Wednesday, July 05, 2006

Joining the Bat Team

Photo of bat caught in mist net

Bioblitz I, June 27, 2003
Photo by Danielle Gustafson

The second Central Park BioBlitz took place from noon Friday, June 23 to noon Saturday, June 24. At the event various experts cooperated with volunteers in a census of all living creatures to be found in Central Park. The event was sponsored by the Explorers Club and The E.O Wilson Biodiversity Foundation and hosted by the NYC Dept. of Parks, the Central Park Conservancy and the Urban Park Rangers.

During the BioBlitz volunteers joined teams searching for various taxonomical groups -- i.e. insects, reptiles and amphibians, birds, mammals. As in the last Bioblitz I joined the Bat team. Our stint: 8:30 p.m. to 11. Our destination: the North Woods.

Our expert was Carl Herzog, a wildlife technician with the New York State Dep't of Environmental Conservation [DEC] and a highly knowledgeable bat person. Also on our team were three local bat experts familiar with the bat population of Central Park, one of them Danielle Gustafson. Then there were four or five others such as me, attracted by the romance of bats and the allure of the park in the dark. We were not disappointed.

We were literally detectives, because we carried bat detectors. Carl also carried a portable computer to which one of the detectors was attached.

What exactly is a bat detector? As you may know all bats use echolocation to help them hunt in the dark. They zero in on their insect prey by emitting high-pitched sounds and then listening to the sounds' ehoes when they collide with objects in the air. The objects are inevitably insects. The echoes provide the bats with information about the direction, size, shape and velocity of their insect prey.

Echolocation is a relatively new concept. Towards the end of the 19th century a Swiss zooologist came up with the idea that bats hunted with their ears instead of their eyes. But his theory was rejected completely. Not until 1944, inspired by military research at the beginning of World War II [Sonar, Radar etc] did Donald Griffin, who was then a Harvard undergraduate, come up with an elegant experiment firmly demonstrating that bats do, indeed, navigate and hunt in the dark via ultra-sonic emissions and their echoes. His book about his discovery, Echoes of Bats and Men, is a classic of science reporting.

But why the detectors? Couldn't we simply find bats by listening for their calls as we listen for bird songs? As you may know the echolocation calls emitted by bats are too high for us to hear. Bats produce sounds in frequncies between 20 and 200 kiloherz [kHz]. while the human ear generally cannot hear sounds above 20 kHz.[Well, very young people can sometimes hear frequencies a bit higher, as recent newspaper articles have reported, an ability teenagers exploit by programming cell phone rings above 22 kHz that their teachers cannot hear.]

Bat detectors are hand held instruments that can pick up those high-frequency sounds and translate them into lower frequency sounds we can easily hear. When a bat comes into range. the bat detector begins to click loudly.

Carl's computer was programmed to record and analyze the particular bat emissions being picked up by the detector at any given moment, thereby giving a clue to that particular bat's species.

The bat team set forth at 8:30 p.m. on Friday from Bioblitz headquarters at the North Meadows Recreational Center. With all detectors turned on we made our way to the nearest body of water, the Pool , as it is called, a little pond between 100th and 103rd St.[ There is another little pond in Central Park, at 59th St., that is called, oddly enough, The Pond.] On our way we looked out for Black Locusts and Shagbark Hickories, both trees with rough, flaking bark. These, we learned, might offers nesting opportunities for local bats. But the bat detectors revealed no bat action in the woods. Several trees had large hollows we thought might serve as bat homes. No, Carl, said, bats avoid large openings. Very small crevices offer far better protection from predators. What predators might bats encounter? Raccoons, among others. Central Park seems to be wall-to-wall raccoon this year.

We were nearing the Pool when we heard our first clicks. It was about 8:45. We stopped near a street lamp and actually caught a few fleeting glimpses of bats, dark, erratically flying shapes swooping between trees and the water. According to the frequency range indicated by the bat detectors, and also by the bat experts' glimpses, these were probably Little Brown Bats. The final report, based on further analysis of the computer recordings, will be available on the Explorers Club website in a few weeks; until then my report here is tentative.

To be sure many bat species emit sounds within a range of frequencies that overlap with other species. The frequency range of the Little Brown Bat is 38 - 62 kHz. The Big Brown Bat's range is 25-51. So if you detect a frequency of, say, 45 kHz, it could be either. Then the visual information is crucial: one of these is a lot bigger than the other. You can guess which one.

We took a path from the Pool down a slope and under the Glen Span Arch. Our next waterbody was at hand -- the Loch, a little meandering stream that flows picturesquely through a deeply wooded area, under another bridge [Huddlestone Arch] and into the Harlem Meer. Sure enough, as we approached the Loch the detectors began to click again, like Geiger counters at a radium lode.

As we approached our last waterbody, the Harlem Meer, we were not surprised to hear another fusillade of clicks from the detectors. One bat here was a surprise. Based on a sighting as well as the computer report, Carl believed he had found a Northern Long-eared Bat, a species never before identified in Central Park. This sort of discovery is what a Bioblitz is all about. Our team felt triumphant, as if we had found a new species of bird in the South American rain forest, or, at least, an Ivory-billed Woodpecker in some lonesome bayou.

On our way back we ran into the Reptile and Amphibian team. They did not seem quite as elated. They had only one species on their list: the bullfrog.