We compared diets of Northern Barred Owls (Strix varia varia) and Northern
Spotted Owls (Strix occidentalis caurina) in western Washington during 1985
-89. Diets of both species were dominated by nocturnal mammals, but diets'
of Barred Owls included a more diverse and more even distribution of prey.
Estimated dietary overlap between the two species based on the Pianka Index
was 76%. Barred Owl diets included more terrestrial mammals, more birds, m
ore diurnal prey, and more prey that were associated with riparian areas, i
ncluding fish, amphibians, and snails. The snowshoe hare (Lepus americanus)
comprised 35% of prey biomass in the diet of Barred Owls. The diet of Spot
ted Owls was dominated by the northern flying squirrel (Glaucomys sabrinus)
, which comprised 51% of prey numbers and 57% of prep biomass. We speculate
that Barred Owls and Spotted Owls compete for food because their diets ove
rlap considerably, their food appears to be limiting in many years, and Bar
red Owls are gradually invading territories historically occupied by Spotte