Association Between Roadways and Cactus Ferruginous Pygmy-owls in Northern Sonora, Mexico
Between 2002 and 2005, we studied the influence of roadways on the ecology and behavior of cactus ferruginous pygmy-owls (Glaucidium brasilianum cactorum) in northern Sonora, Mexico. Our study was motivated by biologists and transportation planners at the Arizona Department of Transportation working in cooperation with the U.S. Fish and Wildlife Service who sought to understand and mitigate adverse effects of roadways on pygmy-owls. Pygmy-owls are currently of profound conservation concern in southern Arizona where their populations have declined to near extinction and where pygmy-owls were listed as endangered between 1997 and 2006.
To identify management strategies to facilitate successful movements of pygmy-owls across roadways, we assessed (1) relationships between flight behavior and environmental conditions along flight corridors, (2) factors that influenced the rate of road crossing by pygmy-owls, (3) vegetation used by pygmy-owls within or adjacent to road corridors, and (4) habitat selected by pygmy-owls within their home ranges. The vast majority of flights by adults and dispersing juveniles were <40 m. Flight distances increased as the height an owl was perched before a flight increased. Pygmy-owls typically flew at heights <2 m above ground that placed them at risk of vehicle collision, although owls that flew longer distances tended to reach higher maximum heights above ground. Flights >80 m were rare; however, owls were capable of crossing larger roadway corridors and other openings in vegetation, such as agricultural fields, in single flights up to 335 m in length. These behaviors, combined with consistent selection of large trees as perches, suggest that maintaining or planting large trees and woodland vegetation along road corridors could reduce the risk of vehicle collision and make roadways more permeable to pygmy-owls.
For pygmy-owls with territories that were adjacent to roadways, the rate at which owls crossed roads decreased as traffic volume and the width of roadway corridors increased. We estimated that traffic volume of approximately 6 vehicles per minute or roadway corridors ≥167 m would completely prevent crossing by pygmy-owls that were nesting 200 m from a roadway. Approximately 21 vehicles per minute would prevent crossing by pygmy-owls nesting 100 m from a roadway. Pygmy- owls that crossed roadways tended to use tall trees adjacent to roads and these tall trees were often associated with riparian vegetation along drainage corridors. No adult (n = 19) or juvenile (n = 54) owls that we radio-marked were struck by vehicles.
Only 24 of the 54 juvenile pygmy-owls (44%) that we radio-marked after fledging survived to initiate natal dispersal; in contrast, none died during the 4 to 6 week period after onset of dispersal. Juveniles dispersed an average of 5.6 ± 0.8 km (± SE, range = 1.1-19.2 km) from nests. The number of roads crossed during dispersal increased as total distance moved during dispersal increased, suggesting that the small- to moderate-sized roads typical of northern Sonora did not pose a significant barrier to dispersing pygmy-owls. Only one radio-marked juvenile encountered Mexico Route 15, the largest road in northern Sonora, and crossed after first remaining near the road edge for approximately 24 hours. Movements of several other dispersing pygmy-owls were apparently slowed by the presence of agricultural fields or the trajectory of dispersal changed so as to avoid large fields. Consequently, large vegetation openings such as roadway corridors may present a significant obstacle for dispersing pygmy-owls.
Pygmy-owls sometimes nested within 100 m of roadway corridors where traffic volume exceeded 5 vehicles per minute. When compared to available nest sites, pygmy-owls typically placed nests closer to roadway corridors than expected by chance after adjusting for the influence of surrounding vegetation structure. This behavior likely reflects an attraction to vegetation edges, yet may augment risk of collisions with vehicles. Nest success and juvenile survival were lower at nest sites
To mitigate potentially adverse impacts of roadways on pygmy-owls, we developed a series of general recommendations to improve roadway design. Although our findings offer both auspicious prospects for mitigating the influences of roadways on pygmy-owls, they also pose new challenges and new questions for transportation planners and conservation biologists.