The Effect of Grass Heights on Bird Presence on Airports

Transcription

1 The Effect of Grass Heights on Bird Presence on Airports John Watterson and Cody Baciuska Since 1988, bird strikes have resulted in 219 deaths and destroyed over 200 aircraft. Damages from these strikes cost over 620 million dollars annually. 1

2 Increasing populations of many different hazardous bird species and increased air travel will continue threat of bird strikes. 80% of bird strikes occur on airport property. Emphasizes the need for continued study of the role of habitat management on airports. Open expanses of grass are typically the most common habitat on airports. Equipment and trained personnel to maintain grass heights are already in place at these facilities. Mowing is one of the most readily available habitat management techniques. Habitat management a long term tool. 2

3 Previous studies have found conflicting results regarding grass height and bird presence. Many results indicate a preference for shorter grass. Others did not detect a difference or found greater bird presence in un-mowed grass. FAA recommends a grass height of 6 12 in. USAF= 7-14 in. Long-term study in UK showed bird presence in long grass reduced by 2/3 (Deacon and Rochard 2000). Problem species of small gulls and lapwing numbers greatly reduced. 3

4 2 month study at JFK showed that fewer laughing gulls used tall vegetation (46 cm) than short vegetation (5 cm) (Buckley and McCarthy 1994). Airport began tall vegetation management program- mowing once in fall or winter. Benefit of FAA recommended grass height based on several factors: -Thought to reduce amount of seed production that attracts granivores. - Reduce presence of flocking species by blocking sight lines between conspecifics and approaching predators. - More difficult to move through vegetation. - More difficult to spot prey in soil. - Less attractive to Canada geese. 4

5 More comprehensive study at JFK found higher presence of birds in un-mowed vegetation (Barras et al 2000). Regularly mowed grass did not pose a strike risk. 2 studies in Ohio showed no difference in bird use between mowed and un-mowed grass plots (Seamans et al 1998). 1 exception more RWBL in tall grass. Methods Data from 8 airports in the eastern US were selected for analysis. Data were collected as part of WHAs over 1 year. Bird counts were recorded during point-count surveys at pre-selected points throughout each airport. 5

6 All birds seen/heard were recorded along with behavior/habitat use. Methods Surveys were conducted for 3 min., 4 times per month (dawn, mid-morning, afternoon, and dusk) Grass heights were recorded during each survey. Methods Grass heights were separated into 10 cm groups. Only birds observed directly utilizing airport grass were included in analyses. - Loafing on ground - Aerial Hunting - Feeding - Perched in vegetation 6

7 # of Birds Observed 3/28/2013 Methods Total counts of birds in each group were directly compared. Mean bird counts in each grass height category were compared using ANOVA. Species compositions (guilds) in each grass height category were compared using Chi-squared analysis. All analyses and data management were conducted using SPSS and Microsoft Excel. Results cm 11-20cm 21-30cm 31-40cm 41-50cm 51-60cm 61-70cm 71-80cm Grass Height Category Figure 1. Total counts of all birds in each grass height category. 7

8 Mean Bird Counts 3/28/2013 Results cm 11-20cm 21-30cm 31-40cm 41-50cm 51-60cm 61-70cm 71-80cm Grass Height Category Figure 2. ANOVA of mean bird counts between grass height categories (F 7, 51 = 7.86, p <0.01). Error bars represent standard deviation. Results Chi-squared analysis of the species composition of each grass height category showed differences in guild presence (X 2 =245.1, p < 0.01). Waterfowl were present in the greatest proportions in the 0-10 cm category. Raptors were present in the greatest proportions in the cm category. 8

9 Results No difference was found for blackbird and gull presence between grass height categories. Few gull observations included in analyses. No difference in columbid presence between grass height categories. With annual increases in air travel, proper management of airport habitat is essential to aircraft safety. Managing grass height has the potential to influence the presence of birds at airports. 9

10 Results partially supported by previous studies. - Selected bird species present in greater numbers in shorter/mowed grass (Seamans et al 1998 and Deacon and Rochard 2000) - Similar studies detected no difference in bird presence between mowed/un-mowed grass (Seamans et al 2007) Fitzpatrick (2003) found that American kestrels, red-shouldered and red-tailed hawks foraged in mowed grassland plots more than in un-mowed plots in the Eastern US. 10

11 Bechard (1998) found that Swainson s hawks foraged more frequently in shorter less dense fields. Vegetative cover more important than prey densities. Greatest counts of birds found in cm fall within the lower ranges of FAA guidelines. Study insufficient to suggest a change in these guidelines. However, this and other studies may call for grass heights to be recommended on a more case by case basis. 11

12 Managing grass at a longer height requires less mowing. - Less stress on equipment - Less cost in fuel - Fewer hours of labor Disadvantages of long grass. - Greater potential for vegetation to produce seeds. - More cover for wildlife on the airport. - Make future mowings more difficult and hamper personnel movements. - May obstruct signs and encroach on perimeter fence. - Less aesthetically pleasing. 12

13 Limitations of study: - Bird observability bias - Data are results of 1 year of data. - Examined only grass height, many more factors influence bird presence. Avenues for future study: - Increase sample size and replication - Features of the surrounding landscape - Vegetation species composition - Insect and small mammal populations - Observability correction 13

14 Questions Cody Baciuska Wildlife.com 14