"If there is a bird target here, you can see it on the radar display as an echo," Larkin said. "You throw a switch and it locks onto the target, it tracks the target, and wherever the bird flies, the radar points at it."
The radar kept track of a target's distance (from the radar), altitude and direction of travel over time. It also provided data used to calculate the frequency of a target's wing beats. Since the radar could also track flying insects and other arthropods, the wing beat data would be important for distinguishing birds from bugs.
In collecting the data, Larkin, Szafoni and colleagues had used the radar in a new way. Once the radar operator had identified a flying object that might be a bird and began tracking its flight, he or she looked for other objects entering the radar's beam. If another potential target appeared, the radar could follow it for a few seconds before switching back to the first. By repeatedly switching back and forth between two targets, the operator could potentially detect the discrete flight details of two birds at a time.
Determining whether two birds were actively traveling together was tricky, Larkin said.
"Even back in the 1970s it hit me that you can have two birds flying absolutely parallel in the same direction and at the same height, but they can be flying at such a different speed that one of them gains on the other and they're just, you know, automobiles passing on the expressway," he said. "They're simply taking the same route and not keeping together."
Similarly, two animals may be going at similar speeds but at a slightly varying angle to one another.
"After a while they would be kilometers apart," Larkin said. This would be clear evidence that the birds were not traveling together.
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign