How to Find a Pacific Wren

“If a Pacific Wren turns up at this time of year, it won’t be singing, but calling,” said Wen as the bird survey team set foot in Canyon Section on October 16, 2019. Everyone turned to Kitty, expecting that our ear-birder extraordinaire would know the calls. She took out her iPod without hesitation and began to play Geoffrey Keller’s recordings of Pacific Wren. After the long and dainty songs, came the thin and scratchy calls of tschet, tschet, tschet.

“Is that what the calls are?” Ellen asked rhetorically. We all tried to imprint the calls in our heads, and walked farther into the shady forest of Canyon Section. Suddenly a small shadow darted to the left, 50 feet below us; the sharp-eyed Allison called out, “Wren!” We heard the tschet-tschet calls that accompanied this bird, but not everyone had a good look because the bird had already hidden itself behind a clump of Sword Ferns (Polystichum munitum). We trained our binoculars to the ferns, focusing our attention on any movement of the fronds that would be indicative of something taking cover there.

After a couple of minutes, the wren moved again, now bolting to the right and uphill. No sooner had it landed at a low shrub, it disappeared into the dense foliage. “I saw its barred tail; it’s a wren all right!” exclaimed Kitty. Again, this wren let out an urgent series of “tschet”s.

The tiny, yet feisty, bird continued its island-hopping maneuver, getting closer and closer to us via stands of undergrowth of the forest, tschetting all the time. “Did we disturb it,” Wen wondered aloud, “or is it checking us out?” By this time the Pacific Wren had become too close to look through binoculars, and it stopped vocalizing. We caught glimpses of a typical wren profile: cocked tail, plump body, and brown plumage. Soon it crossed the trail we were on, and vanished uphill. We looked at one another, amazed by our good luck, except for Ellen, the demanding naturalist who said, “but I didn’t see it through binoculars.”

Listen to the calls of Pacific Wren recorded in Redwood Regional Park.

Pacific Wren (Troglodytes pacificus) is the smallest wren in North America, sharing this title with Winter Wren (Troglodytes himalis) with whom it was conspecific formerly, but now separated. In the field, the two wrens are distinguished foremost by their songs and calls. 

The occurrence of Pacific Wren is usually labeled as uncommon in field guides. Similarly, in our Checklist of the Garden, its sightings spread over many months, but are never frequent. Also in the Checklist, most of the time its habitat is Unknown (= Heard Only). If birding generally consists of equal parts of listening and watching, in the case of Pacific Wren, we rely more heavily on our ears than our eyes to recognize its presence.

The best bet to find a Pacific Wren in the Garden is go to the back of Pacific Rain Forest Section, or walk along Wildcat Creek in Canyon Section. Prick up your ears, and wait for the quick-shifting phrases of its bubbly song, or its scolding-like calls of tschet. Then track the sound using your eyes, and with luck, you will find moving leaves and a flitting shape near the ground. With more luck, you will see a partial or even a complete form of a dark bird. With utmost luck, you will catch a clear image through your binoculars, and declare to the world, “Pacific Wren!”

See photos on iNaturalist of adult and young Pacific Wrens.

My favorite guide book description of Pacific Wren comes from American Bird Conservancy’s Field Guide All the Birds of North America (1997). On Key 104, a general statement under “Curved Bills” goes: “Wrens have big, energetic voices for their small size...They typically scold intruders furiously from the security of their thickets. Wrens are as inquisitive as they are bellicose, and bird calls or simple squeaking noises often coax them into view.” On Key 107, the entry of Winter Wren (at the time of its publication, Pacific Wren was a subspecies under Winter Wren) says: “Scarce; in cool dense undergrowth of conifers in summer, esp. along streams and in boreal bogs...Very vocal, active, but furtive, hard to see.” 

Don’t these words capture vividly and succinctly all the bird’s characteristics revealed in the narrative above?

Townsend’s Warbler

Compared to the Eastern United States, we have relatively few species of warblers in Northern California. Considering that tackling the identification of 56 warblers in North America is worthy of its own guidebook, app and LPs (such as The Warbler Guide and Songs of the Warblers of North America), it is really in our favor that we can select from a small collection of species while craning our necks and arching our backs to identify these tiny birds flitting high up in tall trees.

One thing of note is that nearly all warblers found regularly in our area have some yellow feathers (Black-throated Gray Warbler being an exception). Among these yellow birds, Townsend’s Warbler (Setophaga townsendi) has the most striking facial pattern of yellow and black.

Female top, male bottom; taken respectively on November 24 and December 30, 2018. By Minder Cheng.

According to the checklist of the Garden, there are only two months of a year when we have no records of Townsend’s Warbler. It means that of the six warblers (family: Parulidae) that have been sighted in the Garden, Townsend’s has the honor of being here the longest. However, it is not the most abundant nor the easiest to see. And it does not even breed in our area.

Townsend’s Warblers nest in mature conifers in the Pacific Northwest, in the mountains of Oregon, Idaho, Wyoming and Montana, and in Alaska and the Yukon Territory. Some of them spend their fall and winter along the coast from Washington down to California, and some fly farther south to Baja California, other parts of Mexico, and even Central America. The reason we see them in all months other than June and July is that the Garden attracts both winter residents and those migrating through our area.

Since they do not nest here, we usually don’t hear their songs, but only their soft chip calls emitted as they glean insects in the foliage. If you are familiar with Yellow-rumped Warbler’s chip calls, Townsend’s call is less forceful than that and sometimes in a fast sequence.

When Townsend’s Warblers first arrive in early fall, or in late spring right before they leave for the higher latitude, occasionally we get to hear a male sing. I remember that on the day (August 29, 2015) we celebrated the 75th anniversary of the Garden with an open house, a Townsend’s Warbler broke into song in the morning drizzle as I walked past the shrubs by the pond. Perhaps the unusual chilliness and rain reminded him of his home up north.

For songs and calls of Townsend’s Warbler, Peterson’s Guide to Bird Sounds has a great collection of recordings. If you have the app of The Sibley’s Guide to Birds, check out their recordings of Townsend’s Warbler, which are completely different from Peterson’s. That demonstrates how much variation there can be in the vocal repertoire of a warbler.

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!