Out along the walkway coming down to the main building here at the VIC, we have an old, hollow snag. There’s a perfectly round hole in the side that I’ve often thought was ideal for a chickadee, but I’ve never seen a bird fly in or out of the tree. One year I took our Treetop Peeper, a cavity camera that is mounted on a telescopic pole, and tried to peek inside the hole, but the opening was a just a bit too small for the camera head, so I never found out if it was being used by birds or not. This morning, however, I heard a loud whack-whack-whack as I came down the walkway. I thought for sure a pileated woodpecker was drilling away, but instead what I saw was its much smaller cousin, the yellow-bellied sapsucker (Sphyrapicus varius).

First off, you have to love that name: yellow-bellied sapsucker. It sounds like an insult some rustic dude from the Old West might sling at another rustic dude about whom he had a poor opinion. But, when it comes to birds, it is a pretty apt descriptor. The bird, after all, does have a somewhat yellowish tint to its underside, and it does consume the sap of trees, although not by sucking. More on this in a bit.

The sapsucker is one of the smaller woodpeckers in our area, coming in just behind the hairy. Like its relatives, its feathers are mostly black and white, with a touch of red. Both the male and female sport red caps, but only the male has a red throat patch (see photo). And like all good woodpeckers, the sapsucker has stiff tail feathers that act as supports while the bird climbs trees and whacks away at the wood.

When it comes to excavating trees, the sapsuckers make two types of feeding holes. The first kind is round and deep. Into these holes the bird plunges its beak to extract sap. The second type of hole is more rectangular and shallow. These holes are maintained over the course of several days to keep the sap flowing. The bird uses its brush-like tongue to lick up the flowing sap, and any insects that are stuck to it.

Now, there is an art to this whole hole-making jag. You can spot a sapsucker tree at a distance for it will have a series of horizontal rows of holes going around the trunk. The bird makes these rows, one on top of the next, for a reason: they dam up the flow of the phloem sap in the summer. Phloem sap? Time for a little tree physiology 101.

Trees have phloem and xylem – two “types” of wood. The xylem is the part that provides structure to the tree – most of the wood. It also contains the “vessels” through which water and nutrients rise from the roots to the leaves of the trees. It is xylem sap that maple sugerers tap in the spring to make the sweet stuff we put on our pancakes and waffles. It is mostly water.

Phloem, on the other hand, is the part of the tree (wood) that carries nutrients from the leaves back down towards the roots. It is closer to the outer edge of the trunk. The sap that runs through the phloem is thicker, being chocked full of all sorts of nutritious goodies: proteins, amino acids, sugars, etc. It doesn’t flow in the same manner that xylem sap flows. Which raises an interesting question among tree and bird folks: how does the sapsucker keep the flow, well, flowing?

Apparently scientists have studied this and have tried to come up with an answer with little success. Attempts at mimicking the sapsucker’s techniques have met with failure. The conclusion is that there must be some sort of anticoagulant in the bird’s saliva that keeps the tree’s sap fluid enough to flow. Kind of like vampire bat saliva, which has been found to be useful in medicines for patients suffering from blood clots and heart disease, but that’s fodder for another post.

So, we have these birds making row up on row of holes, creating a backlog of sap in the phloem cells above the holes. Each new row taps into this stored sap, providing nutrients not only to the sapsucker, but to a whole host of other animals, from squirrels and porcupines to warblers, hummingbirds and insects. In fact, the sapsucker has been given the label “keystone species” for the role it plays in maintaining food sources for a variety of lifeforms within its community.

Not only that, but it seems that these birds target trees that are often already in poor health. Apparently trees suffering from insect damage, weather damage (wind, lightning), or disease produce a greater amount of protein and amino acids in their sap – no doubt a last ditch effort to try and heal themselves. This extra nutrition is highly attractive to sapsuckers – a bigger bang for their buck, so to speak. This also means they are less likely to tap into healthy trees, which cuts down on the likelihood of the birds irritating foresters and the timber industry.

If you suspect you have yellow-bellied sapsuckers in your woods, you can find out fairly easily by listening. Not only do they have a cat-like call, but when they are whacking away on a tree, the sound is quite distinctive: you’ll hear a series of about five rapid whacks, followed by three or so slower, quieter whacks: WHACK-WHACK-WHACK-WHACK-WHACK …whack…whack…whack. Add this to the discovery of trees riddled with rows of holes, and you can be pretty sure that yellow-bellies have taken up residence in your neighborhood.