While severe weather's impact on people has long been studied, researchers at the University of Oklahoma have found a correlation between localized severe weather and stress on animals.
The El Reno, Oklahoma, tornado killed eight people and caused millions of dollars in property damage on May 31, 2013. It also stressed birds in the area to the point it became noticeable to biologists studying Grasshopper Sparrows in a nearby research area.
The El Reno tornado, which struck northwest of Oklahoma City, reached 2.6 miles wide at its maximum size, which makes it the largest tornado on record, AccuWeather.com Meteorologist Becky Elliott said.
"It had a peak forward speed of 55 mph, with wind speeds close to the surface of 295 mph measured by the Doppler on Wheels," Elliott said.
While out capturing Grasshopper Sparrows as part of a different project being headed up by Biologist Alice Boyle of Kansas State University, a large proportion of juveniles were found to have synchronous bands of structural and pigment deficiencies across their tails, Executive Director Jeremy D. Ross of the George M. Sutton Avian Research Center at the University of Oklahoma said.
"Generally this is rare in wild bird populations and the markers, which we've since termed 'pallid bands,' were thought to be associated with a past stress response," Ross said. "Intense bouts of stress can lead to pallid bands or the related phenomenon known as 'fault bars' in the developing feathers of birds.
The fault bars are narrow lines extending outward from the feather shaft where the barbules that comprise most of a feather area are completely missing and can cause a bird to be less maneuverable in flight, Ross said.
Ross immediately thought there may be a correlation between the pallid bands and the El Reno tornado. Prior to this study, research was mainly limited to bird stress and larger events such as hurricanes and blizzards.
Hail up to 2.75 inches in diameter fell in the study area in Osage and Washington counties, just northwest of El Reno, researchers estimated using National Weather Service data.
The stress likely, at the very least, slowed the development of nestlings and fledglings that survived, Ross said.
"Of course, our data also indicated that as much as 25 percent of the normal egg-to-fledgling production was either directly killed, crushed, or never even initiated during or after the storm event," he said.
The storm had many smaller "sub-vortices" outside the main circulation and these were where the strongest wind speeds were recorded, Elliott said.
"The tornado stayed mainly over farmland, thankfully, so there was a lack of damage found in the storm survey. Because of this the tornado was rated an EF3 (on the Enhanced Fujita Scale), but wind speeds of EF5 or greater were measured," she said.
The study has inspired the wider development of severe storm ecology at the University of Oklahoma, Ross said.
"We are fortunate enough to have the National Weather Service and the National Severe Storms Laboratory on our Norman campus and we have been working with them to better understand what biological impacts severe storms have," he said. "There are obviously conservation and agricultural implications, but also human safety tie-ins. For instance, our research may help to predict which urban tree species are most vulnerable to failing during a storm and falling on houses."
"Perhaps most importantly, we can actually use biological damage on the ground to better inform weather models as to what the likely wind speeds and hailstone sizes were at the ground. This will help the National Weather Service's early warning system to better interpret their radar data streams, predict possible threats on the ground and [better relay] this information as accurately to the public as possible (i.e., avoid false negatives or false positives)," he added.