Great Horned Owl (Bubo virginianus)
The great horned owl is a year-round resident in the Tittabawassee River floodplain. Sometimes called the tiger of the night, the mighty raptor hunts a widely diverse collection of prey, including shrews, mice, voles, rabbits, muskrats, songbirds, waterfowl and turkeysDuring the daylight hours, the owls tuck themselves in close to tree trunks for a well-deserved nap. By night, however, their powerful “whoo hoo hoo whoooo whoo!” can be heard filling the floodplain with the proud declaration of their presence and strength.
The great horned owl research was intended to determine whether the owl population inhabiting the Tittabawassee River floodplain experienced adverse effects from site-specific contaminants (dioxins and furans); the particular effects those contaminants may have had on the raptors’ health was also determined. This work was structured to examine three lines of evidence: dietary exposure, tissue-based exposure and productivity variables.
Dietary exposure was assessed by analyzing prey remains and tissue concentrations of prey items, such as shrews, mice, voles, rabbits, muskrats and songbirds from the floodplain.
Tissue-based exposure was evaluated by analyzing addled eggs and plasma from blood samples. Blood samples were collected from nestlings by accessing natural and artificial owl nesting platforms and from adults through the use of mist nests.
Productivity measurements included nest success, number of nestlings per nest, fledgling success and species abundance. They were quantified by accessing nesting platforms during the breeding season and utilizing call-response surveys conducted after dusk and prior to dawn.
Great horned owl data were collected in reference areas in Sanford, Michigan, and the Pine and Chippewa rivers at and around the Chippewa Nature Center, and study areas downstream of Midland, Michigan, to the Shiawassee National Wildlife Refuge. Sampling efforts were also conducted on the Saginaw River in Bay City near Saginaw Bay.
The health of the receptor species’ population was determined by evaluating productivity measurements, which included nest success, number of nestlings per nest, fledgling success and species abundance. Estimating the risk of adverse effects to great horned owls inhabiting the Tittabawassee River floodplain was determined by incorporating all the information obtained from the multiple lines of evidence.