Part 1: Birding Acoustics ost birders cannot live without bird sound. Aural identification in - creases efficiency more than any other skill. If all sounds were Mequally incomprehensible to you, you would have to visually iden - tify every robin and towhee to find the visiting Varied Thrush. Anyone who Listen to all the birds in this article! aba.org/birding/v42n4p63w1.html has traveled abroad knows, or once knew, this problem. Moreover, some birds, such as most owls and nightjars and some rails, are unlikely to be de - tected visually because they are active mainly at night. And then there are the species that are easy to see, but still difficult to identify visually—for exam - ple, Chaetura swifts, Catharus thrushes, wood-pewees, and empids. I’m much more comfortable with these aurally than visually. Beyond the identi - Arch McCallum fication quest is the sheer joy (Kroodsma 2005, 2009) of hearing bird sounds Applied Bioacoustics in all their kinds, melodious and noisy, serene and hyperactive; of knowing P.O. Box 51063 the sounds themselves as well as who made them. Eugene, Oregon 97405 The crack birders know almost all species by their sounds. For most of us, [email protected] though, aural skills lag behind visual ones. So why not visualize sound as well as shape and color? We can do this by recording the sounds in the field and using a computer to convert the digitized sound waves to sound pictures All sonograms in this (Kroodsma 2009, Strycker 2009), via audio spectrograms (also known as article appear courtesy of “sonograms”) and oscillograms (graphs of the waveform of the sound). These © Arch McCallum, unless visual representations of sounds help us (1) name different kinds of sounds otherwise indicated. for clearer communication (Pieplow 2007), (2) increase “ear-birding” skill by 50 BIRDING • JULY 2010 engaging visual memory in building a library of known lar-looking notes. I’ll also show how small, easy modifica - sounds, and (3) objectively evaluate recordings that are tions can produce big effects, which can be diagnostic. In presented as documentation for distributional records. a companion article, to appear later this year in Birding , I’ll With improvements in technology and the emergence look at the rules birds use to combine notes into phrases , of a creative commons on the internet (for example, xeno- phrases into songs (Fig. 1 ), and songs into singing per - canto.org and earbirding.com), not to mention free access formances (Fig. 2 ). Such rules produce the broader tem - to traditional archives (for example, tinyurl.com/djfxxe poral patterns of vocalizing that we depend upon for and tinyurl.com/24svbaq), recordings and sound pictures identification of most singing birds. will play an increasingly important role in ear birding. In an article in Birding in 2007, Nathan Pieplow intro - Lest anyone miss out on the fun, I present here visual duced a spectrographically based vocabulary for describ - primers on the two foundations of ear birding: acoustics ing how sounds sound. I aim to complement his important (this article) and syntax (later this year in Birding ). work by shifting the focus from your brain to the bird’s vocal tract, with a nod to the brains of the intended re - he basic unit of bird sound is the note , which, just like a ceivers of these sounds. I believe you will be astonished at Tmusical note, is a sound that is continuous in time. My the variety of sounds birds make and at the variety of tech - goal in this article is to show how different classes of notes niques they use to make them. No other natural sound- look and to outline the different ways birds produce simi - makers come anywhere close to the virtuosity of birds, and Fig.1. Birds’ songs are composed of phrases , which are composed of notes . Shown are single songs of a Carolina Wren from Warren County, Virginia , 9 April 1983 (top ) and an Eastern Towhee from Hayward County, North Carolina, 26 July 2006 (bottom ). Each continu - ous trace is a note. Carolina Wren songs are tandem repeats of a single phrase-type, each of which is a permanent member of the bird’s repertoire of two or three dozen. This phrase- type contains five notes. The fifth phrase in the song is incomplete. The towhee’s song pro - ceeds according to a plan that has been im - mortalized as drink your tea! A brief whistle and sharp click provide the drink , a low whistle is the your , and tea is a slow trill. Each element in the trill is actually a two-note phrase. Carolina Wren. Harris County, Texas; December 2008. Photo by © Alan Murphy. WWW.ABA.ORG 51 BIRDING BY EAR you deserve, as a birder, to know just how good they are. A whistle is flat on a sonogram (Fig. 3 ), which means it Be amazed, but not dismayed, by all the variety. Every does not change frequency. The everyday term for fre - sound is an event in time, and almost all sounds have a quency is “pitch.” The latter is actually a perceptual term, simple and discernible pitch trend, a tune that you could whereas the former is a physical quantity, the number of whistle. The variety comes from what birds do with carrier oscillations per second of the sound source. The inset in frequencies . Fig. 3 is a short segment of the oscillogram, which graphs the waveform of the sound. Notice that it is a simple sine Carrier Frequencies wave—from trigonometry, remember? The oscillations The underlying sound, or carrier frequency, is a simple, above and below zero are a reading of the back-and-forth thin, line on a sonogram. We can characterize this line as movements of a diaphragm in a microphone, which mim - a whistle if it is flat, a click if it is nearly vertical, and a slur ics the movements of membranes in the vocal organ, or or warble if it is in between. These names just describe syrinx, of the bird. The oscillations of the membranes ini - points on a continuum, but they can be useful. tiate pressure waves, which are “read” by the microphone. Many bird sounds, though, are not so simple, because There’s not room for the oscillogram of the entire sound, the bird adds information to the simple pitch trend of the above the sonogram, to show that level of detail, but it carrier. This process of “modulation” gives us derived spec - does show (in volts) the time-varying amplitude (loudness) trographic shapes that I refer to as buzzes , trills , and chords . of the sound. The frequency of 4,000 oscillations (or cy - Fig.2. Shown here are five songs from a bout of singing by an American Robin from Wasco County, Oregon , 9 April 2004. Each song is a slightly different combination of the bird’s fi - nite repertoire of phrase-types, six of which are indicated by letters. Each phrase is composed of one or more notes. The unlettered phrases are the fainter hissely phrases discussed by Kroodsma (2005:30). They are not used by the tanagers and grosbeaks whose songs may be confused with those of robins. American Robin. Lane County, Oregon; June 2009. Photo by © Joe Fuhrman. 52 BIRDING • JULY 2010 cles) per second is referred to as 4,000 Hertz (Hz), or 4 chea (windpipe) divides into the two bronchi that lead to kiloHertz (kHz). It translates to a flat line at 4 kHz on the the lungs. vertical axis of the sonogram, which, as you see, is a graph of frequency against time. If you need to read that over one Raps, Clicks, and Slurs more time, please go ahead. It’s sound production, record - Rotate the flat trace of the whistle almost 90 degrees and ing, and sonography, all in one paragraph. you get a click or rap (Fig. 4 ). The trace of a click can’t be exactly vertical because that would indicate instantaneous Frequency Modulation pitch change, which is physically impossible. The mem - The whistle, or “pure tone,” is the bread and butter of branes in a bird’s syrinx can change their oscillation rates acoustical theory, psychoacoustic research, and western several kHz in ten milliseconds (1/100th of a second), music—and I don’t mean country and western “music.” though, which is very close to instantaneous, at least for The simplest process by which birds dress up their sounds our ears and brains. When stretched out, clicks have dis - is frequency modulation , which produces the changes in tinctive and often diagnostic fine structure. The birds prob - pitch trend that give us clicks, slurs, and warbles. It is ac - ably hear this detail quite well. Many crack birders complished by changing the shape of the syrinx, either apparently resolve enough of the detail to identify birds with the muscles associated with the syrinx (Suthers 2004) accurately by these brief call notes. Sonograms offer hope or by changing the pressure in the air sacs that surround to the rest of us. the syrinx (Beckers et al. 2003, Amador et al. 2008). The A rap, unlike a click, is truly vertical on a sonogram, be - syrinx is a bird’s voice box, which is located where the tra - cause it is actually a very brief chord , discussed later in this Fig.3. Shown here are whistles of a Mountain Chickadee from Dolores County, Colorado , 27 April 1982. This sonogram has very high frequency resolution (at the expense of temporal resolution), so you can see just how close this male came to whistling a perfectly pure tone. Was he trying to? The gray spackling at the bottom of the sonogram is low-frequency noise, compliments of the wind.