Great Horned Owl Distribution and Habitat Use Relative to Post-Release Survival of Endangered Attwater's Prairie Chickens
Type of Culminating Activity
Master of Science in Raptor Biology
Mark R. Fuller
The success of predation by a raptor is determined in a large part by the vulnerability of the prey. Many ecological factors influence the vulnerability of prey, such as habitat quality, prey density, prey health, predator behavior, predator density and the accessibility of an area to predators. I use vulnerability similarly to Craighead and Craighead (1969): the sum of the environmental conditions that allow a species to be preyed on. For example, even with large prey densities in a prairie, if the vegetation provides cover and protects the prey from aerial hunting or perched raptors, the prey population is less vulnerable to predation. Prey vulnerability can be affected by the ecology of the predator species as well. If a predator is unable to use an area then the prey are not vulnerable even if the vegetation does not provide protection for the prey species.
Habitat use by prey and predators plays a crucial part in predator-prey interactions. An ecological approach can be used to modify the hunting behavior of a predator, such as hunting forays from a perch, by altering the natural interactions of predators and prey with their habitats (Herkert 1994, Jimenez and Conover 2001). Habitat can be altered to enhance cover for a prey species or to limit the hunting effectiveness of the predator species (Greenwood and Sovada 1996, Schroeder and Baydack 2001). Habitat alteration to modify predators' ability to search for prey species has been widely used by managers who work with prairie-nesting waterfowl (Greenwood and Sovada 1996). Currently most management plans for prairie grouse recommend management such as increased vegetation around nesting areas, and flooding of marshes to reduce predation (Westemeier and Gough 1999, Schroeder and Baydack 2001).
The Attwater's prairie chickens (Tympanuchus cupido attwateri) are the most endangered birds in the 48 conterminous states. Attwater's prairie chickens (APC) numbers once estimated to be nearly 1 million (Lehmann 1941) have declined dramatically reaching an estimated low of 42 in 1996. On the Attwater Prairie Chicken National Wildlife Refuge (APCNWR), the estimate was eight birds in spring surveys of 1998. This dramatic decline was driven by habitat loss through conversion to unusable habitat, habitat fragmentation, and invasion of natural habitat by woody shrubs and trees (Lehmann 1941, U.S. Fish and Wildl. Servo 1993).
Starting with a pilot project in 1995 captive-raised APC have been released yearly on the APCNWR. In 1997, I participated in radio tracking of released captive-raised APC on the APCNWR. Predation accounted for a significant portion of the mortalities post-release, especially during the first 30 days. Great homed owls (Bubo virginianus) were identified as one of the primary predators (Lockwood 1998, pers. obs.). Current predator management includes exclosures around known nests and lethal removal of coyotes, raccoons, skunks, foxes, and other potential predatory mammal species. Because great homed owls prey on released APC, management attention must be given to them as well. This study focused on evaluation of management practices hypothesized to reduce great homed owl predation.
Two recent publications review predation and predator management. Newton (1998) authored "Population Limitations In Birds" and the 2001 Wildlife Society Bulletin issue 29:1-69, provided a special section to review avian predation of gallinaceous birds and the related predator control. These publications provide examples ranging from predator management being beneficial for increasing prey numbers or breeding success to examples of no significant success. Two conclusions from these publications are particularly relevant to my research. The first is that most predator management programs do not collect data about the target predators prior to implementation of efforts and therefore lack a quantitative basis for planning the management. This void of basic information results in not understanding the specific situation and therefore, whether or not predator management is likely to be effective. I address this issue by collecting basic ecological information about great homed owl use of the APCNWR. The second conclusion concerning management options is that the gains of lethal predator control are limited to the time the target predator(s) is being removed, and therefore, habitat management generally has a longer lasting effect than predator removal (Greenwood and Sovada 1996). I address the issue option of habitat manipulation as a management option with an experiment to reduce hunting perches on sections of the refuge. The reduction of perches might have a positive effect in controlling great homed owl use of the refuge and thus, their ability to hunt released APC.
I collected data with the goal to provide fundamental information about the use of the refuge by great homed owls and assess the feasibility of perch-manipulation as a management option so managers can make informed predator management decisions concerning owls. This research was conducted to address the specific needs of the APCNWR, but the results should have implications in any prairie habitat. In the following chapters I describe this research and make suggestions concerning great homed owl management on the APCNWR.
In Chapter 1, I describe the development and testing of a new survey method to collect information about great homed nocturnal habitat use. I conducted a point count (Ralph et al. 1995) using a spotlight to illuminate a field of vision. Radio tracking (Nicolls and Wamer 1966) and broadcast surveys (Fuller and Mosher 1987) are common methods of collecting information about owls, but often they do not provide exact locations of the owls. I found that the spotlight survey provided information about specific habitat use, such as perching. This survey method also provided information that contributed to understanding the extent of use of the APCNWR by great horned owls and important results from the perch-manipulation experiment I ultimately performed.
In Chapter 2, I address basic questions related to distribution and habitat use of great horned owls on the APCNWR. I collected this information to improve the basis for management options concerning great homed owls on the APCNWR. I found that great horned owls occurred throughout the refuge, while APC use was largely restricted to the prairie. This is important because it allows for a potential alternative to lethal predator removal. That is, management of great horned owl predation can be achieved by restricting their use of the prairie habitat and, thus potentially reducing mortality of APC.
In Chapter 3, I describe the experiment I conducted to test the hypothesis that removing and placing deterrents on great horned owl hunting perches would reduce owl use of manipulated habitat, and thereby reduce their predation of released APC in those areas. I used a spotlight survey, telemetry location estimates, and incidental observations. I also conducted daylight surveys to assess diurnal raptor use of those same areas. I conclude that perch-manipulation reduced use of treatment areas by great homed owls and other raptors.
To summarize my research, I found that a spotlight survey in open habitat is a viable method to locate great homed owls. I documented that great horned owls used virtually all of the habitats within the APCNWR. I found that habitat management, specifically removal of potential perches and placing perching deterrents on fence posts, reduced the use of treated habitat by great horned owls. As a result of limiting the ability of great homed owls to use an area I reduced the number of APC mortalities attributed to owls in those areas. This study also provides an example of the benefit of assessing the ecology of predators prior to implementing management. I conclude that perch-manipulation will be less costly, have longer lasting results, and more effective for managing great homed owls than lethal control or transplanting.
DeHart, Karl Monroe, "Great Horned Owl Distribution and Habitat Use Relative to Post-Release Survival of Endangered Attwater's Prairie Chickens" (2003). Boise State University Theses and Dissertations. 452.