Song of the Summer: Swainson’s Thrush

Around the summer solstice, one of the most acrobatic bird songs in the Botanic Garden comes from a thrush that is brown in plumage, and almost always hidden in vegetation. 

This virtuoso singer is Swainson's thrush. He starts with a sonorous and undulating voice, as if initiating a slow aria. After a short couple of turns of it (whose pitch outlines resemble small hills in a sonogram), he launches into an eerie fine thin line, quickening his pace, reaching higher and higher (outlines of steep mountains in a sonogram), until in ecstasy he goes out of range of our hearing (10 kHz is the upper limit for many human adults).

You can hear a variety of Swainson's Thrush's recordings made available through Peterson's Guide to Bird Sounds; the third song of the page, which is labeled Del Norte, California, best captures the description above.

Sometimes the singer stays at the low range of the song, and doesn't go up to the high range. In the same recording, the second series of notes demonstrate such a song (0:05-0:07). Apparently it takes an inordinate amount of energy to be a male sopranist!

A bird’s voice box is a double structure called syrinx, another novel evolution in this incredible flying animal with so many unique features. 

The New World thrushes in the genus Catharus, which includes Swainson’s, can maneuver two independent systems of musculature on the syrinx at the same time, producing vastly complex voices. That makes the dense growths of their preferred habitat never a hindrance to broadcasting urgent communications to either competitors or objects of desire. 

If we create a singing contest among the three most wide-spread North American Catharus thrushes,  Wood Thrush, Hermit Thrush and Swainson's Thrush, which one will get your vote as the top talent of the show? Listen to a short sample of each of them here. You can probably guess my choice, and surely the evidence is in the hearing!

In addition to songs, Swainson’s Thrushes reveal their presences aurally by short low-pitched whistles and other calls. But to catch sight of them, our best luck is find some ripe fruits at branch tips.

In early July along Wildcat Creek in the Garden, many plants growing on the stream banks and reinforced walls seem ready to take over the waterway. This is the prime time and spot to see a Swainson's Thrush. From the newly formed dense growths below, a thrush may suddenly flutter up and show itself above the green herbage. Dangling just barely on the slightly-bent sprig, it plucks quickly a blue currant with white bloom, or a thimbleberry in crimson red. Almost before you have time to train your binoculars and focus on any field mark, it is already out of sight, savoring its sweet taking behind foliage. We are left with a taste of short bursts of excitement, and a blurred image of Swainson’s Thrush.

Extra songs of the summer

If you are more taken by Swainson's song than its plumage, here’s another snippet that may interest you. It was taped by cellphone on June 12, 2019 at Jewel Lake, Tilden Nature Area, about two miles due north of the main pond in Botanic Garden. It's a soundscape with many voices knitted together, typical of a birdy area. Turning up the volume gives you a fuller range of sounds.

At 0:03, a Swainson’s thrush makes its entrance with a robust sound. He is often preceded and followed by a Wilson’s Warbler’s soft chet-chet-chet-chitchitchit. Our resident breeder Song Sparrow is nearby, emitting a song of trill and more (at 0:19, 0:28, 0:40). The sparrow is loud and forceful. From a high branch afar, an Olive-sided Flycatcher sings a neat phrase of three-note whistles (in a low-high-low pitch pattern, at 0:21, 0:39, 0:55). The flycatcher’s clear and familiar song is always a delight to hear in local woods from spring to summer. But in comparison with the Swainson’s song, wouldn’t you say it’s a little too plain?

Swainson's Thrush in Canyon Section, May 5, 2019. By Minder Cheng.