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
Authors: Bontrager David Roman 
Published: February 2025

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.