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About this time last week I was at Yuba Pass in the northern Sierra, swatting the insatiable mosquitoes and watching a family of red-breasted sapsuckers. (There is a Berkeley connection here: some of these birds spend the winter along the coast, and they’re likely to begin showing up in Tilden Park in a couple of months). 

The group consisted of an adult—whether father or mother I couldn’t tell you, since the only way to distinguish the sexes is by in-hand examination of the tail feathers—and three recent fledglings, recognizable by their brownish heads. What they were doing was sucking sap. The adult was hard at work drilling sap wells in a red fir, and the kids followed him or her around, feeding greedily and bickering among themselves. They were at it for three consecutive days, dawn to dusk. 

“Red-breasted sapsucker” is not the most dignified name for a bird to be saddled with. At least this species has done marginally better in the gravitas department than its close eastern relative, the yellow-bellied sapsucker, the bird that comic birdwatchers in the movies are always looking for. “Yellow-bellied sapsucker” is what the guy in the white hat in a B western would call the guy in the black hat, just before he plugged him. 

But the names are descriptive, at least. Sucking sap is what these aberrant woodpeckers do. The habit has been documented in a number of woodpecker species, but only the four sapsuckers (counting the western red-naped and Williamson’s) make a living at it. They’ve evolved a couple of anatomical specializations for this. Most woodpeckers have extremely long tongues for nabbing wood-dwelling insects; a sapsucker’s tongue is shorter and less extensible, and tipped with stiff hairs to trap the sap. 

You can tell when a sapsucker has been at work by the neat rows of holes it leaves behind. And it’s not just a matter of drilling until you strike sap. These birds take advantage of the annual cycles of tree physiology to get the most nutritious sap available. 

There’s no such thing as just plain sap, it appears. Trees have a kind of circulatory system in which xylem tissues transport water and dissolved nutrients up from the roots into the branches, twigs, and leaves by capillary action, and phloem tissues convey the products of photosynthesis down from leaves to roots. (This is a gross oversimplification, of course). In evergreens, it’s more or less a two-way street, although different conduits are involved. But in deciduous trees, like the quaking aspens that ring the meadow at Yuba Pass, the phloem traffic doesn’t begin until the tree has leafed out, transporting the nutrients produced in all those little green factories. 

I couldn’t find detailed information for red-breasted sapsuckers, but field studies of yellow-bellied and red-naped sapsuckers show that the birds dig xylem wells in conifers during winter and early spring. To reach the xylem tissues, they have to penetrate the outer phloem layer. Then, when the leaves sprout on the deciduous trees, the sapsuckers switch over to them and begin to drill phloem wells, tapping that richer source. Different techniques are involved: xylem wells are circular in shape, phloem wells begin as lateral slits and are expanded into rectangles.  

Both the sounds of a working sapsucker and the marks on the sap tree are fairly conspicuous. So it’s no surprise that freeloaders are attracted to sapsucker diggings. Insects are drawn to the sap, of course, and provide a nice protein bonus for the birds. They sometimes dip ants into the sap, perhaps to kill that formic-acid taste. Red-breasted nuthatches smear sap from the wells around their own nest cavities. Hummingbirds—ruby-throated in the east, rufous, broad-tailed, and calliope in the western mountains—feed at the wells. They often nest nearby, and the timing of their spring migrations may reflect the sapsuckers’ excavation schedules. 

This is what led ecologist Paul Ehrlich to characterize the red-naped sapsucker as a keystone species—one whose activities provide food or shelter for a whole set of organisms—in western forests. Beyond the sap, these birds excavate nest cavities like most woodpeckers; and their old homes accommodate cavity-nesting birds like chickadees, nuthatches, wrens, and bluebirds, not to mention flying squirrels.  

So, silly name notwithstanding, I’d say the sapsuckers of whatever species deserve credit for exploiting a hidden food resource in a fairly sophisticated way, and acting as community benefactors in the process.  

Photograph by Ron Sullivan 

All in a day's work: a red-breasted sapsucker and its sap wells.