A Case of Predation of a Eurasian Eagle-Owl by a Bonelli's Eagle

Superpredation, defined as an interaction in which a predator kills and eats another predator, has been widely studied previously (Lourenço et al. 2011b, 2014) and is a widespread phenomenon in raptor assemblages (Sergio and Hiraldo 2008). A particular case of superpredation is intraguild predation, in which a species preys on another species of the same guild that could be a potential competitor (Polis et al. 1989). Although sometimes overlooked in dietary analyses, superpredation can be an important mechanism regulating population dynamics that leads to behavioral responses among different members of the community (Lourenço et al. 2014). It is well known that the Eurasian Eagle-Owl (Bubo bubo) is one of the greatest top predators of the Palearctic region (Cramp and Simmons 1980, Ferguson-Lees and Christie 2001). However, the opposite situation, in which eagle-owls are predated by other raptors, is extremely uncommon (Mikkola 1976, Lourenço et al. 2011b). The Bonelli’s Eagle (Aquila fasciata) and Eurasian Eagle-Owl (Bubo bubo) are long-lived territorial species that coexist in sympatry in southwestern Europe, and are considered top avian predators in human-dominated Mediterranean ecosystems (Cramp and Simmons 1980, Ferguson-Lees and Christie 2001). Although these species show some degree of dietary and niche overlap (e.g., competition for prey and nest sites), eagles and owls reduce competition via temporal niche partitioning (Lourenço et al. 2011b): Bonelli’s Eagles concentrate their activities during the daytime, whereas owls are crepuscular. On very rare occasions, Eurasian Eagle-Owls prey upon Bonelli’s Eagles, particularly nestlings (Bayle 1987, Real and Mañosa 1990). According to previous work, three raptor species have been recorded killing Eurasian Eagle-Owls: Golden Eagles (Aquila chrysaetos) in Sweden, Finland, and Mongolia (Mikkola 1976, Ellis et al. 2000); White-tailed Eagle (Haliaeetus albicilla) in Sweden (Mikkola 1976); and Steppe Eagle (Aquila nipalensis) in Russia (Vazhov et al. 2013). However, there are no published detailed observations of a case of predation of a Eurasian Eagle-Owl by a Bonelli’s Eagle. To the best of our knowledge, there is only one reported case of consumption of a Eurasian Eagle-Owl by a Bonelli’s Eagle in the literature (Martı́nez et al. 1994), but no details were given that would allow determination of whether it was a case of predation or simply opportunistic scavenging. Here, we report a case of predation of a Eurasian Eagle-Owl by a Bonelli’s Eagle, which we documented during a remote tracking research project conducted in Sierra de Espadán Natural Park, Castellón, Spain (39852.0N, 0817.5W). During 2015, we trapped and equipped a territorial pair of Bonelli’s Eagles with solar GSM-GPS-ACC loggers (48g; e-obs GmbH, Gruenwald, Germany) using Teflon backpack-style harnesses. We programmed the tags to record GPS locations every 5 min from 1 hr before sunrise until 1 hr after sunset. The pair consisted of an adult male (.4 yr old, mass1⁄4 2000 g) that we trapped on 10 June 2015 and a subadult female (2 yr old, mass 1⁄4 2900 g) trapped on 6 November 2015. We determined these birds were paired based on observations of territorial and pair-bond behavior (e.g., often observed flying together; P. LópezLópez, C. Garcı́a-Ripollés, J. Giménez, and V. Urios unpubl. data). The predation event occurred on 28 November 2015, when the subadult female eagle roosted at 1755 H in the middle part of a hillside, very close to a few isolated pine trees (Pinus halepensis). She remained in the same place until 1135 H the next day (29 November 2015) and came back at 1710 H in that evening. She remained overnight and did not move until 1315 H the next day (30 November 2015). On 1 December 2015, the female returned again to the same place from 1000–1045 H, and from 1250–1325 H on 2 December 2015. The male eagle went with the female on 28 November 2015 at 1755 H. He remained close to the female at the predation location until 29 November 2015 at 0750 H. 1 Email address: Pascual.Lopez@uv.es