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Subsong vs. Whisper Song

Subsong vs. Whisper Song

The American Robin frequently gives both subsongs and whisper songs. Photo by Mr. T in DC (Creative Commons 2.0).

Imagine a male robin, treetop in the early morning, belting out his song for all the world to hear, announcing his territory at the top of his avian lungs.  It’s an easy thing to imagine.  In fact, it’s pretty close to the stereotypical image of a singing bird.

Now picture that same male robin, deeper in the foliage this time, singing a song somewhat reminiscent of the usual treetop carol, but far, far quieter — so quiet, in fact, that it can’t be heard at 50 yards, and so subtle that the bird doesn’t even open its bill to sing, a slight fluttering of its throat the only clue to the source of the ventriloquial melody.

If you listen carefully to birds at close range, you’ll find that quiet, complex vocalizations like these are not uncommon.  Often, they are called “whisper songs.”  Some more technically-minded birders might call them “subsongs.”  Both subsongs and whisper songs are fascinating, but they are not the same thing.  Let’s look at the similarities and differences.

Subsong

The term “subsong” has meant a number of different things since it was first coined in 1936, but I have generally thought of it as The Sound Approach described it:

Subsong…is usually given from dense cover, is often full of mimicry, and may bear little resemblance to familiar adult songs. […] Subsongs are typical of birds with a low sexual motivation, for example adults and first-year birds before the breeding season really gets started, or juveniles after it has finished.

In January and February, the flocks of American Robins that descend into the fruit trees around my home provide ample opportunities to hear and study subsong.  I have never made an attempt to age the birds I have recorded, so I can’t comment on whether the subsongs of juveniles and adults are different at this time of year.  (Intuitively, I believe that they should be different, since the avian brain changes as birds mature, but I have no evidence for this at present.)  What seems certain is that almost every robin in these flocks will sometimes get into the sub-singing mood:

American Robin subsong, Boulder, CO, 1/19/2008.

Compare the spectrogram above with the spectrograms of American Robin songs that I posted a few weeks ago, all of which were recorded in April, May, or June.   The January phrases appear more similar to “hisselies” than to “caroling” phrases, but they’re not a perfect match for either one.  Nor are they perfect matches for each other — they’re poorly stereotyped.  Add that to the extremely low volume, from a bird that doesn’t even open its bill, and you’ve got what appears to be a classic subsong — either the practice sounds of a juvenile that hasn’t yet learned to sing, or the “warmup” tunes of an adult whose neural song circuitry has atrophied over the winter, in the absence of breeding hormones.

The general theory about subsong is that the bird isn’t producing stereotyped phrases because it can’t — it either hasn’t learned how yet (as a juvenile) or it’s physiologically unprepared (as an adult outside breeding condition).  Like the babbling of infant humans, subsong provides a window into the process of vocal learning — a complicated, fascinating, messy process that, in a few short months, will result in the crisp, polished performances we know as adult song.

Whisper Song

The term “whisper song” has an even longer history than “subsong,” dating back at least to 1896, when Olive Thorne Miller wrote in the Atlantic Monthly:

A catbird at my back, too happy to be long still, would take courage and charm me with his wonderful whisper song, an ecstatic performance which should disarm the most prejudiced of his detractors.

The phrase appears to have crossed into the ornithological literature as early as 1914, with J. William Lloyd’s letter to Bird-lore titled “The Whisper-Song of the Catbird”:

The performance was like that of a bird in a reverie — like the ghost of a thought of a song. His throat merely trembled, and occasionally the bill parted just a trifle. Yet his song seemed the full repertoire of the Catbird.

Lloyd’s letter seems to have occasioned numerous other published observations of “whisper singing” in other bird species and at other times of year (e.g., Shafer 1916).  Quickly, the notion of a “whisper song” gained broad currency among people interested in birds — mostly, it seems, in reference to the same phenomenon we just described as subsong.

I prefer to restrict the term “whisper song” to another kind of quiet, complex vocalization — one that isn’t heard from juveniles or non-breeding adults, but rather from birds at a peak of sexual excitement.

American Robin courtship "whisper song," Mesa County, CO, 5/3/2011.

Note how different this whisper song is from the subsong above.  For one thing, it matches the “hissely” phrases we’ve seen from other spring robins.  The level of vocal control is much higher; the bird repeats patterns with precision.  For example, both the first and second phrases in the spectrogram include elements that are repeated exactly.  And the whole third and seventh phrases are carbon copies of one another.  This demonstrates that the bird is remembering particular phrases and re-deploying them at intervals, which means that these phrases form a repertoire, a library of remembered behaviors.  This robin isn’t “making it up” as he goes along.  He isn’t subsinging.  He’s singing.

The Sound Approach described such singing as “highly motivated, sexually charged, and ultra-crystallized,” typical of male birds in close-range courtship situations.  Indeed I have heard these whisper songs from male robins only during the breeding season, usually in the presence of females — and when no females were visible, I have suspected their presence.  This is an entirely different phenomenon than off-season subsong, and it needs a different name.  For now, “whisper song” seems like a good way to describe these complex, quiet vocalizations — the avian equivalent of whispering seductively into your sweetheart’s ear.

The Alternate Song of Prothonotary Warbler

The Alternate Song of Prothonotary Warbler

Prothonotary Warbler, Chatham-Kent, Ontario, 5/4/2008. Photo by Gavan Watson (Creative Commons 2.0).
Prothonotary Warbler, Chatham-Kent, Ontario, 5/4/2008. Photo by Gavan Watson (Creative Commons 2.0).

They don’t call it the Golden Swamp Warbler anymore, but name would still be apt, because the flame-yellow Prothonotary is a familiar sight in the swampy areas of southeastern North America.  It’s a wonderful and evocative bird, one that even non-birders tend to notice when it perches or sings nearby.  It’s one of the few warblers that nests in cavities, and it is the sole member of the genus Protonotaria.

In 2006, I joined a volunteer team in Cornell’s search for the Ivory-billed Woodpecker in the White River National Wildlife Refuge of Arkansas.  During the two weeks I spent walking around in the untrammeled, uncampephiled wilderness, I had many spectacular close-up encounters with wildlife, but the most amazing was with a male Prothonotary Warbler.  I was standing in hardwoods on the edge of a flooded swamp when the golden bird popped up over the water and began to sing.  I switched on my microphone to record it, and suddenly it flew into a leafless sapling right over my head.  I didn’t dare breath or move; I just kept the microphone running, hoping the bird would stay in the tree, hoping it would sing again.  Sure enough, after a few minutes of foraging it began to belt out its familiar primary song: a fast monotone series of high clear upslurred whistles, not particularly musical, often transcribed as “sweet sweet sweet sweet sweet.”

Prothonotary Warbler primary song, Phillips County, Arkansas, 4/4/2006.
Prothonotary Warbler primary song, Phillips County, Arkansas, 4/4/2006.

But then the bird did something even more extraordinary.  It began to sing another song, one that was so faint that if the bird hadn’t been within five meters of me, I surely wouldn’t have heard it.  And it was like nothing I’d ever heard from a Prothonotary before:

Alternate song of Prothonotary Warbler (same individual as above).
Alternate song of Prothonotary Warbler (same individual as above). Note that the song has three parts in a strict syntax AAAABBBBBBBCCC. The four calls at the end were not part of the song structure; I only heard such calls a few times, usually right before or after a song strophe. They appear to correspond to the typical flight call of the species, as recorded by Evans & O'Brien (2002).

I went to the literature to find out what I had recorded, and discovered this in Lisa J. Petit’s Birds of North America account (emphasis mine):

Second, less frequent song is sung primarily during interactions with females, often in aerial Flight Display. This extended song (Spector 1992) is longer and slightly more complex than primary song, beginning rapidly and slowing down at end: chwee-chwee-chwee- chwee, teer, teer, teer (LJP) or che-wee-che-wee-chee-chee, chee-chee-che-wee-che-wee (Walkinshaw 1979). Brewster (1878) described this song as resembling song of Canary (Serinus canaria), sung quietly, and consisting of “trills or water-notes interspersed.” Roberts (1899) referred to the song as beginning with the “usual rapid monotone” and ending with a “varied warble.” No sonograms of this song are known to exist.

“No sonograms known to exist,” of course, is the same thing as saying “no recordings known to exist.”  Well, we fixed that, didn’t we?

Or did we?  Upon reflection, the quote above demonstrates some of the difficulties inherent in trying to match old voice descriptions with modern recordings.  Is this really the “alternate” song of males?  After all, no female was present, and the bird wasn’t singing in flight.  Could it be some other type of song?  In her account, Petit also mentions two other potential candidate vocalizations, apparent juvenile subsong and female song:

Two separate observations by LJP during Jul 1985 of immature birds (sexes unknown, both in femalelike plumage-one color-banded, 45 d old; the other unbanded, assumed immature on basis of behavioral interactions with adult pair) singing quiet, raspy, oriolelike warble that did not resemble primary song of adults. Songs were 2-3 s long and were repeated 3-4 times. In Jun 1987, an unbanded female (apparently adult, since it was building nest) was heard to give same type of song, repeated 3 times.

I’m not sure how female song fits into the picture, or whether some of the observers cited in the first quote might have been describing it, but one thing is clear: What I recorded was not subsong.  Subsong is given by juvenile birds between fall and spring as they practice for their big debut on a breeding territory.  It’s poorly structured, with individual notes randomly mixed and rarely repeated, or repeated in garbled form.  The song I recorded, on the other hand, was crystallized: the bird repeated it more than a dozen times, and each time it followed the exact same syntax pattern, with small variations in the exact number of notes A, B, and C, but absolutely no variation in their order, fine structure, or rate of delivery.

Many warblers have complex alternate songs.  Some better-known examples include the “flight songs” of Ovenbird and Common Yellowthroat (for the latter, listen here, between the 21 and 24 second marks).  Paul Driver’s got a neat recording of the extended song of Worm-eating Warbler, and Andrew Spencer has posted to Xeno-Canto what may be an alternate song of Black-and-White Warbler, although it may also be a type of call.  Many of these song types are rarely heard and even more rarely recorded, so I’d love to hear of more examples.

Given that Prothonotary Warbler has attracted a good deal of research attention over the years, I would be surprised if nobody else had recorded the alternate song by now.  If you have any information to share, please let me know.  It’s probably worth writing up a note for publication in a journal.