The highest risk of a bird strike occurs during takeoff and landing in the vicinity of airports, and during low-level flying, for example by military aircraft, crop dusters and helicopters. Some airports use active countermeasures, including a person with a shotgun, playing recorded sounds of predators through loudspeakers, or employing falconers. Poisonous grass can be planted that is not palatable to birds, nor to insects that attract insectivorous birds. Passive countermeasures involve sensible land-use management, avoiding conditions attracting flocks of birds to the area (e.g. landfills). Another tactic found effective is to let the grass at the airfield grow taller (to approximately ) as some species of birds won't land if they cannot see one another.