Mortality and Disease
Adult owls are near the top of the food chain and are not threatened by many animals. Large raptors, such as eagles, may attack owls if food is scarce. Likewise, large owl species may attack smaller ones on occasion.
Owlets are much more vulnerable, however. The more chicks in a given brood, the less likely all will survive disease, malnutrition, or catastrophic weather. Predators of owlets include snakes, squirrels, possums and ravens. Ironically, all of these animals are preyed upon by species of larger owls.
The influence of humans can also lead to chick mortality. Chicks often die directly from human interference, such as deforestation and illegal hunting. Owls also frequently fly into cars, windows, or clotheslines. The mere presence of humans may cause the parents to abandon the nest and the young. Long-term studies have shown that the survival rate of owl chicks goes down as the number of humans in the area goes up.
Young adult owls are also vulnerable because of their lack of hunting experience. They could be pushed by their parents or other adults into unfamiliar hunting territory inhabited by other hostile adult owls. At least half of all young adult owls die before they reach their first birthday. The odds of survival increase greatly as they gain size and experience.
How long can owls live?
Most owls in the wild live about 10 years. However, owls can live much longer if conditions are favorable. Great Horned Owls in the wild have been observed to live for 19 years, whereas Long-eared Owls have lived as long as 27 years. Owls can live for many more years in captivity than they would in the wild. The record for longevity in captive Great Horned Owls, 38 years, is twice as long as the record for longevity in the wild.
Diseases and parasites
Feather lice can torment owls and can also pose health problems. Owls are also susceptible to a variety of parasitic worms. Fleas and flies pester owls, but they only hang around because of the decomposing materials typically found in owl nests.
Hepatosplentitis infectiosa strigum is a virus that is fatal to many owls, but some species, such as Barn Owls, have become immune to it. Owls are also vulnerable to pneumonia and tuberculosis. Pigeons, which are preyed upon by some species of owls, carry a parasitic protozoan that may cause a thick deposit to form in the throats of owls. This deposit can eventually cause an owl to choke to death.