Variability in spatial behaviour and migratory strategies selection in a global warming context: the case of the Hen Harrier at the southern limit of its distribution

Knowledge about the posreproductive movements of species is important for both conservation and ethology. This study investigates the non-breeding or posreproductive movements of Hen Harriers using GPS/GSM data from 30 individuals over 54 periods from 2019 to 2023. Using maximum and mean distances from the reproductive centroid as behavioural proxy, we classified posreproductive periods into in three strategies: migration (22.2%), sedentarism (9.3%), and sedentarism with posreproductive movements (68.5%). Our results challenge the previous belief that the majority of the Spanish Hen Harrier population was sedentary. The study establishes a relationship between distance travelled and latitude of breeding areas, suggesting environmental adaptation. The Spanish Hen Harrier population, being at the southern limit of its breeding range, exhibits less pronounced differences between sexes or strategies. Raptors with extensive migratory and post-breeding periods are prone to be more vulnerable due to the shortening of the area available for posreproductive movements.