WASHINGTON — The blips and blobs caused by flying animals may look like noise to storm-watchers studying Doppler radar data, but these signals could be a big hit with bat biologists.
Bats swirling out of caves show up on weather radar, as do masses of birds and other flying animals. Now that the National Severe Storms Laboratory in Norman, Okla., is merging its radar data onto a single nationwide frequently updated map, biologists have an opportunity to get a new view of aerial creatures, said ecologist Winifred Frick of the University of California, Santa Cruz. She presented a preview of biological studies using the stitched together Doppler data on February 19 at the annual meeting of the American Association for the Advancement of Science.
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Biologically speaking, the air is “a very unexplored part of our biosphere,” said Thomas Kunz of Boston University. Kunz, a bat biologist, has christened this emerging discipline “aeroecology.”
At the meeting, Frick described how, in a radar sequence from southern Texas, she had seen something she had never imagined: swarms of bats following an evening weather front. “Radar is like a really fancy pair of binoculars,” Frick said.
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In the radar view of a storm front rolling over Texas, Frick pointed out a frayed tan bar indicating insect swarms moving with the air mass. The radar doesn’t reveal much about the identity of species in a cloud, but Frick knew that the front was passing over a bat cave. A yellow burst on the radar showed what Frick identified as dense crowds of Brazilian free-tailed bats speeding out of their cave at sunset to hunt for insects. The burst bloomed into a cloud of bats, with an elongated mass sliding west along with the bugs. “It’s a big aerial buffet line,” Frick said.
Radar monitoring of airborne critters had meeting attendee and environmental educator Susannah Graedel of Madison, Conn., grinning at the possibilities. “I was a bird bander,” she said, recalling how much work and luck it takes to fit a band on one bird and eventually, maybe, with a lot of luck, find out where it traveled after that.
Networked radar data also inspired Frick, Kunz and their colleagues to put a weather spin on studies of the tradeoffs bats make in timing their hunts. Two years of radar data showed that after hot days during a drought, bats tended to emerge earlier in the evening. Earlier meant more light in which hawks and other predators could nail a bat. Yet during a dry year, insects are scarcer and bats appear willing to take the risk, showing up early for the evening flights of moths and other insects. Simple thirst may drive bats out of caves earlier too, especially the nursing females. (Baby bats require prolonged nursing “equivalent to if we nursed our young until they’re teenagers,” Frick said.) In a wetter year with presumably better hunting, however, hot days didn’t provoke bats to take such risks.
From a radar perspective, bats are just another kind of cloud. And Frick is now working with meteorological radar physicist Phillip Chilson at the University of Oklahoma in Norman to tweak a method for estimating the number of raindrops in a cloud. The same approach could estimate the number of bat “drops” in a living cloud.
The radar studies help reveal how, much like sea creatures seek out upwellings of nutrient-rich water or surf along currents, flying animals navigate the changing landscape of their ocean of air. “I had never thought of air as a dynamic habitat before,” Frick said.
In the last several years the storm lab has developed combined data from the nation’s 156 NEXRAD Doppler installations and is working on incorporating radars from major airports. These new radar networks will better enable biologists to take advantage of the method to study living things in the air. “Networks — that’s the exciting and powerful thing,” Chilson said.
Weather radar has limitations, Chilson said. For example, coverage fades as the Earth curves down beneath any radar beam. Yet there’s a lot of data available online (at a cooperative NSSL website), including a raw version of interest to biologists. Not so for weather watchers: The signs of bats and birds amount to, Chilson said, “stuff that meteorologists try with a passion to get rid of.”