Intraguild predation and competition between avian top predators: do expanding Eagle Owls (Bubo bubo) contribute to the decline of Northern Goshawks (Accipiter gentilis)?

Abstract Understanding species coexistence is central in ecology. Competition and predation mix during intraguild predation (i.e. predators eliminating competitors). After recovery from historical crashes caused by persecution and pesticides, raptor populations can now be studied in more natural assemblages. We studied two avian top predators in a Dutch-German forest during 1969–2024. Here, Northern Goshawks ( Accipiter gentilis ) first increased (from 4 to max. 61 pairs) but recently decreased (from 40 to 23 pairs), while Eurasian Eagle Owls ( Bubo bubo ) only established in 2019 and increased (to 9 pairs). We studied three aspects of their interaction: competition for territories and food, and intraguild predation. Diet was compared during 2019–2023 (Goshawks, n = 1649 prey; Eagle Owls, n = 655 prey). Diet overlap was moderate at species level (Pianka index, numbers: 0.5, biomass: 0.6). Goshawks ate mainly pigeons, while Eagle Owls mainly took pigeons, rats, and hares/rabbits. Whereas overall intraguild predation frequency was 5% (Eagle Owl) and 6% (Goshawk), predation of Goshawks by Eagle Owls was minimal (n = 1). Accordingly, distance to the nearest Eagle Owl nest did not affect Goshawk territory persistence and breeding success. Still, 3 out of 18 Eagle Owl nest sites were (old) Goshawk nests. The number of Goshawk fledglings per nest was higher further away from Eagle Owls, but this only played at larger distances, likely due to other factors (e.g. urbanisation). Further, Goshawk output was higher closer to the forest edge. Thus, we found no indication that Eagle Owls have contributed significantly to the Goshawk decline. Both currently coexist, but the nocturnal predator thrives while the diurnal predator struggles. Goshawks under pressure may have limited options to cope with increasing Eagle Owls in the future.