Could these mysterious infrasounds -- emanating from the oceans -- be the signposts that guide birds around the wide Earth? And, if so, why were the pigeons at Jersey Hill unable to hear them?

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Following Keeton's death in 1980, the database containing the results of his pigeon releases was made available to all researchers upon request. Hagstrum took that data, and added some more: weather and topography. "The things that really influence sound are temperature, wind speed, and wind direction," Hagstrum said. He continued, "Those three things and then: the terrain."

Using a software program called HARPA (Hamiltonian Acoustic Ray-tracing Program for the Atmosphere), Hagstrum was able to model the infrasound landscape for Jersey Hill, taking into account the topography of the place and the particular atmospheric conditions at the times of the releases. (For the weather data, Hagstrum used a NOAA database of twice-daily wind measurements.) "So you can create this virtual model of the atmosphere and the terrain on the day the birds were released," he explained.

Hagstrum looked at the infrasound waves as they would travel from the Cornell loft to Jersey Hill and he found something very strange: Infrasound waves coming from Ithaca and heading westward skipped right over Jersey Hill, as he reports in his paperin the most recent issue of the Journal of Experimental Biology. Infrasounds from other locations reach Jersey Hill, but the sounds that carried the specific mark of the Ithaca loft -- the ones that announced "Home: This way!" to the pigeons -- they were absent. For the birds of the Cornell loft, Jersey Hill was in an acoustic "shadow zone."

Acoustic shadow zones -- also known as "zones of silence" -- are a normal phenomenon: Sound waves bend up through the troposphere (the lower 10 km of the atmosphere), reach the stratosphere where the temperature gradient changes, and are bent back down. The space in between where the rays go up and where they come back down is mostly silent. This is why in big explosions, such as the eruption of Mt. Saint Helens, people closer in often don't hear anything, while those further out do. Released in a shadow zone, the Cornell pigeons were lost.

But why, then, on August 13, 1969, were they able to navigate so well? When Hagstrum modeled the atmospheric conditions from that date, he found an anomaly: Something -- a wind shear or a temperature inversion at about a kilometer's height -- bent the sounds back down. "It bent it down early enough that it hit at Jersey Hill," Hagstrum said.

No one on the ground would have ever noticed anything odd weather-wise. It was a perfectly sunny day. Keeton and his colleagues took careful weather notes -- all for naught: The weather on the ground was more or less irrelevant. "This was going on a kilometer above their heads," Hagstrum elaborated. "They would have been sitting there completely oblivious." By Hagstrum's calculations, this sort of condition would have happened about 5 percent of the time -- infrequently enough that Keeton would have seen its effects just once.