Habitat Modification

Habitat modification is practiced at a great many airports and airbases around the world. (e.g., U.S. Fish and Wildlife Service 1979, 1984; Transport Canada 1984; Searing et al. 1996; John Floyd, U.S.D.A., Wildlife Services, pers. comm.). Habitat modification involves the removal and/or alteration of habitat features. Typical actions include the pruning or removal of trees and shrubs; removal of standing water (ponds, puddles); revegetation of barren areas with plant species tall enough to prevent use by open country birds such as gulls; planting of crops less attractive to problem birds, and allowing grassy areas to grow taller. Other techniques address nesting and resting sites provided by airport buildings, and removal of perching sites on airfields. Habitat modification should consider habitat features in the vicinity of the airport as well as on the airport itself. An overview of habitat modification issues and techniques is provided in the Transport Canada Manual (Transport Canada 1994). Bird control products/techniques, such as netting and lines, that prevent access to habitat features that attract birds are discussed under "Exclusion Methods" later. Discussed under "Chemical Repellents" are chemicals that control birds' food sources, such as worm control. Some of these alter the habitat for prey species rather than addressing features that birds use directly. The effectiveness of a tall grass policy on airports has been studied at several locations.

Tall Grass

Description. – Airports need to have short grass immediately adjacent to runways so that signs and lights are visible. However, many airfields also have areas of short grass away from runways and in other infield areas. Allowing these grassy areas to grow taller reduces their use by many bird species, especially some particularly hazardous to aircraft safety (e.g., gulls). "Tall" grass generally refers to grass that is at least 15-20 cm in height; "short" grass generally is <10 cm. < p>

Biological Basis. – Tall vegetation appears to be effective because it impedes certain birds' access to food sources (e.g., soil invertebrates) and obstructs the birds' lines of sight to approaching predators, thus creating an unsafe feeding or loafing environment. However, other species of birds are adapted to taller grass habitats and very long grass can attract rodents and attendant raptors (Wright 1968).

Literature. – Brough and Bridgman (1980) compared gull use on plots of longer (15-20 cm) and shorter (5-10 cm) grass on 13 airfields in the United Kingdom. They found that the longer grass was very successful at reducing the numbers of birds, particularly gulls. At Copenhagen Airport, Dahl (1984) found that "medium length" grass (about 20 cm) was much more effective at reducing use by loafing gulls than was longer or shorter grass. The latter grass lengths were not defined and the author did not present his data in the cited paper. The "long grass policy" at RAF stations which operate fixed wing aircraft, requires maintenance of grass at 18-20 cm for most of the year and has proven effective at reducing bird strikes (Deacon 1996). Hupf and Floyd (1995) also found a long grass management policy to be effective in reducing on-airport populations of hazardous bird species (Canada geese, starlings, laughing gulls). "Tall" grass in their context was >14 in (35 cm) and averaged 22-24 in (56-61 cm). Of five species that accounted for 85% of bird strikes at Ottawa/MacDonald-Cartier Airport, four (ring-billed gull, snow bunting, swallows and rock doves) were found to be more attracted to short grass (5-10 cm) areas than long grass (15-20 cm) (Potter 1996). At Vancouver International Airport, extremely long grass (up to 75 cm) has been found to keep geese and ducks out; only great blue herons used such tall grass areas (Dave Ball, Vancouver International Airport, pers comm.). Dolbeer and Seamans (1997) evaluated the response of Canada geese to short (5-10 cm) and tall (16-20 cm) grass in an outdoor pen test. Interestingly, the geese showed a preference for the tall-grass plots.

Gulls, a hazard to aircraft safety at many airports, spend substantial proportions of each day resting, preening, and sleeping. This behaviour is called loafing. Gulls loaf on level areas with little or no vegetation to impede their vision and their ability to detect approaching predators. Many, but not all, major loafing areas have standing water. Thus the attractiveness of gull loafing habitat can be minimized by removing standing water and revegetating barren areas. The importance of loafing areas for gulls was illustrated during a recently completed study in the Tampa Bay area (Patton 1988). A major landfill that had been used by up to 60,000 gulls was closed. The gulls dispersed to five of the six surrounding landfills in the following winter. Numbers at the sixth landfill decreased rather than increased. The decrease was associated with the loss of loafing habitat adjacent to the landfill (Patton 1988). Thus an important feeding area can be rendered less attractive if suitable nearby loafing habitat is not available. Stout and Schwab (1979) also found a very direct connection between gull use of an airbase for loafing and their use of two nearby landfills for feeding. A major, multi-year experiment is being conducted at O'Hare International Airport in Chicago to determine the bird use of large plots (40-50 acres) of grass at different heights. The results are not yet available (R. Sliwinski, U.S.D.A., Wildlife Services, pers. comm.).

An alternative to a tall "grass" policy has been investigated. Dekker and Zee (1996) tested the effectiveness of a "poor grass regime", a herb-rich vegetation community (i.e., more wildflowers and fewer grasses) that commonly is found on poorer soils. Based on five years experience at two airports in The Netherlands, Dekker and Zee found that "bird numbers on poor grass were as low or lower than on long grass. Furthermore the species composition of the remaining birds changed to smaller, less heavy species, thus decreasing the risk of a bird strike causing damage." (p 303). "Poor grass" apparently performs the same structural functions as tall grass (i.e., impeding access to food and detection of approaching predators), but also supports fewer small mammals and invertebrates because it is a nutrient-poor plant community. Therefore there is less food for birds. Botanical expertise would be required to implement this approach.

Habitat modification actions, such as mowing and ploughing, can attract significant numbers of birds in the short-term by making prey more available (e.g., worms and other invertebrates for gulls, rodents for hawks and gulls). Potter (1996) suggested that mowing be conducted either late in the day or overnight to reduce the attractiveness of these activities but did not test this technique. Night mowing has been effective when tried at Vancouver International Airport (D. Ball, YVRAA, pers. comm.).

Evaluation. – The effectiveness of a "tall grass" strategy, or the use of some type of tall ground cover, has been shown to be effective at reducing the use of airfields by several if not all of the most hazardous species (e.g., gulls, waterfowl, rock doves, starlings, snow buntings). Tall-grass habitats do attract some other species hazardous to aircraft safety, such as hawks and owls however.

Recommendation. – A tall grass/vegetation strategy is recommended, with the proviso that the situation should be monitored to ensure that new more serious hazards are not created with the attraction of "tall grass" bird species. Also, the "poor grass regime" reported on by Dekker and Zee (1996) shows promise.

Literature Reviewed. – Brough and Bridgman 1980; Dahl 1984; Deacon 1996; Dekker and Zee 1996; Dolbeer and Seamans 1997; Garber 1996; Hupf and Floyd 1995; Patton 1988; Potter 1996; Stout and Schwab 1979; Transport Canada 1984; Wright 1968; U.S. Fish and Wildlife Service 1979, 1984.