On April 4, I saw my first eastern phoebe of the year; and the next morning, while walking at the farm where I live, I saw a second phoebe.
These intrepid flycatchers are one of our earliest-arriving migrant insectivores. To survive in this season, they rely heavily on sun and flowing water to maintain or open up habitats where insects thrive. My April 5 phoebe was hunting for breakfast along the banks of Dry Run Creek.
The main draw here is probably the aquatic insect fauna -- perhaps mayflies, midges, or other creatures emerging from riffles and other areas of open water. But as I observed the bird, I realized another force besides flowing water was at play here, making conditions good for a bug hunt. You guessed it...the Sun!
In particular, morning sunshine was rapidly heating the exposed soil in the vertical banks of the creek. On this chilly morning, with much of the watercourse frozen over and temperatures well below freezing, you could see "steam" -- evaporating water -- rising in wisps from thin patches of snow on the black soil of the banks. (This was only true on the banks that were oriented in the direction of the morning sun -- roughly speaking, those that faced east.) The billowing tendrils of water vapor were beautiful -- and far too delicate to be captured with my basic photography equipment. Still, I snapped a few photos of the sunny banks for the sake of documentation.
As I watched, the phoebe made multiple dives from hunting perches down onto this sun-warmed earth, attempting to capture invertebrates coaxed into motion by the heat and light. When I stooped to examine one such area, it wasn't long before I saw my first invertebrate -- a good-sized spider, alert, active, and fleet-footed.
Having spent a lot of time down the bird-diet rabbit hole in the preceding days -- dissecting pileated woodpecker excreta and examining these birds' pecked-up snags to see what they'd been eating -- I decided not to invest much effort in determining if there was a particular type of invertebrate (spiders, for instance) that this phoebe was after. (I'm excited to share my woodpecker diet findings in an upcoming post still in progress: Snag life, part 2.) It was satisfying just to catch a glimpse of the phoebe's intelligence and skill: even on a cold early spring morning, it knew exactly where to find its prey.
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| Van Peenan Park, 4 April 2018 |
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| Twin Springs Park, 19 April 2018 |
On April 18, a "winter" storm (it's nearly a month past the equinox!) walloped Decorah with at least 6 inches of snow -- trouble for ground-foraging migrants like American robins, who, as during our previous April snow, appeared on recently plowed roadways in search of water to drink. The day after the storm, driving along Twin Springs Road, I came upon two robins and a hermit thrush hunkered down on the black pavement, which gleamed in the morning sun. They didn't flush as I approached in my vehicle. The first bird, a robin, didn't even move when I pulled up within three feet of it. But it was neither dead nor critically injured. Instead, it was simply resting on the sun-warmed road surface -- its head tucked into its breast feathers.
I am sure that, had I attempted then to drive on, the bird would have never moved a muscle. But leaving it at the mercy of other motorists seemed risky, so I opened my door to get out. Then I changed my mind -- and the sound of me closing my driver's door finally roused the poor bird. It jerked its head up and stared at me with a tired and surprised expression. This was, it seemed, a pretty worn-out animal.
As I slowly drove past them, the other robin and the hermit thrush ambled reluctantly away from my car. And I do mean ambled -- if they felt any urgency, it wasn't translating into their body movements. The thrush seemed to be walking stiffly, awkwardly. I thought of another hermit thrush that had zoomed past me into my house the previous day, when I'd carelessly stood with the front door open for a few moments. (It flapped clumsily against my living room window, then fell into the leaves of my potted plants -- allowing me to capture it with my hands and release it outdoors.) And I thought of the hermit thrush I'd rescued from Madison Road the day before that, shortly after it had been hit by a car. (The bird eventually perished.) Things did not seem to be going so well for these birds.
However, I was consoled to know that they did have certain reliable food sources, even amid such difficult conditions (as I'll discuss in Sun and water, part 3). I drove on, anxious to know the results of a camera trap I'd set recently in an upland woods nearby. What the camera's images revealed would surprise and delight me -- and offer a solution to a mystery I've been wondering about since last spring.
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Sap Flow Nocturna
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| Venus and the crescent Moon, 17 April 2018 (1) |
[W]here is water flowing?
In the maple trees, for one--
water, with a certain
sugary life-force of the trees
intermixed.
-- Sun and water, part 1
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| Yellow-bellied sapsucker at one of its well trees, a black walnut. Beard Century Farm, 18 April 2018. |
I saw my first yellow-bellied sapsucker of the year on April 6 at Van Peenan Park. It appeared near the start of my afternoon hike, on the upper limbs of a walnut tree growing in a ravine. When I returned to my starting point in late afternoon, the bird was still there -- on the same tree. The sun sank low, sending most of the ravine into shadow; but the top of the walnut glowed still in the slanting sun, and that's where the sapsucker was: huddled on a branch in the day's last sunlight, braving a strong cold wind that foretold that night's temperatures (which would drop into the middle teens). It seemed unlikely that sapsuckers -- migratory woodpeckers who return to our area around April 11, on average (2) -- were used to this sort of brusque meteorological welcome. How could my Van Peenan sapsucker eat, I wondered, with temperatures seemingly too low for sap to flow?
On April 15, with four inches of snow on the ground and temperatures in the mid-twenties, I noticed a sapsucker at Dunning's Spring that seemed to be hanging around the same group of trees near the water. When the bird disappeared on some unknown errand, I investigated the trunks of these trees -- and was astonished to find my first active sapsucker wells of the year! Ice clung to the bark just below one such well, and yet liquid still flowed out from the tree's interior. I had been assuming that the sap movement we'd experienced in March had ground to a halt amid our wintry April, and that sunshine and 40s (at least during the day) would be needed to really get sap moving again. Clearly I was wrong. (So much to learn!) And clearly it was time to return to the moth wing trees at Twin Springs Park.
In Sun and water, part 1, I mentioned that certain moths frequent the sap wells drilled by sapsuckers. This first came to my attention early last April, when I came across a curious sign at Twin Springs: amputated moth wings (and the occasional moth leg or head) littering the ground at the base of sapsucker trees. From the base of just one such tree, I collected dozens of moth forewings and hindwings, then glued them to a piece of posterboard.
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Moth forewings collected at the base of a single sapsucker tree (a sugar maple), Twin Springs Park, 9 April 2017. Wisconsin moth guru Kyle Johnson kindly examined this material, and he determined that the forewings belonged to moth species in the genera Orthosia, Lithophane, Eupsilia, and Pyreferra. Some examples: 5 = Lithophane patefacta; 6 = L. antennata; 10 = Orthosia hibisci; 55 = L. scottae; 58 = L. semiusta; 67 = Eupsilia morrisoni; 78 = E. vinulenta; 85 = Pyreferra pettiti. (3) |
Whenever I visited these "moth wing trees" in daylight, I never came across any live moths, so I deduced that the moths -- and the unknown predator that been de-winging them -- were creatures of the night. This suggested that sapsuckers were not the culprit (they're day-active). So what nocturnal animal would be feasting on the moths visiting sapsucker wells?
In an effort to find out, I trained my game camera on one of the moth wing trees, programming it to take a picture every 30 seconds. From dusk until dawn on the night of April 9-10, 2017, the camera snapped away, generating more than 1200 frames. Especially when assembled into video form (I'll get to that), these images showed clearly that the sapsucker tree was a hub of moth activity. Here's a detail from one image, showing several moths on one of the sap flows.
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| Moths at a sap flow on sugar maple, Twin Springs Park, 9 April 2017 |
Caught in flight by the camera's infrared flash, the moths made ghostly forms.
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| Which one is your favorite? |
By and large, this was all my camera trap caught -- lots and lots of bright white moth-blurs. In fact, there was only one image of the twelve-hundred-plus that happened to capture another interesting creature. Here it is:
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| John's Loch Ness Monster photo of 2017 |
At first glance it seems you can make out the two left legs and the curved tail of some kind of mammal, which is hanging onto the tree trunk and facing downward.
However, when I scrutinized the images I realized the "left hind leg" is present on the tree before and after the creature appears; it's actually just a conveniently placed light spot on the trunk (arrow).
The real left hind leg, rather than being extended out laterally (the light spot on the trunk), is apparently stretched back, facing up the trunk. It's the elongate white object between the light spot on the trunk and the base of the tail. Apparently.
All of this suggested to me some kind of mammal -- and in particular, a squirrel. Here's a collection of before-and-after images I assembled into a video, with the alleged squirrel image shown for a bit longer than the others, about halfway through playback. Before the mystery animal arrives, note that moths can be seen on and near the trunk of the tree. However, after the creature departs, the tree trunk is nearly devoid of activity for a few minutes, until moths finally start returning.
As it happens, this is just the response you'd expect from prey animals when a predator shows up: scatter, wait a little while, and then return when it seems like the coast is clear. Writing about wasps they observed during the day at sapsucker wells, Ehrlich and Daily (1988) note, "the yellow-and-black vespid wasps [feeding at wells] were very attuned to the presence of birds and mammals at the wells and rarely fed when they were there. Virtually all of the wasps vanished immediately when a vertebrate entered the clump [of vegetation], only to return just as quickly upon its departure" (p. 363).
Of course, my one blurry and overexposed image of a supposed moth predator at a well wasn't enough to draw any solid conclusions as to its identity or its intentions. All that seemed certain was that some mysterious being had visited the sap flow that night.
Overall, I was unhappy with these results, so I set the camera trap again -- but to no avail. Sugar maple sap-moth season drew to a close, and I was forced to wait until the spring of 2018.
Which brings me to the past few days of this snowy April.
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After discovering the active sapsucker wells at Dunning's Spring on April 15, I headed to Twin Springs Park. There, I found wells recently opened on the trunks of mature sugar maples. The freshly drilled sapsucker holes possessed a rich golden brown color and leaked sap continuously…
…whereas previous years' wells had long ago been healed over by the trees, producing corky "belly buttons."
Here's an active well from a bit further away, showing the trunk moistened by sap.
As with the trees described in Sun and water, part 1, these maples' trunks had been stained black by years of sap flows induced by sapsuckers. Can you spot the sapsucker tree in the image below?
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| Twin Springs Park, 19 April 2018 |
Here's a closer view of this tree's trunk, showing an active sap well. Recently drilled sapsucker holes are visible at the top of the image; at bottom, sap has flowed onto the snow.
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| Twin Springs Park, 19 April 2018 |
The preceding image is from the morning of April 19. For the previous two nights, my game camera had watched the trunk of this very tree. And guess who had glided in to the tree trunk at about 9 pm on the first night?
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| Twin Springs Park, 17 April 2018 |
A flying squirrel!
It now seems likely to me that the sapsucker tree visitor I caught on camera in 2017 was also a flying squirrel. Whether this nocturnal rodent is also the mystery moth predator remains to be seen: the current batch of sapsucker wells is quite fresh, and I have not yet seen any moth wings at the base of sapsucker trees. (As temperatures warm I intend to keep my game camera running.) In fact, with the extended cold I have yet to see a single insect at a sapsucker well this year -- which strongly suggests to me that the flying squirrel shown above was feeding on sap rather than bugs.
Flying squirrels are certainly not the only vertebrates to enjoy sugary drinks from sap wells. A 1988 paper by Gretchen Daily and Paul Ehrlich (4) includes photographs of an orange-crowned warbler, a rufous hummingbird, and a red squirrel feeding at active wells maintained by red-naped sapsuckers in Colorado. The authors also mention visits to the same wells by broad-tailed hummingbirds and chipmunks. Another Colorado author, Dave Leatherman, reports that hairy woodpeckers will visit sapsucker wells -- and he also documents downy and hairy woodpeckers making their own wells to drink from (rather than just waiting around for some sapsucker to do it!). (5)
And then, of course, there are the invertebrates. Once things get a bit more springlike around here, I hope to make a post highlighting some of the insect and arachnid fauna at sap wells, as they begin to show up (which should be quite soon). For now, we'll have to be content in the knowledge that our local flying squirrels and yellow-bellied sapsuckers -- and perhaps other vertebrates too? -- are finding flowing tree-water to be a helpful source of nourishment in this time of transition.
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In Sun and water, part 3, we'll visit still other places where sunlight and flowing water help bring things to life at this time of year.
Coming soon: Sun and water, part 3
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NOTES
1. Thanks to P. van der Linden for sharing the identity of the planet (Venus).
2. From a phenology article (including bird migration tables) written by Tex Sordahl and published in a Decorah newspaper (copy of the article provided to me by a friend)
3. Kyle Johnson, pers. comm.
4. Ehrlich, P.R. and G.C. Daily. 1988. Red-naped sapsucker feeding at willows: possible keystone herbivores. Am. Birds 42:357-365.
5. See https://cobirds.org/cfo/ColoradoBirds/HungryBird/79.pdf
