Two bird species common to parts of the northern U.S. have attracted the semi-morbid attention of scientists and engineers at Florida Atlantic University and the U.S. Geological Survey. Using carcasses of common loon and lesser scaup species, the team conducted experiments to determine how water current, wind velocity and the amount to which the carcass is submerged affect its drift.
USGS is using the results to develop computer simulations to potentially pin point the location of toxic sources in the Great Lakes region. One of the prime threats to birds frequenting the Great Lakes is type E botulism, a neurotoxin-producing bacterium that causes paralysis and death when ingested. USGS estimates that over 80,000 birds have died from botulism intoxication in the Great Lakes region since 1999. The sites of exposure remain unknown.
The team submitted an overview of their work in an abstract for the upcoming 66th Annual Meeting of the APS Division of Fluid Dynamics to be held in Pittsburgh, PA where they will present their results in detail.
“The rationale behind the two bird species we used was that they’re representative of two different body forms most prevalent of waters birds,” said Karl von Ellenrieder, an associate professor at the Department of Ocean and Mechanical Engineering at FAU and lead author of the abstract. “The lesser scaup has a short neck and body and the common loon has a long neck and larger body.”
The team conducted their experiments in FAU’s towing tank, which gave them control over the conditions of the water and air speeds inside. The longer that bird carcasses are adrift, the more water logged their feathers become and the more the carcass sinks into the water. To account for this, the team submerged carcasses to various levels and then calculated the resistance, or drag coefficient, of the carcass against varying current and wind speeds similar to those of Lake Michigan.
“What makes the birds move after they die and are afloat is the current to some extent, but its largely the wind because often more of the carcass is exposed to the air than is under water. Winds can get up to 15 to 20 knots (17.3 to 23 Mph) while currents are around 2 knots (2.3 Mph),” Ellenrieder said.
The team's measurements showed that the drag coefficient against both water and air was consistently smaller for the larger of the two species, the common loon. This means that the loon experiences less drag but does not necessarily move faster through the water than the lesser scaup. In addition to drag force, surface area plays an important role.
“Though larger birds have lower drag, they also have larger surface areas,” Ellenrieder said. “So depending on relative size of the two birds you may have a loon or scaup drift at the same speed even though the drag coefficients are different.”
The computer simulations, Ellenrieder expects, will incorporate modified versions of search and rescue software used to calculate drift velocities of people gone overboard at sea. Although other scientific publications explore drag and drift for objects such as boats and bodies, this is the first publication to address drift coefficient of bird carcasses.
“We’re hoping that if people spot [dead] birds on the shore, they can use recently recorded measurements of current and wind velocities to back track where the animals came from,” Ellenrieder said.
If scientists know the drag coefficient and couple that with meteorological data, then they can readily calculate the drifting speed of the carcass, providing some information of how far it may have traveled before reaching shore. Determining direction is a bit trickier since waves can contribute to the direction of motion in addition to current and wind. Addressing wave interference is the next hurdle, which the team will tackle using the follow-up grant they now have to continue their research.