Great Horned Owl

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 rabbits, muskrats, turkeys, mice, and voles. During 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 intends to determine whether the owl population inhabiting the Tittabawassee River floodplain is experiencing adverse effects from site-specific contaminants (dioxins and furans); the specific effects those contaminants may be having on the birds’ health will also be determined. This work is structured to examine three lines of evidence: dietary exposure, tissue-based exposure, and productivity variables.

Dietary exposure is assessed by analyzing prey remains and tissue concentrations of prey items, such as mice, voles, shrews, rabbits, muskrats and birds from the floodplain.

Tissue-based exposure is assessed by analyzing addled eggs and plasma from blood samples. Blood samples are collected from nestlings by accessing natural and artificial owl nesting platforms and from adults through the use of mist nests.

Productivity variables including nest success, number of nestlings per nest, fledgling success, and abundance. They are quantified by accessing nesting platforms during the breeding season and utilizing call and 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 downstream of Midland, Michigan, to the Shiawassee National Wildlife Refuge. Sampling efforts were also conducted on the Saginaw River in Bay City near the Saginaw Bay.

The health of the receptor species’ population is determined by evaluating productivity, including 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 is determined by incorporating all the information obtained from the multiple lines of evidence.