Risking Fatal Collisions: Linking Crane Flight Habits to Habitat near Power Lines
By Kim Ness
People have long admired cranes for their tall and stately posture, their monogamous pair bonds involving ritual courtship dances and territorial unison calls, and their majestic flight. In some cultures, cranes symbolize peace and freedom. The ubiquitous power lines that stream electricity to our televisions and microwaves, however, place cranes in grave danger. As they fly each dawn from roosting wetlands to foraging fields and back again at dusk, the cranes risk striking these lines in the dim morning light, fog, or high winds.
Both species of North American cranes - the abundant sandhill crane and the endangered whooping crane - are vulnerable to power line strikes. About one whooping crane dies each year from a power line strike in the eastern migratory flock of 106 individuals - a significant loss considering that fewer than 400 individuals are thought to be left in the wild. In fact, power lines are the highest known cause of mortality of fledged whooping cranes. For sandhill cranes, the number of deaths from power line strikes is less well-known. However, with an estimated 600,000 greater and lesser sandhill cranes migrating and breeding throughout North America, a mortality rate similar to that of eastern migratory whooping cranes would indicate several thousand sandhill deaths each year from power line strikes.
Working with resources and researchers at the University of Wisconsin-Madison and the International Crane Foundation (ICF) in Baraboo, Wisconsin, I studied power line risks to sandhill cranes in local populations in Wisconsin to develop a predictive model of where, when, and why power lines are threats to cranes. To understand where power lines pose a collision risk, this project aimed to assess how sandhill cranes reacted to power lines and identify the habitat conditions that correlate with abrupt reactions.
By collaborating with crane experts, we discovered that cranes, when within 20-30 feet of a power line, exhibited abrupt reactions by immediately flaring upwards or changing directions to avoid striking the line. This is consistent with the findings of other researchers, who observed cranes reacting abruptly just before a power line that the crane ultimately failed to clear, clipping a wing, leg, or foot on the line. Therefore, we considered these abrupt flight behaviors as indicators of a potential collision risk. While clipping power lines may not kill cranes instantly, they can inflict serious injuries that cripple the birds and make them more vulnerable to predators.
After observing such abrupt reactions, we then identified and quantified habitat conditions surrounding each power line. I compared the flight behaviors to habitat types. From previous land surveys of habitat, we knew cranes preferred foraging in row-crop agriculture or pasture, roosting in emergent, shallow wetlands, and strongly avoided densely forested areas. Data also suggested that cranes avoided forested areas when flying at low altitude. Such patterns in habitat use may provide a key link in understanding where power lines pose a collision risk to cranes. Eventually, this information might be used to devise effective mitigation strategies and reduce mortality in the U.S. and other parts of the world.
Fieldwork took place during the summer and fall of 2009 and spring of 2010. I observed cranes flying near power lines in two wetland-agricultural communities in southwestern Wisconsin. Equipped with binoculars, a video camera, and strong coffee, I meticulously recorded crane reactions, flight direction and height, and weather. Sitting in my car, which served as the "blind," on small roads for two-hour shifts at dawn and dusk, it took 46 days and about 184 hours to record cranes flying near the entire 30 miles of low-voltage power lines. ICF interns helped search these power lines for dead birds weekly. Fortunately, we found no dead cranes during the study, but discovered a female mallard with a broken neck directly under a line near a cut wheat field in the fall, where cranes commonly foraged.
Though we found no dead cranes, the following observation illustrates the danger cranes face: At dawn on an overcast, windy March morning, almost 200 sandhill cranes flocked in freezing temperatures to feed on the previous fall’s corn. Three cranes flew straight at a power line surrounded by cut corn. Within 15 feet of the line, they abruptly "put on the brakes" and veered right, flapping wildly and practically striking each other in the process. Throughout my study, I recorded many of these reactions. Further, just after completing the study, an ICF intern observed a crane strike a power line near the study area. Not only did this crane continue flying, but it turned around and clipped its leg on the same power line again!
Throughout the rest of the study, of the total 1,300 individual cranes, I recorded 381 flights ranging from solitary birds to flocks of up to 26 cranes, with an average of three to four cranes per flock. Birds within a flock commonly mirrored the leading bird’s behavior, like geese in migration. On the approach, 150 feet before a power line, flocks gradually gained altitude to fly, on average, 54 feet above a line. Of these, 56 flocks (15 percent) displayed abrupt reactions, and four flew under a power line, which indicated a higher collision risk near these locations.
Observing these sudden changes in behavior, and mapping power lines and associated land cover, we found a close correlation between pasture and row-crop agriculture and abrupt flight reactions. Initial results indicate that cranes reacted abruptly near power lines over agricultural fields with cultivated crops (soybean, wheat, or corn) or pasture lands (alfalfa or grass) with less wetland and forested areas within a 0.25-mile radius centered on the power line. By contrast, around more forested areas, cranes flew at higher altitudes and did not alter their flight near power lines.
As this study continues, we hypothesize that proximity of agricultural fields or pasture lands and wetlands to power lines during varied weather events (e.g. cloudy or windy weather) increase the likelihood of cranes abruptly reacting near power lines. Such predictive modeling projects can serve as a tool in the conservation planning toolbox. We hope this information will aide electric companies, state and federal wildlife agencies, crane advocates, and other conservationists as they seek to create and coordinate strategies to establish and protect safe flyways for cranes.