Understanding Pacific Chorus Frog Calls

["davem98607" davem98607]

I am not a biologist or nature recordist, but I do enjoy sound and
make crude recordings with my camcorder every once in a while.
Someday when time and budget allow I would like to get into it more.

I recently recorded some Pacific Chorus Frogs (Pseudacris regilla) in
my backyard pond (http://freesound.iua.upf.edu/packsViewSingle.php?
id=3D2029).  I used a simple binaural mic connected to a digital
camcorder.  I set the mics on the ground right at the edge of the
pond, and let the camera record for 45 minutes.

In listening to the recording I noticed that quite often the frogs
seemed to begin their "chorus" when a distant sound began to increase
in volume.  e.g. from an airplane or train.  I am guessing the frogs
are trying to scare off the "competitor" - or was this just a
coincidence?