What we're hearing during nautical twilight in the Denver metro region

[Ted Floyd]

Hey, all.

One of my favorite locations in spacetime is nautical twilight in residential and urban districts in the Front Range metro region in early September. The plains harvest flies, Megatibicen dealbatus, have largely quieted down by that time, leaving the Twilight Big Three (TBT) to do their thing. The TBT are: the low-frequency snowy tree crickets, a.k.a. "thermometer crickets," Oecanthus fultoni; the mid-frequency fall field crickets, Gryllus pennsylvanicus; and the high-frequency greater angle-wing katydids, Microcentrum rhombifolium. Here's the sound of all three of them going at once:

https://www.inaturalist.org/observations/32419411

I made that recording in Lafayette, eastern Boulder County, at 8:18 pm on Sat., Sept. 7. 2019. The temperature was 71 degrees Fahrenheit. Key point: I was in a residential district, away from the marshes and meadows of, say, Greenlee Preserve, where many other orthopterans join the fray and complicate matters. This is the sound of the city (also the sound of the suburbs) in the region. Call it the symphony of the city. The snowy tree crickets have a distinctive, musical, pulsating quality; count the number of chirps in 15 seconds and add 40 and you have the temperature in degrees Fahrenheit. The fall field crickets are higher and shriller, the classic "nothing but crickets" of 20th-century television and movies. (21st-century, too, perhaps, but I haven't watched TV since about 1990. I hear there's something called flatscreen and reality shows??) And the greater angle-wing katydids are the spooky "clicky night bugs," like a little ghoul--a DIA lizard person, perhaps--chattering its teeth in the treetops.

I find spectrograms to be useful for understanding and enjoying natural sounds, so here's a spectrogram of the recording above:

The top panel is, in essence, a measure of how loud the sounds are. The fascinating and--for me--somewhat depressing thing is that the katydids (indicated by the vertical lines) are unbelievably loud. I don't hear them all that well anymore. But my wife and kids tell me they are as loud as the flickers that are slowly but surely denuding the house of its siding and roofing. The steady pulses, barely visible above the baseline trace, are the snowy tree crickets; and the baseline trace itself is the chorusing of the fall field crickets.

The lower panel is, if you will, the musical score of the orthopterans' songs. A musical score is a plot of frequency against time (really! it is!), and this lower panel is precisely the same thing. Different units and scales, but it is trivial to "translate" from one notation to the other. Anyhow, you can see the high clicking of the katydids (so-called "carrier frequency" way up there just under 10 kHz), the mid-range trilling of the fall field crickets (around 5 kHz), and the distinctively low-frequency grinding of the snowy tree crickets (just under 3 kHz).

There's a nice tutorial in the physic literature on all of this, although applied to the songs of common American birds, not orthopterans. The paper came out just a week ago:

https://aapt.scitation.org/doi/10.1119/1.5124271

Some of the content is behind a paywall. If you want the PDF of the full article, please get with me backchannel, and I'll be happy to send it along.

Or simply enjoy the songs, just as they are! Remember: low, pulsing = tree crickets; higher, shrill = field crickets; very high, clicking = katydids. And this: Listen in busy neighborhoods; tree-lined boulevards are the best.

Ted Floyd
Lafayette, Boulder County

[Martha LKJ]

I found your article and recording fascinating. Thank you for sharing!!

Martha Jones, Arvada, Jefferson County