Owls possess the best hearing and the best vision in dim light among birds.  Their eyes are so large that they cannot move inside the head so owls have evolved extra mobility in their neck to compensate.  In the retina of owls, the ratio of rods (the receptors which detect dim light in black and white) to cones (which perceive color in bright light) is 12.6 to 1 which is far greater than other birds such as chickens where this ration is 2.5 to 1.  Owl vision is so precise that they could navigate an area the size of four football stadiums using the light from a single candle.  Owls also have excellent hearing.  The hearing of owls is so developed that some owls (such as great gray owls) can hear mice under several inches of snow from 20 meters away well enough to plunge into the snow and capture them.  These owls have been observed to capture mice in snow as deep as 45 cm (Johnsgard, 1988). 

     Owls lack a crop and therefore they are not as able to digest hard materials as efficiently as other birds.  As a result, owl feces (known as owl pellets) contain more bones than do the feces of hawks, for example.  Owls can swallow large bites of food and often swallow small prey (such as mice) in one gulp.   The feet of owls are unusual among birds because they have two toes which face forward and two which face rearward.

      In almost all species of owl, females are larger than males.  In larger owls there is a greater degree of sexual dimorphism.  While large owls tend to stay in one habitat all year round, smaller owls often migrate.  The great horned owl probably has the greatest diversity of habitat and prey among owls. (Johnsgard, 1988). 

The feathers of an owl’s wing are especially adapted for silent flight.      Owls can vary considerably in size, as is evident in the South American species pictured below.

     Larger owls possess a larger vocal apparatus and therefore produce lower pitch sounds than smaller owls.  Lower pitch sounds travel farther through vegetation, which corresponds to the larger territories of larger owls.

Owls do not build nests, although they may excavate an area in a tree.  Most owl nests are holes in trees into which round, white eggs are laid.  Average clutch size varies in owl species: 2-3 eggs in great horned owls and barred owls, 4-5 in barn owls, and 6-7 in short-eared owls.  Reproductive success varies from under 30 % (short-eared owls), 30-40% (barn owls), about 70% (screech owls), to over 70% (great horned owls).  Between half and 75% of owls typically die in their first year (Johnsgard, 1988).  

     Owls seem to be related to nighthawks but it is possible that similarities between the two groups have resulted from convergent evolution.  Owls also may be related to oilbirds which have adapted to life in caves (Steatornis).

eagle owl