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Listen to part of a lecture in an Animal Behavior class.

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We know that birds are extraordinary navigators.

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Many species make long migrations, sometimes 1000s of kilometers, to reach their breeding and feeding grounds.

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What we don\'t know, though, is exactly how birds navigate these long distances.

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Some hypotheses say that birds navigate using Earth\'s magnetic field.

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Others say they rely on airborne odors.

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But no one single hypothesis seems to account for what we observe.

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So I want to tell you a story.

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Some time ago, between 1968 and 1987 researchers from Cornell University in the state of New York were studying homing pigeons.

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All pigeons can navigate long distances, but homing pigeons are bred to be particularly good navigators.

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Now between 1968 and 1987 the researchers at Cornell raised homing pigeons at the university.

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They released them periodically from various sites at different distances from the university, and tracked the birds to see if they could find their way back to Cornell.

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And most of the pigeons did return home without difficulty.

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But there was something about one site, Jersey Hill.

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A large majority of the birds released at Jersey Hill never returned home.

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And remember, these are homing pigeons.

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Jersey Hill is about 119 kilometers west of Cornell University.

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Over the course of the 19 year study, 90% of the pigeons released at Jersey Hill disappeared.

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Time after time they vanished and never made it home.

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That is, except for one day.

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August 13, 1969.

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On that day, all the pigeons navigated home successfully.

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What was so special about that day, and why did the pigeons released at Jersey Hill disappear every other time?

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Well, for years, no one had an answer.

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But in 2015 a scientist named Jonathan Hagstrum looked at the previous data and did a new study.

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Hagstrum thinks the answer is sound waves.

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We\'ve known for a long time that homing pigeons can hear what are called infrasounds.

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Infrasounds are sounds at a lower frequency than humans can hear.

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Hagstrum thought maybe homing pigeons use infrasounds to navigate.

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Okay, so let\'s back up a bit.

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Infrasounds can come from any number of sources, but Hagstrum was interested in infrasounds generated by the Atlantic Ocean.

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Deep waves in the ocean pound against the seafloor, causing energy vibrations.

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These vibrations produce infrasound waves that can travel through the land or through the atmosphere.

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Now what does this have to do with bird navigation?

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Well as infrasound waves travel through the atmosphere, they bounce off mountains or high buildings and get redirected downward by cold air and wind high up.

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So the landscape and prevailing weather conditions of a location give it its own unique acoustic signature.

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Hagstrum thought that maybe birds hear these acoustic signatures and recognize the sound of home.

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Since infrasounds can travel 1000s of kilometers in the atmosphere, birds might be able to navigate by listening for familiar acoustic cues.

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Hagstrum hypothesized that the pigeons at Jersey Hill were for some reason unable to hear these acoustic cues from Cornell University.

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To test this hypothesis, he used a software program that creates three dimensional models of infrasound waves interacting with the terrain, the physical features of the land and weather.

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In other words, the computer makes a kind of visual infrasound map.

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He could see the way infrasound waves bounced off the terrain in the area around Jersey Hill.

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And he found that jersey Hill sits in a spot where infrasound waves from Cornell cannot usually reach.

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It\'s in what\'s called an acoustic shadow zone.

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It seems infrasound waves from the Cornell site skip right over Jersey Hill.

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Okay, so that would explain why so many pigeons had disappeared after leaving Jersey hill.

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But what about that one day when the birds did make it home?

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Well, Hagstrum looked at 19 years of climate data and found that atmospheric conditions were unusual on that particular day.

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As you know, the higher we go in the atmosphere, the cooler it gets.

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But on August 13, 1969 warm air sat above the cooler air.

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The infrasound waves from Cornell bounced off the low cool air and were able to reach Jersey Hill.

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And Hagstrum findings don\'t just solve the mystery of Jersey hill.

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They strongly suggest that Infrasound is important to bird navigation, and go a long way in explaining how birds find their way.

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