Back in 1994, Diane and I were taking an animal behavior course from Dr. Jane Packard. Part of the course involved doing a project concerning observation of Greater Prairie Chickens (Tympanuchus cupido pinnatus) at the SURF facility in College Station. The males put on lekking displays, even in captive facilities like SURF. The official project had something to do with photoperiod or something, but what got our attention was a project that came out of another class we were taking, which was bioacoustics with Dr. Robert Benson.
Dr. Benson assigned us to use some of the recording gear at the Center for Bioacoustics, make some recordings of organisms, and report on it. He wasn’t particular. So, we decided to multi-task a bit and set up the recorder while we were at the SURF facility, capturing the sounds of the male prairie chickens doing their thing.
The first day that we recorded there were two males who made the preponderance of sounds, with a third male making occasional contributions. So we had 45 minutes of prairie chicken sounds to somehow analyze. Not really wishing to get much, much more familiar with the Kay Sona-Graph, I wrote a simple event recorder program, worked out a set of keys for the acoustic events on tape, and set about keying in event values based on closely listening to the tape. At the end, I had a bunch of timestamps and event codes. I wrote another program, this one in Perl, to make a report of the three call types. Then we ran some statistics… and everything was clearly significant. I’ve never had a another dataset like that, before or since.
To let you in on this, you need to know that each male puts on a stereotyped display behavior for the females. Both the behavior and the resulting sound are called a boom, but it really sounds like a low “whoo-hooo-whoo” noise. Males also cackle, which is familiar if you’ve ever heard a chicken do so. And the third vocalization type is a short, sharp noise called a pwoik. That’s the sort of word you only find because you’ve had success using some other search keywords to find relevant papers. It is the boom, though, that gets the girls, if at all.
So, start reeling off the hypotheses… do males preferentially emit booms when other males are quiet? Yes, that’s significant. Do males preferentially emit pwoiks in the midst of other males’ booms? Yes, that’s significant. Do they emits cackles preferentially during other males’ booms? Yes, that’s significant.
There had been some time that had passed between when we recorded that first batch of data and when we next recorded there. By then, there were half a dozen calling males. While it was possible to keep up with keying events in the event recorder program for two, sometimes three, males, there was no way to do the same for six at a time. A different approach had to be used. It wasn’t until 1996 when we came up with it. Our acoustic data had been recorded on a camcorder, so step one was to copy that to standard audiocassettes. Step two was to use a table of random numbers to come up with a tape offset index, and then move the tape in the deck to the indicated offset. Step three: digitize a 30 s sample of sound from the tape into the Canary sound analysis program from Cornell. Step four was to then find and log every vocalization in the sample. When I first did this, I found that I could completely process a 30 s sound sample in about an hour’s time. With practice, I got this down to about forty minutes, as did our special projects student, Lissa Davis. Repeat steps two through four…
Note the difference in time efficiency between the event recorder and the sound analysis approach. After 45 minutes of event recording, there was a dataset ready for processing. We processed 44 sound samples the other way, representing a probable investment of about 60 hours of work, which gave us a total of 22 minutes of examined time. And, when we ran the statistics again, the results were very similar to those obtained in the original dataset produced by event recording.
We presented on this at the 1996 Animal Behavior Society meeting in Flagstaff, AZ. And events conspired to keep us too busy to do anything further with this work, until now. It took a bit of search to locate our original data and analysis files, but now that I’ve got those in hand I’m working this up into a manuscript.
But what does it mean? The male Greater Prairie Chickens appear to prefer to emit their mate-attracting boom call when nobody else is currently making noise. That makes sense. Other males preferentially emit pwoiks and cackles while another male is producing a boom. This bears some thought. Does a sort of acoustic competition occur here, such that distracting a female in assessment of another male’s boom call helps make it more likely that one will, in the aggregate, receive more attention from other females? To address that would require a lot more observation of lekking prairie chickens, such that one could quantify patterns of female assessment and whether there is a higher rate of post-display copulation when the display was uninterrupted than when interruptions occur. For now, what we’ve got is the data on the males and their apparent competitive use of sound on the lek.