On May 19, 1993, two little openings in the bark of a Michigan tree had sap flowing from them. One was just above the other. A day later you could still put a fingertip on either spot, but a yellow-bellied sapsucker wanting a drink would have had better luck elsewhere. Both holes had gone dry.
We know that much because Laurie Eberhardt kept track of individual holes at the University of Michigan Biological Station in Cheboygan County. Her drawing of one patch fills less than half a page. It has a two-centimetre scale bar and four dates. Pick out the chain of openings at the far left, and a question that’s hard to answer from a photograph becomes a fair bet. If you had to set your binoculars on one small piece of bark tomorrow, where would you aim?
Where would you look on May 21?
Read down a column to follow the trunk. Read across to follow one spot through time. Each symbol stands for one observation at that spot.
| Position | May 19 | May 20 |
|---|---|---|
| Highest | No hole shown | No hole shown |
| Upper middle | No hole shown | Flowing |
| Lower middle | Flowing | Dry |
| Lowest shown | Flowing | Dry |
Check May 21 and 22
You found it. There is a new flowing hole one place higher.
Both lower openings stayed dry. Follow the row at the very top instead.
The May 20 opening did keep flowing. Look one place higher for the new one.
| Position | May 19 | May 20 | May 21 | May 22 |
|---|---|---|---|---|
| Highest | No hole shown | No hole shown | Flowing | Flowing |
| Upper middle | No hole shown | Flowing | Flowing | Flowing |
| Lower middle | Flowing | Dry | Dry | Dry |
| Lowest shown | Flowing | Dry | Dry | Dry |
On May 21 the new opening was above the one that had supplied sap on May 20. Both were wet. On May 22 those same two positions were still wet. In this chain the bird could get another day’s sap without adding another hole.
Stray Press transcription of four positions in the far-left chain of Laurie S. Eberhardt’s field diagram. Symbols show flow and relative position. Their spacing and size are schematic.
What a dot meant in the field
Each dark mark in Eberhardt’s drawing meant the hole was yielding sap when she checked. That distinction matters on a tree with dozens of perfectly good-looking holes. An opening can keep its shape long after the bird has taken its last drink there. The outline tells you where something happened. To find out where it was still happening, Eberhardt needed to check the contents.
For her measurements of sap flow, she emptied a hole with a small syringe, waited a measured interval, and collected the sap that had come in since. Across 51 holes she could collect sap for an average of about three days per hole. Now imagine trying to recover those three days from a single photograph. Two dark holes side by side might have opened on different days, and the one that looks newest might already be dry.
What the next cut buys the bird
The tree has more than one place a woodpecker can get a drink. Mississippi State’s field guide describes the deep, round holes a sapsucker probes, and the shallow, more rectangular wells it keeps working with its bill. The four-day Michigan record concerns those feeding cuts into the phloem, the living tissue just under the bark that carries sugar made in the leaves.
Eberhardt watched a bird start a well as a small oval, then take successive cuts at its upper edge. A place that began as a pinprick could grow tall enough to look like a different kind of mark. Run several of those chains alongside one another and the bark takes on its ruled appearance. Each hole was once a place to feed. Only a few needed to be paying out at any one visit.
Why keep the next cut so close? Eberhardt tested whether the bird might be changing the patch it fed from. On some trees she covered established wells to keep the birds off and left other wells open. Bark just above the openings the birds had continued to work held more moisture than bark at the covered wells. She also tried making wells herself. Even with a drill and a chisel, she couldn’t reproduce a bird’s steady supply of sap on her own.
One possibility she proposed is that repeated cuts interrupt the passage of sap, making the tissue just above a wound a better place for the next one. The moisture measurements give that idea some support. They leave a nice loose end, too: a person who knew where the holes belonged and had the tools to copy them still couldn’t make the arrangement work the way a woodpecker did.
For a row you can follow on actual bark, look at the yard tree in figure 2 of Mississippi State’s field guide. Try picking one opening you could recognize again by its neighbors. That would be the one to photograph on a second visit. If you’re still deciding who made the marks in the first place, try the three examples in Who left those marks on the tree?.