Breeding spatial behavior of an Iberian dimorphic raptor (Circus cyaneus) based on GPS tracking

Abstract The Hen Harrier ( Circus cyaneus ) is a medium-sized, dimorphic raptor from Eurasia which usually hunts birds and small mammals. Studies on the spatial behavior of the species based on GPS-tracking devices are extremely scarce at the global level. Thus, here we GPS-tracked 24 adult Hen Harriers ( Circus cyaneus ) to study the overall patterns of the breeding spatial ecology of the species in Iberia, including home range sizes, cumulated distances, temporary variability, and the effect of latitude. The breeding season period of the tagged Hen Harriers started on 21 March ± 25 days (mean ± SD) and ended on 04 July ± 21 days, with a duration of 104 ± 18 days. We found an effect of latitude on the phenology, with northern individuals breeding later than southern individuals. Breeding home ranges (95% KDE as kernel density estimation ) averaged 53.7 ± 68.2 km 2 , while core areas (50% KDE) averaged 6.2 ± 11 km 2 . Home ranges were smaller during the central weeks of the breeding period, when females were arguably incubating, and progressively increased as chick-rearing progressed. Regarding fidelity to breeding areas, of the four females analyzed over 2 consecutive years, all returned to the same area to breed in the second year, with a measured overlap in home ranges of 51 ± 22 %, and core areas in 34 ± 40%. Future studies combining comprehensive field monitoring with remote tracking are necessary to further explore the behaviour of the species. Nonetheless, given that there are not GPS-based studies of Hen Harrier breeding ecology worldwide, our work provides a basic overview of the species that can be used as starting point for further studies.