Human–Wildlife Confl icts 1(1):97–105, Spring 2007 Comparison of 2 vegetation-height management practices for wildlife control at airports THOMAS W. S EAMANS, U.S. Department of Agriculture/Wildlife Services/National Wildlife Research Center, 6100 Columbus Avenue, Sandusky, OH 44870, USA [email protected] SCOTT C. BARRAS, U.S. Department of Agriculture/Wildlife Services/National Wildlife Research Center, P.O. Drawer 6099, Mississippi State, MS 39762, USA GLEN E. BERNHARDT, U.S. Department of Agriculture/Wildlife Services/National Wildlife Research Center, 6100 Columbus Avenue, Sandusky, OH 44870, USA BRADLEY F. B LACKWELL, U.S. Department of Agriculture/Wildlife Services/National Wildlife Research Center, 6100 Columbus Avenue, Sandusky, OH 44870, USA JONATHON D. CEPEK, U.S. Department of Agriculture/Wildlife Services, 6100 Columbus Avenue, Sandusky, OH 44870, USA Abstract: Vegetation-height management is a potential method to reduce bird numbers at airports. Based on studies in Europe, researchers recommended vegetation heights around 25 cm; however, preliminary studies in the United States produced confl icting results regarding the effect of tall (18 to >25 cm) vegetation on bird numbers at airports. From 1999 to 2002, we compared birds and other wildlife use of 4 short-vegetation plots (mean maximum height of 15.6 cm ± 5.1 SE and visual obstruction reading of 4.6 ± 3.0 cm) and 4 tall-vegetation plots (mean maximum height of 26.9 ± 8.4 cm and visual obstruction reading of 10.0 ± 5.0 cm) in Ohio. We surveyed bird use of the plots 2 to 3 times/week and observed 6,191 birds in short-vegetation plots and 5,962 birds in tall-vegetation plots. We detected no difference between short-vegetation and tall-vegetation plots in the probability of avian use of the plots when evaluated as a binary response of presence and absence. Small mammal capture rates in 100 adjusted trap nights were 0.0 in short-vegetation plots and 0.3 in tall-vegetation plots. We found no difference in the number of deer observed in the plots during sunset and spotlighting counts. There was slightly greater insect biomass in tall- than in short-vegetation plots. Mowing negatively affected small mammal use. The generalization that tall vegetation (18 to >25 cm) alone would reduce bird use of an airport is not supported by the results of this study. Further research on vegetation density, composition, palatability, and nutritional value is necessary to accommodate airfi eld requirements for habitat that is pleasing to the public and repellent to wildlife. Key words: aircraft–bird hazard, airport management, bird–aircraft collisions, bird strike, eastern meadowlark, European starling, human–wildlife confl ict, Sturnella magna, Sturnus vulgaris, wildlife hazard When mammals and birds collide with and other areas where birds pose problems. aircraft they pose serious safety hazards to One method oft en suggested for reducing bird people. Additionally, aircraft -bird collisions numbers at airports is to maintain tall vegetation (i.e., bird strikes) cost the U.S. civil aviation (18 to >25 cm), as opposed to standard mowing industry > $496 million annually and resulted in practices that maintained short vegetation 9 fatalities from 1990 to 2004 (Cleary et al. 2005). (5–10 cm; U.S. Department of Transportation Approximately 78% of all bird strikes occur < 1993, Transport Canada 1994, U.S. Department 244 m above ground level (AGL) and 90% of Agriculture 1998, Civil Aviation Authority occur < 610 m AGL (Cleary et al. 2005). Gulls 2002). Tall vegetation is thought to interfere (Larus spp.), waterfowl such as Canada geese with visibility and ground movements of (Branta canadensis), hawks (Falconiformes), fl ocking birds such as European starlings owls (Strigiformes), blackbirds (Icteridae), and gulls (Solman 1966, Blokpoel 1976, U.S. and European starlings (Sturnus vulgaris) are Department of Transportation 1993, Transport the species of great concern at U.S. airports Canada 1994, Dekker and van der Zee 1996, (Wright et al. 1998, Dolbeer et al. 2000, Wright U.S. Department of Agriculture 1998). The U.S. 2001, Cleary et al. 2005, Blackwell and Wright Air Force implemented a policy (AFI91-202) 2006). Management techniques that reduce in 1998 requiring vegetation to be maintained bird use of habitats on and around airports are at 18 to 36 cm tall when possible. The basis for therefore critical for safe airport operations. these recommendations comes from studies Habitat management provides a nonlethal conducted in Great Britain (Mead and Carter technique for reducing bird use of airports 1973, Brough and Bridgman 1980), where the 98 Seamans et al. Human–Wildlife Confl icts 1(1) bird species of concern in the United States were mow, but this simulated mowing at airports not present. For example, there is limited data practicing short-grass management. on how Canada geese and various raptors that are signifi cant problem species on many U.S. Bird surveys airports react to tall-vegetation management. We began bird surveys on July 23, 1999, and In addition, tall-grass management in Great conducted surveys 2 to 3 days/week starting Britain involves a rigorous regimen of at randomly-chosen plots and times from mowing, thatch and weed removal, and the sunrise to sunset. We conducted 2 rounds of use of fertilizers to maintain an erect, dense observations approximately 1 hour apart during stand of grass (Civil Aviation Authority 2002). each observation period, such that when the fi rst This type of vegetation management is not survey or round of all 8 plots was completed, we generally practiced on United States airfi elds immediately began a second round of surveys. because previous studies on tall-vegetation We observed each plot for 5 minutes from a fi xed management at airports in the United States point <30 m outside of the plot. We counted the have produced confl icting results (Buckley and number of birds and mammals by species that McCarthy 1994, Seamans et al. 1999, Barras we observed on the ground in the plot, within et al. 2000). Additionally, tall vegetation at 1 m of its edge, or fl ying over the plot. By the airports may be undesirable from aesthetic and end of the fi rst growing season, we recognized security viewpoints, particularly if benefi ts of that we could not see all birds in the plots due such management are questionable. to the vegetation height. Therefore, during May Our study included observations of bird and 2000 we initiated fl ush counts of each plot once mammal activity in vegetation plots maintained every 2 weeks (May to November) to account within set height ranges. Our objective was for birds that were in plots but blocked from to determine if bird and mammal use of plots our view by vegetation. We conducted fl ush diff ers due to vegetation height. Our goal was counts aft er the second-round observation was to provide airport personnel with objective completed on a plot. Prior to entering the plot, recommendations for vegetation management we counted all birds in the plot, and then 2 to minimize bird and other wildlife strikes. people walked circular paths through the plot 30 and 40 m from the perimeter, respectively. Methods When possible, a third person observed the plot We conducted our study from May 1999 to count fl ushed birds and look for new birds through July 2002 at the National Aeronautics entering the plot aft er initiation of the count. and Space Administration’s Plum Brook We did not record any birds that fl ew into the Station (PBS), Erie County, Ohio (41°37′N, plot aft er the fl ush count had started. 82°66′W). The 2,200-ha facility is enclosed by a 2.4-m-high chain-link fence with barbed- Plot vegetation wire outriggers. Habitat within PBS diff ered Despite proximity and moving regimens from the surrounding agricultural and urban for our plots, we could not be sure that area and consisted of dogwood (Cornus spp., plant composition and structure would be 39%), grasslands (31%), open woodlands (15%), approximately homogeneous within treatment. and mixed hardwood forests (11%; Rose and Therefore, we measured vegetation height Harder 1985). Birds commonly observed at weekly from July 26 to October 21, 1999, April PBS include American goldfi nch (Carduelis 10 to October 23, 2000, April 23 to October 9, tristis), American robin (Turdus migratorius), 2001, and April 22 to July 22, 2002. Vegetation Canada goose, eastern meadowlark (Sturnella measurements began at the start of the study magna), European starling, and red-tailed hawk (July 26, 1999), when grasses started growing in (Buteo jamaicensis). The vegetation management the spring, and we ceased measuring at the end regime we followed simulated that of airport of the growing season (i.e., fi rst killing frost) fi eld management. when the study ended (2002). We selected 10 In May 1999, we established 8 circular plots, sample points from the center of each plot along each 1.5-ha and all ≥ 0.4 km apart from each a randomly selected compass heading using a other. We randomly assigned 4 plots each to random numbers table. At each sample point, treatments of short vegetation (9–15 cm) or we measured the maximum vegetation height tall vegetation (15–30 cm) management. When by placing 2-m sticks vertically 1.5 m apart plots exceeded their maximum height (15 cm with a string connecting the sticks and in line for short and 30 cm for tall), we mowed the with the compass heading. We kept the string plot to their minimum assigned height (9 cm parallel to the ground and adjusted its height for short and 15 cm for tall).