GENEVA COURTHOUSE OWLS, 2008

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This is the fourth year this pair of owls have used the courthouse site.
2005: They fledged 2 owlets
2006: They fledged 3 owlets on April 11, but one was later found dead
2007: They fledged 2 owlets on April 12 & 16
2008: On nest by January 12

Great Horned Owl, Bubo virginianus
Ht. 18-25"
Wingspan 36-60"
Weight 32-63 ounces
Females are always larger than males (this is generally true of all raptors).
Chiefly nocturnal
Call: 3-8 deep hoots (usually 5)
Ear tufts (wide apart)
Iris yellow
White throat
Breast streaked side to side (long-eared owl is up and down)
 
Prey: as large as skunk but usually rabbits, mice, birds, small inverts, insects

Habitat: forest, city, open desert (a pale form found on tundra)

Nest: in trees, caves, or on the ground
In Illinois they usually usurp an abandoned nest of a crow or hawk.
Philopatric - an animal or species tending to return to or remain near a particular site or area.
Monogamous pair bond
2-3 eggs  in our area (1-6 eggs possible)

Incubation: Both male and female incubate eggs from 30 to 37 days, averaging 33 days. Brooding is almost continuous until the offspring are about 2 weeks old, after which it decreases. Semialtricial 2 – The owlets are born immobile, downy, with eyes closed and must be fed. Young hatch asynchronously. This is an important adaptation of raptors, because when prey base is low, the strongest will survive. This also means that incubation begins as soon as the first egg is laid and the eggs will hatch on different days. Passerines, like our American robin, do not do this.
Young owls move onto nearby branches at 6 weeks and start to fly about a week later (some texts say 65 days). This is variable because of available food (prey base) and weather.
 
Feathers: Silent flight of owls is caused by two structural modifications to their feathers. The forward edge is serrated, which disrupts airflow over the wing eliminating the vortex noise created by airflow over a smooth surface. The very long barbules help minimize the rubbing of overlapping feathers and create a soft fuzzy feather texture.
 
Eyes: Eyes are immobile, “fixed” in their large head. Eyes are forward and create binocular vision. This provides a three dimensional sight (depth of field).
Nocturnal owl retinas are packed with rods, which are light receptors important for night vision.
Owls have the ability to rotate head 270° from side to side. They have 14 vertebrate in their necks, which help with flexibility while we only have 7 in ours. Remember, we can shift our eyes and they cannot.
 
Ears: Most highly developed sense of hearing of any bird. Hears sounds inaudible to humans. Ears are large asymmetrical vertical openings just below and beyond eyes. Large parabolic facial discs, “ruffs,” are feathers to amplify and channel sound to ears. Owl pinpoints location of prey within 1.5 degrees in both horizontal & vertical planes. Cue used to determine if sound comes from left, right or straight ahead is the difference in time it takes to reach each ear.
 
Head bobbing: When sound is received equally in each ear, prey is found.
 
Feet: Talons and feet are powerful, and are used to pith and kill prey. They have zygodactyl feet. This means the outer toe can be turned, creating a stronger ability to grasp.
 
Pellets or Castings are regurgitated. Pellets are formed in the gizzard (muscular stomach) within 6 hours after a meal. A freshly formed pellet passes from gizzard to the proventriculus (glandular stomach) where it remains until ejecting. The contents of a pellet cannot be digested or may harm the intestinal tract of the bird. Ejecting a pellet is a brilliant adaptation. Many species of birds use it. The owl is just the most famous.
 
Songbirds and crows will often "mob" an owl. Mobbing is a defensive behavior that occurs when birds locate an avian predator in their midst. Much is lacking in our understanding of mobbing. It is not clear why predators don't simply turn on their tormentors and snatch up one or two of the mobbing birds.
 
Resources used:
The Birder's Handbook, Ehrlich, 1988.2.
The Audubon Soc. Encyclopedia of North Am. Birds, Torres, 1982.
Ornithology, Gill, 1995.
www.wikipedia.org
 
Any mistakes are mine alone.
Comments and corrections to Ann Haverstock
Ann Haverstock, 02-18-08