Chiricahua National Monument
Chiricahua National Monument is part of the Chiricahua mountain range in southern Arizona.  The monument hosts many of the bat species that are found throughout the Sonoran Desert and is an ideal place to gather data and recordings on a wide variety of species.   In May of 2005, I joined biologist Karen Krebbs of the Arizona Desert Sonora Museum and her research team at two of her favorite field sites.  Karen is currently engaged in a large scale, multi-year, bat population survey which involves netting and cataloging the various species at the monument.  This was an ideal opportunity to make controlled recordings of individual species.

Karen organized each of the sites into three separate areas, a netting area, a cataloging area, and a release and recording area.  The recording area was chosen to be far enough away from the nets and cataloging areas that we could make good quality recordings without much interference.  It was also far enough away that the recording activity would not interfere with the main survey.

 All recording was performed without holding or “zip-lining” the bats.  Instead, the bats were released by hand and where recorded while being released and during the initial phases of flight.  One of the most notable benefits was that it allowed the recording task to be added with little impact to the netting and cataloging effort.  This technique is also easier on the bats and can potentially provide recordings that better represent bats vocalization dynamics in the wild.

For recording, I used two AR125 receivers and a lap-top computer with SPECT’R software.  One receiver was placed on a tripod just a few feet from the release point and the other was placed on another tripod twenty feet back.  The idea was to record the bats just prior to release with the close up receiver and, at the same time, to record them as they flew away using the far receiver.  I tried to positioned the far receiver in the direction I expected the bats would fly.  Obviously, the bats had their own preferences, but after a few positioning “adjustments”, I was able to find a reasonable compromise.   In general, the results were good, but next time I may include additional “far” receivers.

The bats were recorded one at a time.  Once identified and cataloged, a bat would be carried to the recording area and released by hand.  While the bats might be angry or distressed as they are brought over, once the grip is released and the hand is opened, they quickly calm down.  Few take flight right away.  Most are fairly cautious and will scurry around on the hand and perform some initial echolocations to check out the surroundings before they take flight.   These sites were very productive.  Over the course of two nights, we made recordings of twelve difference species.  For several of these, we were able to make recordings of both males and females.

I will like to acknowledge and thank Karen Krebbs for her support of this effort and for her expert guidance.

Examples to ponder

Big Brown Bat (Eptesicus Fuscus) - Gender: male
I chose these particular snippets to illustrate how a big brown vocalizations change as they are released and take flight.  Typically the initial calls are very short duration chirps, but the duration is increased and the vocalizations are stretched as the flight progresses.  The sonogram segments below illustrate these vocalization changes.  The initial vocalizations of this brown bat were only 2 msec long and contained three strong harmonics.  As the bat took flight, the both the duration and the fundamental frequency were increased.  The last of the vocalizations were on the order of 6 msec as the bat flew out of the area.  One other interesting note is that some of the initial (at rest) vocalizations start at a low fundamental frequency, but that the 2nd harmonic is actually dominant.

Sonogram sequence illustrating the vocalization changes when taking flight

This is a recording of a big-brown just as it was being released.

This is a recording of the same big-brown as it was in flight.