Listen to part of a lecture in an Animal Behavior class.We know that birds are extraordinary navigators.Many species make long migrations, sometimes 1000s of kilometers, to reach their breeding and feeding grounds.What we don\'t know, though, is exactly how birds navigate these long distances.Some hypotheses say that birds navigate using Earth\'s magnetic field.Others say they rely on airborne odors.But no one single hypothesis seems to account for what we observe.So I want to tell you a story.Some time ago, between 1968 and 1987 researchers from Cornell University in the state of New York were studying homing pigeons.All pigeons can navigate long distances, but homing pigeons are bred to be particularly good navigators.Now between 1968 and 1987 the researchers at Cornell raised homing pigeons at the university.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.And most of the pigeons did return home without difficulty.But there was something about one site, Jersey Hill.A large majority of the birds released at Jersey Hill never returned home.And remember, these are homing pigeons.Jersey Hill is about 119 kilometers west of Cornell University.Over the course of the 19 year study, 90% of the pigeons released at Jersey Hill disappeared.Time after time they vanished and never made it home.That is, except for one day.August 13, 1969.On that day, all the pigeons navigated home successfully.What was so special about that day, and why did the pigeons released at Jersey Hill disappear every other time?Well, for years, no one had an answer.But in 2015 a scientist named Jonathan Hagstrum looked at the previous data and did a new study.Hagstrum thinks the answer is sound waves.We\'ve known for a long time that homing pigeons can hear what are called infrasounds.Infrasounds are sounds at a lower frequency than humans can hear.Hagstrum thought maybe homing pigeons use infrasounds to navigate.Okay, so let\'s back up a bit.Infrasounds can come from any number of sources, but Hagstrum was interested in infrasounds generated by the Atlantic Ocean.Deep waves in the ocean pound against the seafloor, causing energy vibrations.These vibrations produce infrasound waves that can travel through the land or through the atmosphere.Now what does this have to do with bird navigation?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.So the landscape and prevailing weather conditions of a location give it its own unique acoustic signature.Hagstrum thought that maybe birds hear these acoustic signatures and recognize the sound of home.Since infrasounds can travel 1000s of kilometers in the atmosphere, birds might be able to navigate by listening for familiar acoustic cues.Hagstrum hypothesized that the pigeons at Jersey Hill were for some reason unable to hear these acoustic cues from Cornell University.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.In other words, the computer makes a kind of visual infrasound map.He could see the way infrasound waves bounced off the terrain in the area around Jersey Hill.And he found that jersey Hill sits in a spot where infrasound waves from Cornell cannot usually reach.It\'s in what\'s called an acoustic shadow zone.It seems infrasound waves from the Cornell site skip right over Jersey Hill.Okay, so that would explain why so many pigeons had disappeared after leaving Jersey hill.But what about that one day when the birds did make it home?Well, Hagstrum looked at 19 years of climate data and found that atmospheric conditions were unusual on that particular day.As you know, the higher we go in the atmosphere, the cooler it gets.But on August 13, 1969 warm air sat above the cooler air.The infrasound waves from Cornell bounced off the low cool air and were able to reach Jersey Hill.And Hagstrum findings don\'t just solve the mystery of Jersey hill.They strongly suggest that Infrasound is important to bird navigation, and go a long way in explaining how birds find their way.