Population recovery and spatial determinants of occupancy and breeding success in the Saker Falcon (Falco cherrug): A study from Western Romania

Despite the lowland area in Western Romania offering high-quality habitats for Saker Falcons, with abundant mammalian prey like the European Ground Squirrel (EGS), and proximity to the strong Hungarian population, Sakers mostly visited the region with few breeding attempts recorded before 2014. This changed after installing 83 aluminum nest boxes on high-voltage transmission line towers in a European Union funded conservation program, which encouraged settlement and led to a rapid growth in the Saker population. Our study, covering 2016–2023, analyzed population changes, focusing on nest box occupancy, nesting success, and brood size. Furthermore, we also investigated covariate effects by using generalized linear (mixed) models. By 2023, the population reached 43 pairs, with the nearest neighbor distance between occupied nest boxes decreasing from 9.9 km in the second year to 6.1 km in 2023. We evaluated 255 breeding attempts, 192 of which were successful, resulting in 794 young, while 63 attempts failed. The mean nesting success rate was 0.752±0.432 (n = 255), mean brood size per successful pair was 3.135±1.15 (n = 192), and mean brood size per all breeding attempts was 2.361±1.682 (n = 255). Evidence suggested that nest box occupancy positively correlated with the distance to nearest settlement, the cumulative grassland area within ten km, and the number of EGS colonies within 5 km of the nest boxes. The distance of nest boxes to the nearest EGS colony significantly and negatively impacted nesting success. Our results confirm that installing nest boxes can quickly establish a healthy population if nesting sites are the only limiting factor. These findings contribute to a more effective planning of future Saker conservation measures.