A radar blast a day… keeps the birds away?

Human-wildlife interactions are perhaps more tightly controlled and more intensively monitored at airports than just about anywhere else in the world. Bird strikes – when birds collide with airplanes – are an important conservation concern for endangered species, and are of course a public safety and financial concern for the aviation industry. As I wrote in April, since 1990, the US has logged more than 127,000 bird strikes, with many more probably going unreported. Those strikes have cost some $700 million in losses, and between 1990 and 2011, have led to 23 human deaths and 223 injuries, not to mention the deaths and injuries among the birds themselves. Many airports attempt to actively mitigate the possibility of bird strikes through removing attractive avian habitats, hazing birds that do show up, trapping and relocating birds, and in some cases through lethal means. But these are all fairly broad approaches, and they have varying levels of success. To be more precise and more effective, it helps to know how birds respond to the airport landscape.

One of the most salient features of airports, for wildlife at least, is radar. As a result, both aircraft and control towers emit quite a bit of microwave radiation, both to keep track of aircraft and to monitor weather. Typical airport radar emits microwaves at a frequency capable of penetrating 4mm into biological tissue. That’s enough for an animal to detect microwaves either through thermoreception, because the animal becomes a bit warmer, or through auditory detection, because microwaves can alter air pressure enough to generate a discernible sound wave in some cases.

Of course, just because an animal can detect radar doesn’t mean it will necessarily alter its behaviors in a meaningful way. To find out how different types of radar might influence wild birds, Purdue University biologists Eleanor Sheridan, Jacquelyn Randolet, and Esteban Fernández-Juricic, together with US Department of Agriculture wildlife researchers Travis Lee DeVault, Thomas Walter Seamans, and Bradley Fields Blackwell, rounded up a few hundred brown-headed cowbirds from NASA’s Plum Brook Station in Ohio and brought them into captive aviaries at Purdue.

In one experiment, the researchers exposed the birds to emissions from a stationary solid-state radar, to mimic the type of radar used by control towers. In a second experiment, they exposed the birds either to an approaching vehicle outfitted with either a solid-state radar or a magnetron radar. Both are commonly used on aircraft; the solid-state radar is more commonly found on large commercial planes, while the magnetron is more often installed on smaller jets and on helicopters. Rather than use an airplane, the researchers used a Ford F-150, and drove it at speeds similar to the ones airplanes use to taxi while on the ground. While that’s not a perfect analogue for aircraft, Sheridan and her team argue that it comes sufficiently close to provide useful information.

In both experimental settings, the researchers observed changes in the birds’ behavior that was most likely associated with the radar. In the stationary radar experiments, the birds moved around more while exposed to the radar, but seemed to decrease their vigilance behaviors. In some ways, that’s contrary to their predictions, which assumed the birds would become more vigilant while exposed to microwave radiation. It’s possible that the decreased vigilance reflects habituation to the experimental enclosure, but more work is necessary to confirm that possibility. Still, the radar appears to have caused the birds to alter their behaviors.

When it came to the approaching radar experiments, the results were a bit more intuitive. The cowbirds responded more quickly to the more powerful solid-state radar than they did to the less powerful magnetron, but they did respond to both, affirming the hypothesis that radar increases vigilance. However, they responded to the two forms of radar in different ways; they were more likely to escape the approaching vehicle perpendicular to the road for the magnetron, and parallel to the moving truck for the solid-state. That’s actually quite reasonable. To escape an approaching vehicle, flying away from the road required a short flight (2 meters) than over the truck and parallel with it (3 meters). Because the birds became aware of the solid-state radar sooner, they had the extra time to avoid it using a longer flight path. Meanwhile, since the birds had less time to anticipate the truck’s approach when outfitted with the magnetron, they opted for a quicker escape route. According to the researchers, this pattern suggests that “radar increases alertness or attracts attention to the threat, making the threat seem riskier.”

That the birds moved around more when exposed to stationary radar could mean that they were attempting to find a spot free of microwave radiation. Combined with visual cues, that could be a useful method for airports to discourage birds from foraging or roosting in dangerous spots. The approaching radar experiments underlined the usefulness of active radar on moving aircraft. Assuming that the cowbirds flew away from the approaching truck similar to the way they’d avoid an aircraft, then any aircraft outfitted with a solid-state radar would have an extra 2-6 seconds to avoid a strike than one without an active radar at all. Potentially, that’s enough to increase the number of successful escapes, though the potential for the birds to escape goes down as the speed of the taxiing aircraft increases. Together, these findings offer wildlife managers at airports with several detailed options for attempting to mitigate bird strikes. – Jason G. Goldman | 30 September 2015

Source: Eleanor Sheridan, Jacquelyn Randolet, Travis Lee DeVault, Thomas Walter Seamans, Bradley Fields Blackwell, & Esteban Fernández-Juricic. (2015). The effects of radar on avian behavior: Implications for wildlife management at airports. Applied Animal Behaviour Science 171, 241–252. DOI: 10.1016/j.applanim.2015.08.001.

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