Digital sound systems have moved from the living room to the forest. In Yosemite, Calif., researcher are using digital mp3 recorders to used to study a rare species of great gray owl.
Trapping and banding them is traumatic for the
birds, and the Joe Medley, a PhD candidate in ecology at the University of California, Davis,
wanted to find a way to avoid that. So Medley decided to use
digital audio recorders to pick up the owl's calls.
The recorders are called Autonomous Recording Units (ARUs) are powered by batteries and have two-high gain microphones. The ARUs are put inside waterproof cases and hung off of tree branches. They can record for about two weeks at a time. The particular ones Medley used were purpose-built, but there are commercial versions available, he said. (The detectors are only six to eight feet off the ground, so no tree-climbing was needed).
At first he ended up with 50 terabytes of owl calls mixed with background
sounds. So the next step was to tease out the owls' calls. That required sophsticated software, Medley told Discovery News. It's called Raven Pro, developed by the Cornell University Laboratory of Ornithology. Medley had to write the owl-specific parts of the program himself, though. The algorithm he developed searched recording data for a certain amplitude — essentially, the amount of energy in the sound — within certain times and frequencies.
"The detectors are very good at detecting target signals, but also detect a lot of false positives, so we had to develop a secondary processing method where we used a classifier (using a statistics program) to differentiate actual owl calls," Medley wrote in an email.
program could ultimately pick out males and females from juveniles, and even identify nesting
females calling for food. The results are still being analyzed.
Great gray owls are the largest owls in North America, and the ones in Yosemite
are a subspecies that split off from their cousins relatively recently –
during the last ice age, about 30,000 years ago. Great gray owls generally are
more common, with a range that extends through much of Canada and the taiga
forests in Asia. But the group in Yosemite seems to be a genetically
distinct population. They also have differences in behavior such as where
they build nests, migrate and what they eat. Only about 200 still exist today,
and they face threats from humans such as habitat destruction.
Medley added that while owls have relatively low-frequency calls, the technology could also be applied to other animals as well, such as frogs, that have distinctive noises. The methods would be the same — the only difference would be what the software is programmed to pick up.
Credit: National Park
Service / Joe Medley