What Kills Owls In Hawaii?
Biology: Hawaii has two species of owls, the non-endangered barn owl and the endangered Hawaiian owl or Pueo. The barn owl is slightly larger with lighter colored feathers wheras the pueo is smaller and darker. Both species are found on all the main islands. Both owls have a “radar shaped” face and excellent vision allowing them to use both sound and sight to locate prey. Barn owls can be heard screeching as they hunt at night. Both owls are carnivorous and eat small rodents and insects.
Barn owls were introduced into Hawaii the 1950s as way to control rodents. They are very common, and if you see a owl in Hawaii, it is most likely to be a barn owl. Like most owls, barn owls hunt during the night and are particularly fond of hunting in ecotones. Ecotones are transition zones from one habitat type to another (e.g. forest to grassland, grassland to dirt road, etc.). In Hawaii, edges of roads and forest or grassland are great ecotones frequented by rodents and insects. As a result, owls will often hunt and forage in those areas.
Threats to native species: Barn owls are not known to pose significant threat to native species in Hawaii, and it is not clear how or if they compete for resources with pueo.
Causes of mortality: Mortality surveys by the Honolulu Field Station of owls statewide have revealed three major causes of mortality.
Trauma: The most significant cause of mortality in owls is trauma, usually secondary to getting hit by cars. Because owls often hunt along roads, they are prone to getting hit by passing cars. This usually results in severe trauma including internal bleeding and broken bones. About 50% of owls that have been submitted for necropsy (autopsy for animals) have died from trauma.
Infectious disease: Diseases are an important cause of mortality in owls killing about 25% of birds. The most important disease in owls is a parasite called Trichomonas. The parasite causes large sores on the roof of the mouth making it very painful for the owl to eat. Affected birds will appear listless and depressed and will often have foul-smelling fluid leaking from the mouth and staining surrounding feathers. The easiest way to tell if a bird is affected is to look inside the mouth for the characteristic sore. The disease is invariably fatal, and owls probably get it from infected prey.
Starvation: About 20% of owls die from simple starvation. This may sound somewhat counterintuitive given that we do not lack for rats and mice here, however, the key is whether these prey are available to owls. Most owls that die from starvation are young birds that are perhaps inexperienced hunters. Also, owls form territories, and it is likely that adults displace some young birds from prime hunting habitat.
Miscellaneous: A minority (5%) of birds die from things like cat bites, drowning, or rodenticide poisoning. We have found little evidence that pesticides play a major role in owl mortalities.