Publication Date
8-2024
Date of Final Oral Examination (Defense)
6-19-2024
Type of Culminating Activity
Thesis
Degree Title
Master of Science in Raptor Biology
Department Filter
Biology
Department
Biological Sciences
Supervisory Committee Chair
Jennyffer Cruz Bernal, Ph.D.
Supervisory Committee Member
Jennifer D. McCabe, Ph.D.
Supervisory Committee Member
T. Trevor Caughlin, Ph.D.
Abstract
Habitat change is the leading cause of species extinctions and declines worldwide. In addition to species losses, habitat changes can also alter species behavior, diet, and the way they interact with other species. Less than one percent of coastal grasslands and prairies in Texas, U.S.A. remain intact as a result of agriculture, urbanization, and woody plant encroachment. This loss of Texas grasslands has resulted in bottom-up impacts, including the extirpations of multiple species that inhabited these ecosystems, such as the Northern Aplomado Falcon (Falco femoralis septentrionalis), Attwater’s Prairie Chicken (Tympnuchus cupido attwateri), and Pronghorn (Antilocapra americana). Besides direct loss of grasslands, some grassland areas in the region are also experiencing woody plant encroachment that may be benefiting Great Horned Owls (Bubo virginianus). This owl is a generalist predator known to prey on fledgling Northern Aplomado Falcons. After its extirpation in 1952, the Northern Aplomado Falcon was reintroduced to the U.S. beginning in 1977, and a small, isolated breeding population currently persists along the South Texas coast. However, Great Horned Owls may be hindering the recovery of this population of Northern Aplomado Falcons. We aimed to first evaluate the best approach to estimate Great Horned Owl distribution by comparing two single-method (callback survey and autonomous recording unit) occupancy models against an integrated occupancy model that combines two detection methods in a single model to make the most informed inference. Second, we aimed to use the best approach from aim one to determine the distribution of Great Horned Owls and its relationship with vegetation cover (grassland, cropland, shrub, aquatic vegetation) and vegetation structure (tall vegetation > 3 m in height) using satellite and LiDAR data. We used data collected during the winters of 2022 and 2023 using callback surveys and autonomous recordings units at 65 survey points on Laguna Atascosa National Wildlife Refuge, Texas. We found that the callback surveys provided more conservative and reliable estimates of owl occupancy and chose the callback-only model for further inference. Results from this callback-only model suggested that Great Horned Owls were more likely to occupy areas with tall vegetation (> 3 m) cover above 5 % and with shrub cover above 30 %. Conversely, Great Horned Owls were less likely to occupy areas with grassland cover over 30%. Overall, Great Horned Owl distribution was widespread within the refuge, and nest cameras confirmed that owls come in close contact with nesting Northern Aplomado Falcons, with 4 of 10 cameras capturing owls perched or consuming prey on Northern Aplomado Falcon artificial nest structures. Thus, results suggest that increases in woody plant encroachment likely benefitted Great Horned Owls and that owls pose a major threat not only to adult falcons, but especially naïve, newly fledged young. Our findings can be used to inform where restoration efforts should occur. We recommend prioritizing removal of woody plant encroachment in areas of high owl occupancy and near falcon nests to reduce the risk of predation. Furthermore, future Northern Aplomado Falcon release efforts should consider owl distribution and the level of tall vegetation and shrub cover when selecting release sites in order to maximize their survival. Reintroduction efforts of other species would also benefit from assessing how habitat changes may impact species interactions when returning a species to an altered landscape.
DOI
https://doi.org/10.18122/td.2229.boisestate
Recommended Citation
Bontrager, David Roman, "Distribution of Great Horned Owls (Bubo virginianus) Near Breeding Grounds of the Reintroduced Northern Aplomado Falcon (Falco femoralis septentrionalis) at Laguna Atascosa National Wildlife Refuge in Coastal South Texas" (2024). Boise State University Theses and Dissertations. 2229.
https://doi.org/10.18122/td.2229.boisestate