected offshore, as the number of offshore windfarms that can cause fatalities amongst bats in the North Sea is increasing rapidly. We performed ultrasonic acoustic monitoring at multiple nearshore locations at sea between 2012 and 2016 for, in total, 480 monitoring nights. We modelled the offshore occurrence of Nathusius’ pipistrelle in autumn as a function of weather conditions, seasonality, and the lunar cycle using a generalized additive mixed model (GAMM). We investigated which covariates are important using backward selection based on a likelihood ratio test. Our model showed that important explanatory variables for the offshore occurrence of Nathusius’ pipistrelle are seasonality (night in year), wind speed, wind direction, and temperature. The species’ migration is strongest in early September, with east-northeasterly tailwinds, wind speeds < 5 m/s, and temperatures > 15 °C. Lunar cycle, cloud cover, atmospheric pressure, atmospheric pressure change, rain, and visibility were excluded during the model selection. These results provide valuable input to reduce bat fatalities in offshore wind farms by taking mitigation measures.

To determine home range and habitat use of common noctules, we equipped 56 individuals with GPS tags: 31 females (8 young of the year [YOY] and 23 adults [AD]) and 25 males (13 YOY and 12 AD). Of these, four females (AD) and two males (1 YOY and 1 AD) were tagged twice. Of the resulting 62 ‘deployments’ (i.c. deployed devices) 40 GPS tags with data could be retrieved. On average each recovered deployment yielded six monitored nights (range 1–11), whereas the number of GPS fixes per deployment was on average 29 fixes (range 3–75). The GPS fixes (n=1145) were used to determine flight paths per tagged individual and per group (YOY and AD females, YOY and AD males). Of these GPS fixes 17 fixes were obtained above sea, of which three occurred close to the Wadden Sea coast and 14 above the North Sea up to 2.7 km from shore. Home range sizes were estimated using the AKDEc method for 31 individuals. Home range estimates of adult females only were used for the habitat use analysis, since the number of GPS fixes and the number of individuals in the other groups was too small for an analysis of habitat use. From our current study we conclude: • that the AKDEc home range of adult female common noctules is on average 45 km2 and extends not further than a few kilometres onto the North Sea. • that adult females are almost completely ‘terrestrial’ and show the least preference for the maritime habitat. When they occur at sea, this is probably driven by specific weather conditions. • that adult females face low risks of potential negative impacts from offshore wind farms outside the 12 mile zone. • that home ranges could not reliably be assessed for adult male, young male and young female common noctules due to a lack of data. In comparison to adult females, however, the home range of adult males seem to be larger, while the home ranges of young males and young females are even larger than those of adult males. • that habitat use could not reliably be assessed for adult males, young males and young females, due to small sample sizes. • that adult male common noctules face possibly a slightly higher risk in comparison to adult females from offshore wind turbines. • that young male and young female common noctules face uncertain risk of potential negative impacts from offshore wind farms outside the 12 mile zone • that our results are valid for Ananas and ‘t Wildrijk, and extrapolation of these conclusions to home ranges and habitat use of common noctules in other areas in the Netherlands is not necessarily straightforward. Though outside the scope of this study, our previous bat detector work concluded: • that common noctules are detected in the offshore wind farms outside the 12 mile zone To answer remaining questions we recommend: • to obtain more data from the study area in late summer, put more emphasis on young males, young females, and adult males • to tag common noctules in other areas, in particular close to the current offshore wind farms.

Offshore wind farms' impact on vulnerable species, such as birds and bats through mortality, habitat loss, and barrier effects, is receiving growing attention worldwide. Focus on the effects of wind energy on species is important since the European Union, as well as the international community, strive to halt biodiversity loss through the Nature Conversation Directives and the Sustainable Development Goals. We argue that it is necessary and possible to protect vulnerable species better by quantifying the impact of wind energy on populations, avoiding sensitive areas for species as wind energy development areas, and developing guidelines and measures to mitigate the impact of wind energy. The recognition of existing uncertainties requires more investments in (at least) nationally coordinated monitoring, reporting and adaptive management. This article uncovers barriers in the existing legal framework based on state-of-the-art ecological research. It provides several policy recommendations for improvement, such as a policy guideline, adaptive permitting, and prioritizing (cumulative effects in) environmental impact assessments in wind energy decision-making procedures.

Aerial surveys to estimate the abundance of Harbour Porpoise Phocoena phocoena were conducted on the Dutch Continental Shelf in summer 2019. These surveys followed predetermined track lines in four areas: A - Dogger Bank, B - Offshore, C - Frisian Front & D - Delta. Between 16 July and 4 August the entire Dutch Continental Shelf (DCS) was surveyed. Marine mammals were assessed using line transect distance sampling methods. Density and abundance estimates were calculated. In total, 150 sightings of 189 individual Harbour Porpoises were collected. Porpoise densities varied between 0.54-1.76 animals/km² in the areas A-D. The overall density was 0.66 animals/km². The lowest density (0.46 animals/km²) was recorded in area A – Dogger Bank. The densities in the other areas were in the same order of magnitude, ranging between 0.68-071 animals/km². In summer 2019 the total number of Harbour Porpoises on the Dutch Continental Shelf (areas A-D) was estimated at 38,911 individuals (CI = 20,791-76,822). This estimates falls in the range of abundance estimates since 2010, with a minimum of 25,998 (CI = 13,988 – 53,623 in 2010) and a maximum of 76,773 (CI = 43,414-154,265 in 2014) individuals. The confidence intervals of the abundance estimates overlap, indicating no statistically significant differences between the years. The time series, however, is relatively short to measure trends. These abundance estimates show that up to a fifth of the North Sea population, estimated at 345,000-361,000 individuals, has been present on the Dutch Continental Shelf during the summer surveys in 2010-2019. The results of these aerial surveys will feed into the OSPAR MSFD indicator on abundance and distribution of marine mammals. In total 26 sightings of other marine mammal species than Harbour Porpoises were recorded. These comprised 22 sightings of seals (Grey Seal Halichoerus grypus and Harbour Seal Phoca vitulina). The majority of the seals was observed in coastal waters off the Wadden Isles. Three single Minke Whales Balaenoptera acutorostrata were seen (feeding) in area A – Dogger Bank and B – Offshore, with another one sighted off effort in the same area. One sighting of a pod of two White-beaked Dolphins Lagenorhynchus albirostris was made in area B – Offshore. This research is part of the BO-project 'monitoring bruinvis'.