Sorry to keep you in suspense! Here’s the final photo:
It was a sapsucker after all.
Of course, you all knew that, didn’t you? If not by the photos/evidence, then simply by the title of yesterday’s post and which way it was clearly going. But still, I was surprised – and relieved, I have to admit – to discover this, some ways up one of the trees. An answer. Classic (to me) sapsucker sign: stacked, rectangular holes.
But that didn’t explain why five trees were absolutely peppered with little holes, as if someone had come out for shotgun practice there. I’ve seen some worked-over trees, but I’ve never seen sapsucker feeding sign like that. Could these be the favoured trees of a pair that nested in the little swampy bit just a couple dozen meters away? Since sapsuckers don’t winter here, and would be unlikely to remain in one spot long enough during migration to create such a multitude of holes, it would have to have been birds present during the summer months.
To try to find an answer, I pulled out the Birds of North America account for Yellow-bellied Sapsucker and began reading. It makes the note that pairs will usually return to the same breeding site as in previous years, and also indicates that the longevity record for a wild bird (these are usually inferred through the time between two captures of a banded bird, whose age was known when it was first banded) is approaching 7 years. So perhaps if the same pair came back to this site for four or five years, they’d have time enough to riddle a few trees.
Something else I came across, though, which I found interesting, and which may also help to explain what I was seeing here: sapsuckers make two types of wells, one that penetrates to the xylem, and the other to the phloem.
Each ring of a tree ring has both xylem and phloem; the xylem is found closer to the center of the tree, while the phloem is found outward, closer to the bark. The xylem is a network of capillary tubes used primarily for the transport of water, and some nutrients, from the roots of the tree to its leaves. Movement of fluids upwards (against gravity) in this network is passive. Water molecules climb the sides of these tiny tubes through capillary action, drawn upward (usually) by transpiration (the tree “breathing”, losing water through its leaves), or sometimes pushed upward by excess water pressure in the root system.
The phloem, the outer layer, transports sugars and other nutrients throughout the plant, to wherever they need to go. Unlike the xylem, which is actual tubes, the phloem is composed of cells, which pass contents from one to another like a bucket brigade. The phloem is where the sap flows, and where the sugar content is highest. The reason that girdling will kill a tree is that with the phloem destroyed all the way around the trunk, the sugars have no way of making it down to the roots to keep the roots alive.
Consider the two systems as your arteries and veins, with the roots as the heart and the leaves as the lungs. The water starts in the roots (your heart) and is pumped up through the tree’s xylem (your arteries) to the leaves (your lungs). There it picks up sugars (oxygen, in this analogy) which it then carries back through the rest of the tree by the phloem (your veins) before returning to the roots to be cycled through again. Of course, this isn’t a perfect analogy because in our bodies the switchover between arteries and veins isn’t at the lungs as I’ve drawn here but at the tissues where the oxygen is dropped off. But you get the idea. Also, just as there remains a small amount of oxygen in the blood that the veins carry back to the heart and lungs, so too is there a little big of sugar in the water going back up from the roots in the xylem.
Back to the sapsuckers; the wells that they drill into the xylem are circular, usually in horizontal rows of 3 to 15 holes. Those bored into phloem, however, are typically rectangular and stacked vertically for as much as 20 cm (8 in). What I’m used to thinking of as “sapsucker holes” are their phloem wells.
I’m not entirely clear on why they bore two types, but I would guess it has to do with energetics. Although phloem sap contains >10% sucrose and xylem sap only 2-3% (according to the BNA), xylem sap, by virtue of its density/viscosity and its transport method, flows faster. Although the bird may not need to consume as much phloem sap to obtain the same energy, it would need a lot more wells because of the flow rates. (That said, the BNA indicates that sapsuckers will preferentially choose trees with higher sugar content over trees with lower sugar but higher sap flow. Perhaps in the summer, when they don’t want to be flying all over the territory, they settle for the faster-but-lower-sugar xylem sap flow.)
Incidentally, in the spring the tree has no leaves yet, and so the sugar movement is all unidirectional – from the storage location in the roots, up through the xylem to the branches so the tree can use it to start making new leaves. Maple syrup is made from xylem sap. As soon as the leaves start to bud out and begin photosynthesizing, and the phloem sap starts flowing, the taste of the collected sap changes (not for the better) and maple syrup season is over. [In the spirit of full disclosure, I deduced this paragraph from everything I’ve just learned about the two types of sap, except for the bit about maple syrup season ending at leaf out because the taste changes, which I already knew.]
What about why sapsuckers arrange the two types of wells differently? This, too, isn’t specifically stated in the BNA account but I have a guess. Because the xylem operates passively, putting a second well above the first does you no good – the sap would leak out at the first well it gets to and the well above wouldn’t function. So they put them side-by-side. They could also do this with the phloem wells, and you do sometimes see multiple stacks of phloem wells side-by-side. But (and here the BNA account does say) sugars tend to back up in the phloem above a wound/well where the transport cells have been damaged (remember, they’re headed down toward the roots, so they accumulate above the well), so when the first well starts to seal up, the sapsuckers drill their second well above it, where the sugars have become concentrated.
I feel I’ve learned a lesson here. (Ironic, because I’m usually pretty careful about double-checking my info, or using language such as “probably”, “might be”, or “I think” if I’m not thoroughly certain.) Hopefully you have, too, and without having had to eat your words! Still, I feel it was a worthwhile lesson, to have learned that cool stuff about sapsucker wells.