Flight behaviour characteristics such as flight altitude and avoidance behaviour determine the species‐specific collision risk of birds with wind turbines. However, traditional observational methods exhibit limited positional accuracy. High‐resolution GPS telemetry represents a promising method to overcome this drawback. In this study, we used three‐dimensional GPS tracking data including high‐accuracy tracks recorded at 3‐s intervals to investigate the collision risk of breeding male Montagu's Harriers Circus pygargus in the Dutch–German border region. Avoidance of wind turbines was quantified by a novel approach comparing observed flights to a null model of random flight behaviour. On average, Montagu's Harriers spent as much as 8.2 h per day in flight. Most flights were at low altitude, with only 7.1% within the average rotor height range (RHR; 45–125 m). Montagu's Harriers showed significant avoidance behaviour, approaching turbines less often than expected, particularly when flying within the RHR (avoidance rate of 93.5%). For the present state, with wind farms situated on the fringes of the regional nesting range, collision risk models based on our new insights on flight behaviour indicated 0.6–2.0 yearly collisions of adult males (as compared with a population size of c. 40 pairs). However, the erection of a new wind farm inside the core breeding area could markedly increase mortality (up to 9.7 yearly collisions). If repowering of the wind farms was carried out using low‐reaching modern turbines (RHR 36–150 m), mortality would more than double, whereas it would stay approximately constant if higher turbines (RHR 86–200 m) were used. Our study demonstrates the great potential of high‐resolution GPS tracking for collision risk assessments. The resulting information on collision‐related flight behaviour allows for performing detailed scenario analyses on wind farm siting and turbine design, in contrast to current environmental assessment practices. With regard to Montagu's Harriers, we conclude that although the deployment of higher wind turbines represents an opportunity to reduce collision risk for this species, precluding wind energy developments in core breeding areas remains the most important mitigation measure.

SummaryCurrently, renovation and thermal insulation of buildings is happening at a high rate in many European countries, driven in part by the political aim to reduce greenhouse gas emissions. Unfortunately, building renovations often lead to a loss of structures, such as accessible cavities, on which synanthropic species, for example house-nesting birds, depend. In Germany, due to legislative regulations, it is a common practice to install nest-boxes as compensation for destroyed nest sites of birds. However, studies on the efficacy of such measures remain sparse. We monitored the occupancy of 477 compensatory nest-boxes for Common Swifts Apus apus, predominantly placed on renovated prefabricated buildings in a city in Germany. We found 24.3% of the boxes occupied by Swifts. On most buildings, the number of occupied boxes was as high as or even higher than the assumed number of breeding sites prior to renovation. Furthermore, in a district where nearly all buildings had been renovated in the past 10 years, we recorded a remarkably high density of Swifts breeding in nest-boxes. Using boosted regression trees, we analysed whether eight different nest-box properties influenced box occupation probability. The number of neighbouring boxes was the most important. Additionally, box age, facade orientation, city district, relative and absolute height, and manner of installation (external/internal) also played a role. Between different nest-box types, we found only negligible differences in occupation probability. Our findings suggest that installing nest-boxes is likely to be an appropriate measure to compensate for nesting sites of Swifts lost during building renovations. Based on our results, we recommend mounting the boxes a few metres apart from each other and close to the roof edge to maximise success. Further studies should be carried out to assess whether our results and conclusions can be confirmed in other situations.

Summary Hen Harrier Circus cyaneus and Short-eared Owl Asio flammeus are open-country birds of prey with overlapping distributions. Although both species face similar conservation threats across their ranges, work to date has largely been undertaken at a national scale with few attempts to collate and assess factors relevant to their conservation at an international scale. Here we use an expert knowledge approach to evaluate the impact of conservation threats and the effectiveness of conservation strategies for each species across Europe. We report results of responses to a questionnaire from 23 Hen Harrier experts from nine countries and 12 Short-eared Owl experts from six countries. The majority of responses for both species reported declines in breeding numbers. The perceived impact of threats was broadly similar for both species: ecological factors (predation, extreme weather and prey availability), changes in land use (habitat loss and agricultural intensification) and indirect persecution (accidental nest destruction) were considered to be the greatest threats to breeding Hen Harrier and Short-eared Owl. Short-eared Owl experts also highlighted lack of knowledge and difficulties associated with monitoring as a major conservation challenge. Despite broad-scale similarities, geographical variation was also apparent in the perceived importance of conservation threats, with some threats (such as direct persecution, large-scale afforestation or habitat degradation) requiring country-specific actions. Implementation of different conservation strategies also varied between countries, with the designation of protected areas reported as the most widespread conservation strategy adopted, followed by species and habitat management. However, protected areas (including species-specific protected areas) were perceived to be less effective than active management of species and habitats. These findings highlight the overlap between the conservation requirements of these two species, and the need for collaborative international research and conservation approaches that prioritise pro-active conservation strategies subject to continued assessment and with specific conservation goals.

Capsule: We developed a protocol for efficient monitoring of potential Common Swift Apus apus nest sites which considers variation in nest visit frequency across the breeding season and in relation to time of day and weather. Aims: To investigate patterns of nest visit frequency in Common Swifts in order to improve the efficiency and reliability of the monitoring of nest sites threatened by building renovations. Methods: We derived information on nest attendance from light data recorded by geolocators from ten adult Common Swifts during three breeding seasons (n = 686 individual sampling days) and analysed how nest visit frequency varied across the breeding season and in relation to time of day and weather. Results: The mean nest visit frequency was 5.63 visits per bird per day (0.32 visits per hour of daylight). The daily number of visits was highest at the beginning of July during chick-rearing. Moreover, it was positively correlated with temperature and negatively correlated with rainfall and wind speed. Nest visit frequency showed a distinct peak around sunset, while also being relatively high in the morning and around noon. Conclusion: We recommend monitoring potential Common Swift nest sites in Central Europe between the end of June and mid-July during good weather between 0.50 and 7.75 h after sunrise or between 3.00 h before sunset and sunset, when observation bouts of 0.5-2.0 h provide an encounter probability greater than 90%. Our study shows that repurposing geolocator light datausually used to study bird migrationfor investigating nest attendance in cavity-breeding birds can provide important information for bird conservation.