(Turkey vulture, Cathartes aura)
This is what motivated a team of researchers from the United States Department of Agriculture to use GPS satellite transmitters to study the flight behavior and activity patterns of vultures near the Marine Corps Air Station in Beaufort, South Carolina. Vulture populations in the state are quite healthy, having recently risen by 9.9% and 11.9% for black and turkey vultures, respectively; further, individuals of both species are resident year-round. As a result of this high potential for collisions, 4 vulture-aircraft strikes have been reported at Beaufort since 2006.
The researchers used a baited walk-in trap to capture 16 vultures, after which they also measured, individually tagged, and attached transmitters to the birds. Unfortunately, several of the original focal birds died before the end of the study, so the scientists redeployed transmitters on new individuals, bringing the total sample size to 22; 4 of these were individuals that remained alive for the entire 2-year study. The transmitters were attached to the vultures with a Teflon tape backpack harness. During every hour between dawn and dusk, on the hour, the transmitters recorded latitude and longitude, altitude, speed, and direction of movement; any time a bird was moving more than 0 km/h, it was assumed to be in flight.
(Black vulture, Coragyps atratus)
The transmitters logged a whopping 106,554 locations over the duration of the project. The scientists used these data to investigate variations in behavior between species and among seasons, calculate the proportion of time an average bird spends in flight, and estimate the home range size of each bird.
Both species were most active in the winter and least active in the summer, which the authors link to the shorter day length in the winter and the increased need for foraging efficiency at that time of year. Black vultures spent less time in flight than turkey vultures (8.4% of the day vs. 18.9% of the day, respectively). Black vultures have long been thought to watch turkey vulture activity in order to find sources of food; plus, black vultures are very social birds and probably communicate with each other to share information about feeding locations. Thus, it was not surprising to find that they spent less time in transit.
Turkey vulture activity peaked in the middle of the day and then gradually declined; black vulture activity was similar in pattern, but lagged behind by about an hour. Overall, the busiest time of day for vultures was during the 4-9 hours after sunrise. Both species of birds spent the bulk of their time <100 m in altitude; 48% of 3,992 black vulture data points fell within this range, and 60% of 9,622 turkey vulture data points were within this range. However, black vultures were more common than turkey vultures >200 m; while the peak turkey vulture altitude was 1,378 m, black vultures were recorded as high as 1,578 m. Again, this is probably related to the black vultures' tendency to watch turkey vultures in order to find food--a high cruising altitude likely gives them a good vantage point from which to see their red-headed relatives. Throughout the day, the birds distributed themselves at different altitudes; although most flights were <50 m during the first 4 hours of daylight, they rose to 100-150 m by the middle of the day, depending on species.
(Damage to a plane after a bird strike--the plane was confronted by an entire flock of storks in flight.)
Across all seasons, turkey vultures had larger home ranges than black vultures--a couple individuals of the latter species even flew all the way down to Florida. The vulture mortality observed throughout the 2-year study period resulted from traffic, shooting, and electrocution--but, luckily for pilots, not collisions with aircraft.
Because vulture flight activity is associated with the birds' efforts to locate food, the authors suggest that a more detailed analysis of food habitats would help further elucidate the species' foraging behaviors. For instance, turkey vultures prefer small, solitary prey items that are unpredictable and dispersed throughout the habitat; this requires them to potentially spend more time in the air, covering larger distances. Although there have been some studies on how the birds locate their prey, these natural history details remain contested. Additionally, a better understanding of the movements and population numbers of their most common meals--squirrels, rabbits, opossums, etc.--might also help managers predict where and when vulture flight activity will be heaviest.
The scientists used their data to create an 11x15 grid plotting altitude by hour after sunrise; each grid square was colored to indicate the occurrence of vultures at that combination of time and height. This relatively simple technique provides a clear indication of the most dangerous times and heights to fly--the red "danger zone" at <100 m between 4 and 9 hours after sunrise and the orange zone at <400 m between 2 and 12 hours after sunrise indicate combinations that pilots should actively seek to avoid. This particular matrix can be made even more accurate as additional vulture behavior data are collected; similar matrices can also be developed for other locations or other species, potentially minimizing flight danger to pilots and passengers across the country.
Avery, Michael L. Humphrey, J.S. Daughtery, T.S., Fischer, J.W., Milleson, M.P., Tillman, E.A., Bruce, W.E., Walter, W.D. 2011. Vulture flight behavior and implications for aircraft safety. The Journal of Wildlife Management 75(7):1581-1587.
Thanks to the following websites for providing the images used in this post: