Bubo virginianus



          Bubo virginianus are oviparous, meaning they bear their young in the form of eggs (Walters 1994). Before the female owls lay their eggs, they must find their mate and select a location to nest. About two months prior to mating, the owls find their mate (Alderfer 2005, Robbins Jr. 1994). Like most birds, the males attract the females (Kinstler 2009). The male advertises themselves by a variety of calls; the most common call heard to attract a partner is a short excited hoot (Elphick et al. 2001, Kinstler 2009, Robbins Jr. 1991). To download owl calls and hear this call, click here. Once the female has selected her mate, it is common for the pair to remain partners until one dies (Elphick et al. 2001). Next, the pair must find a site for their nest. Bubo virginanus usually do not make their own nests; instead they will choose a natural or manmade cavity or take over old nests. For example, great horned owls will take over an old red-tailed hawks nest (Elphick et al. 2001, Robbins Jr. 1994). These nests are typically found at mid canopy where cover is offered away from other species (Rohner et al. 2000). After mates are selected and nests are chosen, the female may lay her eggs; this usually occurs between January and April (Alderfer 2005).
         Actual reproduction of Bubo virginianus begins within the females' reproductive system. A male bird first fertilizes the egg; in the ovary the egg then becomes a developing embryo. As the egg gets carried into the magnum region, layers of yolk travel closely behind (Ritchison). Yolk serves as a food source for the embryo; it is made of lipids and proteins (Ritchison, Walters 1994). After an egg has been laid, the yolk also rotates and helps keep the embryo in an upright position (Walters 1994). After the embryo enters the magnum, the egg is coated with a layer of albumen (Ritchison). The albumen layer is the clear part of an egg; it is made of water and proteins that provide an extra coating of protection (Walters 1994). Next, the egg passes through the isthmus and gains a shell membrane (Ritchison). The egg continues in to the uterus where the hard shell is applied making the spherical egg shape specific to owls (Elphick et al. 2001, Ritchison). The hard outer shell is made of calcium, which provides protection for the embryo; it also includes an outer cuticle providing further protection from bacteria. The outer shell is also porous to allow for gasUsed with permission exchange (Walters 1994). Lastly, the egg passes through the vagina and cloaca to be laid. It is typical for great horned owls to lay 2-3 eggs per clutch (Robbins Jr. 1991). For further detail on how the eggs are made and general anatomy of birds, you can visit Professor Ritchison's webpage Avian Biology.
         After an egg has been laid, it must be incubated while it develops or it will not survive. Too high of temperatures will kill the embryo, where low temperatures will slow the development of the embryo and potentially kill it. Mothers achieve keeping their young warm by shedding feathers on their chest area and laying their exposed skin on the eggs; this gives direct heat to the eggs (Ritchison, Walters 1994). The female will incubate the eggs for about 35 days; the male will hunt and provide food for her during this time (Elphick et al. 2001). After complete development, the baby bird pecks creating cracks in the shell until it can eventually pull itself out (Ritchison). For the first couple weeks, the fledglings do no leave the nest; the father hunts and provides food for the young (Elphick et al. 2001). After about three months of the fledging period, the young owls are ready to leave the nest, but they cannot fly until they are about ten weeks old (Alderfer 2005, Elphick et al. 2001).  




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