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.

Woodpecker with a Sweet Tooth (or Tongue): Red-breasted Sapsucker

The Island Oak (Quercus tomentella) in front of the glass house is the first tree in the Botanic Garden that I came across to have rows of holes drilled by a Red-breasted Sapsucker. It was Celia Zavatsky who pointed them out to me years ago. Celia, who passed away last year, was the quintessential docent, having a phenomenal knowledge of, and an infectious enthusiasm for California native plants. Since that initiation by her, I have found numerous sap wells on trees in the garden, both on hardwoods and conifers. 

The best display of sap wells on conifers in the Garden belongs to a Western Red Cedar (Thuja plicata) in Pacific Rain Forest section. The holes climb its trunk from 12 feet above ground, and up without stop for as high as you can see. They do not seem to affect the health of the tree. From their dark color and dry circumference, the holes look old, and tree sap no longer flows out of them. In fact, the majority of the sap wells we see in the garden are remnants of years past. 

If you see sapsuckers flying around and follow their moves closely, you will likely be rewarded with the discovery of fresh wells. The new wells are light brown in color, maintained diligently by the birds to keep the sap flowing. That is how the bird surveying docents first saw the sap wells on Leather Ash (Fraxinus velutina) four years ago. Sapsuckers have come back to the ashes at the north end of Southern California section adjacent to the grassy glade every year since then. All three ash trees growing there sport lines of old and new wells interspersing with each other, going up from the base of stems to where the stems branch.

There are four species of sapsuckers in North America, and what we have in the garden is Red-breasted Sapsucker. Lee Friedman summarizes that our sapsucker breeds to our east, and spends fall and winter in the Bay Area. Sapsucker is in the woodpecker family (Picidae), with the physical characteristics of zygodactyl toes (two pointing forward and two pointing backward) for grasping, a fortified skull for protecting its brain from being damaged by their hammering movements, and a sharp drill bit for a beak. It differs from other woodpeckers in its sugar-dense diet. The tip of its tongue is brush-like, specialized for lapping up the sap by capillary action.

Most of the holes we encounter in the Garden are xylem sap wells. Xylem is the tissue in vascular plants that transports water, minerals and other molecules from roots to other parts of the plants. Many deciduous trees have sap run in spring (such as maples, birches, and walnuts) which moves a large quantity of fluid through their xylem. (Check out this post on tapping various trees for syrup-making, drinking, and fermentation. Another well-written blog talks about mechanisms of xylem transportation, and explains in detail the relationship between sap flow and syrup production.) 

The fact is all trees have sugar in their xylem sap throughout the year, but the sugar concentration varies widely, depending on environmental factors as well as inter- and intra-species differences. Figure 1 shows that sucrose percentage of xylem sap in a willow peaks in August, and can be three times higher than in other months.

Figure 1 Sucrose level of xylem sap extracted from Salix fragilisat monthly intervals except December. 

Source of Figure 1: “Sugar content of xylem sap and susceptibility of willow to Chondrostereum purpureum,” by Sandra D. Stanislawek, P. G. Long and L. K. Davis. New Zealand Journal of Botany, 25:2, 263-269. 1987.

The other transportation tissue in vascular plants is phloem, which is closer to the bark and conducts nutrients from leaves to the rest of the plants. In general, phloem sap is a lot sweeter than xylem sap. Birds of North America Online (a paid account is needed for full access) cites that on average xylem sap has 2-3% sucrose, while phloem sap has over 10%. Again, sugar content of phloem sap differs across seasons, species, and individuals. See Table 1 for the great diversity of sucrose levels. One species, White Ash, marked by light blue color, shows a range of 8.5-25.0%.

Table 1 Sucrose in Phloem Sap

Species	Common name	% (wt/wt)
Salix viminalis	Basket Willow	5.0
Fraxinus americana	White Ash	8.5
Fraxinus pennsylvanica	Green Ash	11.1
Pinus sylvestris	Scots Pine	11.5
Acer platanoides	Norway Maple	17.9
Acer pseudoplatanus	Sycamore Maple	20.6
Fraxinus americana	White Ash	22.1
Fraxinus americana	White Ash	25.0
Eucalyptus globulus	Tasmanian Bluegum	25.1
Acer platanoides	Norway Maple	25.2
Prunus persica	Peach	37.9

Table 1 is excerpted from a larger table in “Optimal concentration for sugar transport in plants,” by K.H. Jensen, J.A. Savage and N.M. Holbrook. Journal of the Royal Society Interface. 2013.

Considering the average higher sugar content in phloem than xylem, we would expect that sapsuckers favor the sweeter phloem sap and dig more phloem wells. Yet, so far we have found only one tree in the Garden with extensive phloem sap wells. It is a Catalina Cherry (Prunus ilicifolia ssp. leonii) in Channel Island section. Pressed against the wire fence along Wildcat Canyon Road, this small hardwood tree with shiny green leaves has large rectangular sap wells on its trunk and a large branch.

Red-breasted Sapsucker on Catalina Cherry, March 1, 2017. Taken by Wen Hsu.
The rectangular shape of wells on the left branch is the hallmark of phloem wells.

Why do we see more xylem wells in the Garden? Could it be that the xylem sap in the seasons when Red-breasted Sapsuckers come to the Garden (late fall through early spring) has enough sugar to warrant their effort of drilling? Could the phloem sap at this time of the year be too sticky to flow, and thus less favored by them? There do not seem to be ready answers to these questions. In the section Priorities for Future Research, BNA Online says, “The criteria by which trees are chosen for sap wells are not known. …Most studies have been made during the breeding season...much remains to be learned about behavior and ecology during other times of the year.” That means our sapsucker observations in the Garden can make important contributions to the study of these beautiful birds!

Willow Flycatcher: In Praise of Bird Voice

If there is a yardstick of frequency of occurrence for flycatchers in the Botanic Garden, Black Phoebe will stand at the top, being a year-round resident of the garden. Pacific-slope Flycatcher, Olive-sided Flycatcher and Western Wood-pewee (in descending frequency) will perch in the middle; all of them breed in our area from spring to summer, then go south to spend their winter. Occupying the bottom of the yardstick will be Willow Flycatcher (Empidonax traillii), a spring and fall transient, passing through the San Francisco Bay Area on its way to and from breeding ground and wintering ground.

On our regular garden bird surveys for nearly 4 years, we have yet to record a Willow Flycatcher. But I am lucky to have seen it twice in the garden, both times in early September. And both times I was alerted to its presence first by its calls. Thanks to eBird, which faithfully keeps my bird reports on the cloud for free and allows me to access the data any time, I can say for sure that three years ago on September 6 the bird was on a Button Willow (Cephalanthus occidentalis).

When I first saw the bird, I thought it looked like a Pacific-slope Flycatcher. However, the series of sounds coming from the bird were unfamiliar, and unlike any calls made by a Pacific-slope Flycatcher. When I looked closer, I could see that this small flycatcher did not have a large eye-ring, its head did not sport a peaked crown, and it looked gray. These three features contrast with a Pacific-slope Flycatcher’s tear-shaped eye-ring, obvious crown and olive-green back.

Willow Flycatcher on Western Spicebush (Calycanthus occidentalis), September 3, 2018. By Minder Cheng. 

Pacific-slope Flycatcher on Hollyleaf Cherry (Prunus ilicifolia), September 3, 2017. By Minder Cheng.

The genus Empidonax is notorious for having many species that are hard to tell apart visually. Field guides tell birders that the surest way to identify them is by their voice. Willow Flycatcher and Pacific-slope Flycatcher are not the toughest pair of Empids to distinguish by sight. But as they flit in trees and shrubs, shrouded by leaves and shadows, we often don’t have unobstructed views of them, so their sounds are indeed the best clues.

After encountering the first Willow Flycatcher in the garden, I went home and clicked through recordings at xeno-canto for the best match of what I had heard in the garden. I chose one but felt dissatisfied. One reason is that what I heard were calls, not songs. Songbirds use a variety of communicative calls, which are often quite different from their characteristic songs that are usually delivered by males to court and establish territories. It takes a bird-watcher more effort to learn and recognize bird calls than songs. It’s no surprise that many calls have been overlooked and not recorded.

The two Willow Flycatchers I saw this year on September 3rd in the garden also called, but their sounds were different from that last one from three years ago. As they flew into Western Spicebush (Calycanthus occidentalis) growing on the bank of Wildcat Creek, they made several single notes. To my ear, they sounded like quick whistles, not as fluty as those of Swainson’s Thrush, but thinner, more crisp, and rising in pitch. This time I found the exact match on the website of Nathan Pieplow’s Peterson Field Guide to Bird Sounds of Eastern North America (2017). To listen to Willow Flycatchers that visited our garden, go to “pip” call taped in California. The first 4 notes in “whit” call, also of California, are similar, too. The location is crucial. Pips and whits from other areas sound distinctly different. (Willow Flycatcher has its range over the whole North America, and despite the Eastern focus of the book, the author has generously uploaded songs and calls recorded in many places.)

Speaking of the importance of flycatcher voice, I will never forget a lesson from Denise Wight, instructor of Birding by Ear class sponsored by Golden Gate Audubon. On a spring field trip to Sunol Regional Wilderness, we came upon a Hammond’s Flycatcher. She tilted her head and purposefully looked away.  She said, “Listen!” All the students’ eager eyes glued to their binoculars became unhinged—no looking, and only listening? As Donald Kroodsma writes in Listening to a Continent Sing: Birdsong by Bicycle from the Atlantic to the Pacific (2016), “Seeing birds is highly overrated….No, for me it is their voices that are so special….So, who cares what they look like?”

Staking out the Birds

Like hunters, bird watchers who have patience and perseverance often capture more birds by their binoculars than those who don’t. An especially fruitful practice for bird-hunting by either guns or glasses is to stake out our objects of desire at promising spots. Food is the foremost lure of wild animals. So finding (or setting up) a place with things they like to eat is an obvious first step.

For plant lovers, your keen observations of the yearly cycle of leafing, blooming, fruit-setting, leaf-shedding, etc., come handy in choosing a stake-out point. In fall, the ripened fruits on many plants attract birds large and small, from band-tailed pigeons, cedar waxwings, American robins, red-breasted sapsuckers to towhees, sparrows and chickadees. A tree, shrub or vine with a heavy load of bright-colored ripe fruit is a perfect location for a birder to sit and wait.

Early this week, we saw hermit thrushes galore; they were taking Summer Holly’s (Comarostaphylis diversifolia) orange drupes up in the trees and down below the trees. They have a discriminate taste, picking only the healthy fruit.

Hermit Thrush on Summer Holly, November 7, 2017, by Minder Cheng.

Last week, a hermit thrush was in the bush of Snowberry (Symphoricarpos albus) by alpine granite bed, sometimes perching gingerly on the thin branch, sometimes hovering in the air, and sometimes walking around on the ground, all for the purpose of getting the most palatable fruit. In my eye, the waxy white drupes looked the same. Endowed with greater visual acuity, the bird knew which ones were best. Once it inspected one fruit that had dropped, and left it there. We walked up after it flew off, and even we could see that this berry had a yellowing spot at one end, apparently an indication of its inferiority.

The Snowberry rejected by Hermit Thrush. November 1, 2017.

For me, it’s often the birds that alert me to the fruits that I would have missed. Without seeing a hermit thrush flying into Hackberry (Celtis reticulata) two weeks ago, I wouldn’t know that the small berries were ripening. Two years ago, we also saw a fox sparrow pecking at the berry’s thin pulp.

This year from October up to now, several other bird species were also seen eating fruits in the garden: purple finch on Twinberry (Lonicera involucrata), golden-crowned sparrow on Snowberry, pine siskin on Mountain Alder (Alnus incana ssp. tenuifolia) seed, and Steller’s jay carrying acorn (Quercus) in its beak. The latter two kinds of food, botanically speaking, are parts of fruits borne by angiosperms.

If you want to experience intense observations of bird feeding (other than setting up a bird feeder), go to those plants in the botanic garden with fruits at their prime, stand or sit at some distance, and wait. Your patience will be handsomely rewarded.

Tail of Black Phoebe (Sayornis nigricans)

Thursday volunteers who propagate plants for the garden are familiar with the nest of Black Phoebe. It’s a sloppy-looking mud nest with plant material sticking out here and there. One such nest is attached to the wall under the eaves and above the door to Juniper Lodge (or the potting shed). In fact, wires were added by humans to the nest to make it structurally sound (and even seismically safe?).

But the garden has another Black Phoebe nest, which is hidden from view to the general public, yet well known to gardeners: it was affixed to the outside wall of their office located at the far corner of Pacific Rain Forest section. And this year a pair produced fledglings successfully.

Being a year-round resident in the botanic garden, Black Phoebe is a familiar sight to us. It’s conspicuous in many ways, first and foremost being its choice of perch: an out-stretched branch, the tip of a conifer, a trail signpost, a plant label in the garden, or any other thing that sticks out and gives it a vantage point. Its black and white plumage is elegant and offers no camouflage. Its preference for ponds and riparian habitats make it a lot more visible than other flycatchers that hunt in shrubs and trees. What is more, it often announces its presence by loud calls either perched or flying. Its adaptability in using artificial structures as nest sites is certainly another help for people to find it. Black Phoebe is often the first flycatcher that many learn to recognize in California.

Black Phoebe sometimes moves its tail up and down when it is on a perch. David Allen Sibley quotes a study that suggests the tail-pumping be a deterrent to predators--it is a signal sent by phoebes to hawks saying: “I see you.” Granted that there is a statistically significant association between the presence of predators and the bobbing of phoebe’s tail, I have reservations on the idea that the latter is an active signal. Unless the predators are shown to be attuned to the tail-bobbing and be so mesmerized by it as to forget about attacking, I doubt the validity of the theory.

Watch videos on the internet of tail-bobbing Black Phoebes, and you may come up with an alternative hypothesis.

Immature Black Phoebe in Sierran section, July 30, 2017. By Minder Cheng.

Moon Eclipses Bird Action in the Garden

Eclipse Day! I was amazed at all the bird activity in the garden at 9 a.m. this morning - singing, calling, chasing, feeding - in all garden habitats. Very "birdy." With the temperature at 59 degrees and a heavy cloud of fog overhead, I expected a quiet start, even quieter during the eclipse, which I came to observe. Instead, bluebirds were sallying out over the lawn below the sierra granite bed and poking around in the grass, next to robins both mature and speckled immature. The Black Phoebe was noisy as it picked insects out of the air. Dark-eyed Juncos were the most prevalent including immature following their parents, both in the Sierra and along the Freeway below the Island Section. Both mature and immature Spotted Towhees were noisily foraging and calling. A Red-tailed Hawk cruised low over the garden, possibly seeking one of the voles we've been observing recently. Nine other species were present including Wilson's Warbler, Song Sparrows, Nuttall's woodpecker but only one Anna's hummingbird, vocalizing.
I could find no pollinators, and none of the poppies were open, so most of my observations were focused on the garden birds.
As darkness fell towards totality (76% here), the birds fell silent except for one adult Spotted Towhee who at last was quiet. A chill set in. But as soon as the light started to return, within six minutes, all the birds became active, flying and vocalizing. Yet after half an hour, it was overall much quieter than when I had arrived. Curious!
Did anyone else have an observation of bird behavior through the eclipse?

Fruit- and Insect-Eaters Abounding

Birds were very busy in the garden on a recent day in mid-July. Robins and their speckled young were literally mobbing the fruits of a Hawthorn shrub (Crataegus) located at the southwest side of the pond, providing amusement for us birders. We were able to approach quite closely as the birds gorged themselves.

Insects were in high demand by four species of flycatchers, by hummingbirds, and by warblers and a vireo, among others. While the warblers and vireo were busy gleaning insects from the trees and shrubs, the flycatchers and hummingbirds were sallying out to catch insects on the wing.

With all the competition, you have to ask if there are enough insects to go around. The flycatcher species are exemplary. Olive-sided flycatchers sit at the top of the tallest trees, calling and sallying out.  The Pacific-slope flycatchers tend to sit mid-tree or shrub adjacent to open areas where they may spy flying insects. The Western Wood Pewee tends to sit lower in trees and shrubs. Potentially, they are eating insect species that separate themselves by habitats high, medium and low in the environment.

The garden's resident Black Phoebe seems to be flexible, perching at a variety of levels from low fences to the taller trees.

In an exciting flash of yellow, red, black and white, we had a quick view of a male Western Tanager darting over the upper creek. In fall and winter, these birds eat berries and other fruit. In summer, their diet is dominated by insects, especially if they are feeding chicks.

The brilliant red head feathers are due to pigments. Unable to make the pigment themselves, Western Tanagers probably obtain it from insects in their diet. Great information about this and other birds may be found at Cornell's website: www.allaboutbirds.org.
Here's a photo by George Grover from Flickr.