Dietary biology of the Eurasian eagle-owl
In rare cases, other predators may kill a Eurasian eagle-owl. In Europe, there is one case of a white-tailed eagle killing an eagle-owl and there are at least four known incidents of eagle-owls being killed by golden eagles. Also, an eagle-owl was found among the prey remains at a golden eagle eyrie in Mongolia. An eagle-owl was also found amongst the prey in the nest of steppe eagles ("Aquila nipalensis") in Altai Krai, Russia. In 2016, a large female subadult Bonelli's eagle ("Aquila fasciata") apparently preyed upon a three-year-old male Eurasian eagle-owl in Spain. Pine martens ("Martes martes") and stone martens ("Martes foina") are opportunistic nest predators. Although martens could probably reach most eagle-owl nests due to their agility and climbing abilities, they probably will attack nests when disturbance or low food supply result in lower nest attendance by the parent owls. Cases where nests have atypically been accessible on foot (for both humans and wild predators) have resulted in Eurasian eagle-owl nests being predated. Eurasian badgers, wild boars and raccoon dogs and raccoons, the latter two both where non-native in Germany, have also reportedly preyed on nests that they can access. In coastal areas of Spain, red foxes were reported to have preyed on four eagle-owls, including one unsexed brooding adult, at nests found to be reachable on foot by researchers. Cases of red foxes killing Eurasian eagle-owls at nests have reported elsewhere albeit very rarely, normally the eagle-owl is a greater danger to the foxes (and many of its other nest predators) than vice versa. In May 2017, a red fox was filmed killing two chicks and raiding the larder of an eagle-owl in Denmark.
Eurasian eagle-owl
Apparently, the point at which the chicks venture out of the nest is driven by the location of the nest. In elevated nest sites, chicks usually wander out of the nest at 5 to as late as 7 weeks of age, but have been recorded leaving the nest if the nest is on the ground as early as 22 to 25 days old. The chicks can walk well at five weeks of age and by seven weeks are taking short flights. Hunting and flying skills are not tested prior to the young eagle-owls leaving the nest. Young Eurasian eagle-owls leave the nest by 5–6 weeks of age and typically can be flying weakly (a few metres) by about 7–8 weeks of age. Normally, they are cared for at least another month. By the end of the month, the young eagle-owls are quite assured fliers. There are a few confirmed cases of adult eagle-owls in Spain feeding and caring for post-fledgling juvenile eagle-owls that were not their own.
A study from southern France found the mean fledgling number of fledgling per nest was 1.67. In central Europe, the mean number of fledglings per nest averages between 1.8 and 1.9. The mean fledgling rate in the Italian Alps was 1.89, thus being similar. In the Italian Alps it was found that heavier rainfall during breeding decreased fledgling success because it inhibited the ability of the parents to hunt and potentially exposed nestlings to hypothermia. In the reintroduced population of eagle-owls in Eifel, Germany, occupied territories produced an average of 1.17 fledglings but not all occupying pair attempted to breed, with about 23% of those attempting to breed being unsuccessful. In slightly earlier studies, possibly due to higher persecution rates, the mean number of young leaving the nest was often lower, such as 1.77 in Bavaria, Germany, 1.1 in lower Austria and 0.6 in southern Sweden. While sibling owls are close in the stage between leaving the nest and fully fledged, about 20 days after leaving the nest, the family unit seems to dissolve and the young disperse quickly and directly. All told, the dependence of young eagle-owls on their parents last for 20 to 24 weeks. Independence in central Europe is from September to November. The young leave their parents care normally on their own but are also sometimes chased away by their parents. The young Eurasian eagle-owls reach sexual maturity by the following year, but do not normally breed until they can establish a territory at around two or three years old. Until they are able to establish their own territories, young eagle-owls spend their life as nomadic "floaters" and, while they also call, select inconspicuous perch sites unlike breeding birds. Male floaters are especially wary about intrusion into an established territory to avoid potential conspecific aggression.