Broadcast Writer

By Vicki W. Kipp
Society of Broadcast Engineers (SBE) Chapter 24 Newsletter 
July 1, 2001

This month, we will examine the phenomenon of birds colliding with broadcast towers. At this time, there is much speculation about causes of and solutions for bird collisions with towers, but the theories still need to be thoroughly investigated.

At first glance, the phenomena of birds flying into towers may seem like a rare occurrence. But research reveals that birds fly into towers and guy wires in greater numbers than one might expect. On several recorded occasions, thousands of birds have died on a single tower in one night. For example, up to 10,000 birds, mostly Lapland longspurs, died at a 420-foot tower during a West Kansas snowstorm on January 22, 1998. From 1955 – 1980, 42,386 birds died at a 1,010-foot tower near Tallahassee, Florida. Federal officials estimate that 4 to 5 million birds are killed each year by communications towers.

Although reports of birds colliding with towers go back 100 years, this seems to have become a hot-button topic within the past few years. There are several reasons for this sudden awareness. Citizen resistance to new tower construction is increasing, with tower opponents seeking any reasonable argument against new construction. Environmental organizations, spurred on by the staggering number of bird deaths on a single night in 1998 at a Kansas tower, have stepped up their fight against what they call “tower kill.” Meanwhile, former President Bill Clinton issued an executive order on January 11, 2001 that requires Federal agencies to actively avoid take steps to protect birds and their habitat.

CAUSE

Theories abound as to why birds crash into towers, guy wires, and the ground surrounding towers. Suggestions include attraction to tower lights, weather factors and low visibility, blind collision due to lack of tower light, tower height, effects of RF, and Darwinism.

ATTRACTION TO LIGHT

Towers present the greatest threat to migratory birds that fly at night. Some scientists believe that birds are attracted to flashing red lights or the strobe effect of lights. Many experts claim that once birds enter the lighted area of the tower, they are reluctant to leave that lighted area. Observers claim that birds often successfully fly through the tower frame and on towards the edge of the tower’s lighted area, only to turn and fly back toward the light source. The birds circle the tower, flying around the light. Predictably, some of the circling birds will strike the guy wires of the tower. As more and more birds flock into the small lighted area, the tower circumference becomes congested. Birds may collide with other birds or spear each other with their beaks. In the experiment “Attraction of nocturnal migrants by lights on a television tower,” researchers William Cochran and Richard Graber alternated between turning the tower lights on and off. They found that birds swarmed around the tower when the lights were on, and quickly separated from the tower when the lights were turned off. According to data reported at http://www.towerkill.com, “It is important to clarify that the lights apparently do not attract birds from afar, but rather tend to hold birds that pass within a certain illuminated vicinity.”

WEATHER

Weather is thought to be a significant factor in bird collisions with towers. There is evidence that significantly more birds fly around towers on overcast nights than on clear nights. Migratory birds appear to actively avoid towers on clear nights.

According to many ornithologists, birds navigate using celestial beacons. When skies become cloudy or foggy, birds’ views of the stars are blocked. If there is a low cloud ceiling, birds are also deprived of any broad orienting perspective that the landscape would have otherwise provided. To a bird, tower lights may mimic the stars. Birds may be attracted to towers on overcast nights because tower lighting aids their vision. When it comes to navigation, birds are suspected to rely more heavily on visual cues than on an internal magnetic compass. Wisconsin DNR environmental analyst Steve Ugoretz says, “Birds flying in these low-light conditions may circle the lights and inadvertently fly into guy wires, just as they navigate in relation to the moon: they keep the light at right angles to their flight to keep going in the same direction.”

LACK OF LIGHT

While many people blame tower lights for bird deaths, others feel that unlighted towers are a threat to birds. Some researchers even suggest that unlit towers should add lighting to protect birds from blind collision. When flying in the low visibility, the birds may not see a tower in time to avoid it. Researchers have reported hearing alarm calls from birds as they finally do spot the tower and attempt to swerve. Characteristics such as vision and flight agility determine how vulnerable birds are to blind collision. Faster flying birds such as waterfowl and shorebirds are more susceptible to blind collision than slower, more agile birds such as songbirds.

TOWER HEIGHT

While larger migrant birds are known to ascend to altitudes above the clouds, smaller migrants are more likely to fly underneath clouds at levels interrupted by tall towers. A short tower built on a hilltop is suspected to have the same risk to birds as a taller tower built on flat ground.

DISRUPTION BY RF

RF energy may disrupt birds’ navigation skills and cause them to crash. However, at least one study contradicts this theory. In 1976, researchers Avery, Springer, and Cassel conducted a study on two overcast nights where they alternately turned the transmitter on and off, and then observed how many birds congregated at the tower. Their results showed that the presence of RF energy at the tower had very little, if any effect on the gathering of birds at the tower.

NATURAL SELECTION

Another possible explanation of bird collisions at towers is Darwinism as it applies to birds. Charles Darwin’s theory of evolution asserts that through natural selection, individuals with characteristics advantageous for survival in a specific environment form an increasing proportion of their species in that setting with each successive generation. Translated to birds and towers, towers are a device of natural selection which weeds out birds that are less observant. One tower industry expert suggests that some birds will collide with the first tower they come across, while other birds will fly past many towers without colliding once.

MORTALITY

Bird mortality at towers can occur in several different ways. A bird may crash into the tower structure or guy wires, dying immediately from a broken neck or subsequently dying from the impact of falling to the earth. A bird might survive the impact of colliding with a tower and falling to the ground, only to be killed by a predatory animal while lying stunned or injured on the ground. Birds may fly right into the ground as a result of disorientation. Or a bird may be accidentally speared by another bird’s beak.

TOWER DEFENSE

Although towers are being scrutinized for their role in bird deaths, far more birds die from other hazards such as cats, automobiles, high rise buildings, and power lines than from towers. Stealth towers can even become a habitat for birds. Environmentalists have called for tower restrictions that would discourage the use of guy wires for towers. But this raises the issue that tall towers without guy wires would require more steel and concrete, and would cost more. In a memo to the US Fish and Wildlife Service, FWS Director Jamie Rappaport Clark says, “…it must be recognized that some birds may be killed at structures such as communications towers even if all reasonable measures to avoid it are implemented.”

SOLUTIONS

With the goal of making towers as harmless to birds as possible, independent researchers, the US FWS, and the Communications Tower Working Group are searching for solutions. Since bird migration routes are busiest in the eastern two-thirds or the country, the majority of research about bird – tower collisions will occur there. Radar, acoustical monitoring, and night vision goggles will be among the research tools used by ornithologists, including experts in bird vision and navigation.

In hopes of alleviating the problem of birds colliding with towers, some novel ideas have already been suggested. These include covering towers with fluorescent paint, placing mirrors at towers to shine light skyward; and installing giant netting, devices that spin in the wind, or devices that broadcast low frequency sounds. One researcher wants to study the role that flight exhaustion might play in bird deaths by measuring the body fat and water content of birds killed at tower sites.

The US FWS has listed some voluntary guidelines for tower construction. Their suggestions include: installing antennas on existing towers; installing white strobe lights on towers instead of pulsing red lights; reducing the number of guys when possible; and constructing unlit and unguyed towers that are less than 200 feet above ground level.

Comprising about 50 Federal officials, conservationists, researchers, and tower industry members, the Communications Tower Working Group (CTWG) is researching tower kills. They have published a guide called “Service Interim Guidelines for Recommendations on Communications Tower Siting, Construction, Operation, and Decommissioning.” Suggestions include: collocating antennas on a single tower; avoiding guy wires for towers less than 199 feet above ground level; not locating new towers near wetlands, migratory flyways, habitats of endangered species, or areas with much fog, mist, or low cloud ceiling; minimizing amount of FAA required warning and obstruction avoidance lighting used for towers more than 199 feet above ground level; Using only white (first choice) or red (second choice) strobe lights at night; Limiting the number of lights, intensity, and number of flashes per minute to the FAA required minimum; Avoiding solid red or pulsating (beacon) red lights since they attract night-migrating birds at a much higher rate than other light types; Adding daytime visual markers to guyed towers in areas of significant bird population to protect day-migrating birds. Recommending relocation of a proposed tower site if that site is consistently used by a sizeable number of birds; Down shielding site security lighting to prevent light from trespassing beyond the site boundaries; Allowing CTWG researchers access to a tower site, to use monitoring equipment, and to place catch nets below the tower, but above the ground; and removing obsolete or no longer used towers within 12 months of the end of use.

CONCLUSION

The phenomenon of birds colliding with towers is a complex issue. At this point, the exact causes have not been confirmed, and researchers are still in search of reasonable solutions that succeed.

Information for this article came from the following sources: Newhouse News 2000 Seeman, Bruce Taylor, “Scientists, Communications Industry Struggle With Bird Kills at Towers ”; http:// http://www.towerkill.com; http://dnr.state.wi.us. “Wisconsin participates in effort to reduce bird deaths at towers.”; Personal Communications Industry Association “Memo from Sheldon Moss”; Communications Tower Working Group “Service Interim Guidelines For Recommendations On Communications Tower Siting, Construction, Operation, and Decommissioning”