The Eurasian eagle-owl is one of the largest owls in the world, and a Los Angeles Zoo page says it is the world's heaviest owl. Animal Diversity Web lists adult mass as 1,600 to 4,200 grams, length as 58 to 71 centimeters and wingspan as 1.5 to 2 meters.

Eagle-owls are elusive, which makes them difficult to study in their natural habitats. In Spain, researchers monitored 253 territories from ten subpopulations between 2004 and 2012, and genotyped 235 eagle-owls from ten subpopulations using eight microsatellite markers. Bayesian analyses of those markers revealed three genetic clusters, grouping subpopulations from central, south-western and south-eastern Spain.

Philopatry is the tendency to breed close to birthplaces, and the Spanish authors list it among traits reported to limit how far eagle-owls spread.

In Spain, the European rabbit crashed after two viral disease outbreaks: myxomatosis in the 1950s and rabbit haemorrhagic disease in the late 1980s. Scrub areas favour rabbits, which positively influences eagle-owl breeding success.

The Spanish authors write that, in recent times, less direct persecution and some conservation schemes have allowed the eagle-owl's demographic recovery. Unlike most large raptors such as eagles or vultures, the eagle-owl has high reproductive rates, which may have favored a rapid recovery in Spain.

Researchers who modelled eagle-owl territories in Spain found that breeding success in a territory in one year was the most important factor in whether it was reoccupied and bred successfully the next year. Occupancy models account for imperfect detection, which is failing to detect a species when it is in fact present. Past success and individual experience probably matter more for territory occupancy than climate, habitat and human activity, according to the Spanish study's authors.

Compared with other large birds of prey, eagle-owls, with their cryptic and nocturnal behaviour, seem to tolerate human closeness and human-caused disturbance fairly well.

In Norway, five of seven GPS-tracked adult eagle-owls made long excursions of more than 20 kilometers away from their breeding territories during autumn. The authors say this had not previously been reported for adults of the nominate subspecies, Bubo bubo bubo.

Owl pellets are very useful to scientists because they help them understand what these hard-to-find birds eat. A pellet study in southeastern France found that the pellets showed a broad dietary range, which reflects the generalist predator behaviour of the Eagle Owl. A 2019 study of pellets in Scientific Reports says the Eagle Owl is one of the main potential predators of small prey found in archaeological sites.

Eagle-owls are largely solitary except when pairing up to breed, and they often pair up for life. A Eurasian eagle-owl pair's young leave the nest area and usually stay in their parents' territory for another 3 to 4 months before looking for a territory of their own. A zoo raptor group says eagle-owl young are independent at 150 to 160 days of age.

Words to know

• philopatry: staying near birthplace
• occupancy: being present
• elusive: hard to find
• cryptic: hard to notice