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第1段
1 .Listen to part of a lecture in an Animal Behavior class.
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2 .We know that birds are extraordinary navigators.
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3 .Many species make long migrations, sometimes 1000s of kilometers, to reach their breeding and feeding grounds.
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4 .What we don\'t know, though, is exactly how birds navigate these long distances.
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5 .Some hypotheses say that birds navigate using Earth\'s magnetic field.
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6 .Others say they rely on airborne odors.
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7 .But no one single hypothesis seems to account for what we observe.
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8 .So I want to tell you a story.
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9 .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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10 .All pigeons can navigate long distances, but homing pigeons are bred to be particularly good navigators.
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11 .Now between 1968 and 1987 the researchers at Cornell raised homing pigeons at the university.
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12 .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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13 .And most of the pigeons did return home without difficulty.
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14 .But there was something about one site, Jersey Hill.
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15 .A large majority of the birds released at Jersey Hill never returned home.
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16 .And remember, these are homing pigeons.
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17 .Jersey Hill is about 119 kilometers west of Cornell University.
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18 .Over the course of the 19 year study, 90% of the pigeons released at Jersey Hill disappeared.
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19 .Time after time they vanished and never made it home.
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20 .That is, except for one day.
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21 .August 13, 1969.
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22 .On that day, all the pigeons navigated home successfully.
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23 .What was so special about that day, and why did the pigeons released at Jersey Hill disappear every other time?
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24 .Well, for years, no one had an answer.
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25 .But in 2015 a scientist named Jonathan Hagstrum looked at the previous data and did a new study.
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26 .Hagstrum thinks the answer is sound waves.
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27 .We\'ve known for a long time that homing pigeons can hear what are called infrasounds.
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28 .Infrasounds are sounds at a lower frequency than humans can hear.
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29 .Hagstrum thought maybe homing pigeons use infrasounds to navigate.
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30 .Okay, so let\'s back up a bit.
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31 .Infrasounds can come from any number of sources, but Hagstrum was interested in infrasounds generated by the Atlantic Ocean.
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32 .Deep waves in the ocean pound against the seafloor, causing energy vibrations.
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33 .These vibrations produce infrasound waves that can travel through the land or through the atmosphere.
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34 .Now what does this have to do with bird navigation?
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35 .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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36 .So the landscape and prevailing weather conditions of a location give it its own unique acoustic signature.
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37 .Hagstrum thought that maybe birds hear these acoustic signatures and recognize the sound of home.
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38 .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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39 .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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40 .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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41 .In other words, the computer makes a kind of visual infrasound map.
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42 .He could see the way infrasound waves bounced off the terrain in the area around Jersey Hill.
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43 .And he found that jersey Hill sits in a spot where infrasound waves from Cornell cannot usually reach.
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44 .It\'s in what\'s called an acoustic shadow zone.
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45 .It seems infrasound waves from the Cornell site skip right over Jersey Hill.
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46 .Okay, so that would explain why so many pigeons had disappeared after leaving Jersey hill.
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47 .But what about that one day when the birds did make it home?
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48 .Well, Hagstrum looked at 19 years of climate data and found that atmospheric conditions were unusual on that particular day.
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49 .As you know, the higher we go in the atmosphere, the cooler it gets.
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50 .But on August 13, 1969 warm air sat above the cooler air.
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51 .The infrasound waves from Cornell bounced off the low cool air and were able to reach Jersey Hill.
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52 .And Hagstrum findings don\'t just solve the mystery of Jersey hill.
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53 .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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