Yellow-bellied Sapsucker: An Ecological Keystone

Each spring, the end of the maple sugar season is marked by the return of one of nature’s own sap harvesters – the Yellow-bellied Sapsucker. While its name may seem more fitting as an insult that Homer Simpson might sling at a co-worker, it is an apt description of this small woodpecker. Moreover, the ecological contributions this species provides are worthy of such a memorable name.

Most conservation biologists consider sapsuckers to be a keystone species – one whose existence is vital for the maintenance of a community, and whose loss would have a significant effect on other species’ populations or on ecosystem function. Common examples of keystone species include the beaver (for creating wetlands), starfish (as a predator of sea urchins and shellfish that have few, if any other, natural predators), and prairie dogs (for creating burrows that are utilized by many other species).

The Yellow-bellied Sapsucker plays two vital roles in woodland communities. Like all woodpeckers, they excavate new nest cavities each year that subsequently are used by a wide variety of animals, from other birds to squirrels and spiders. Sapsuckers also drill sap wells from which other animals obtain nutritious sap, as well as the insects that are attracted to it. Hummingbirds, which are particularly reliant on sapsuckers, often nest near sap wells and have been observed following sapsuckers, presumably to locate sap wells. In addition, it is thought that Ruby-throated, and possibly Rufous hummingbirds, time their spring arrival in northern New England and Canada to coincide with peak sapsucker activity, and that the northern limit of their breeding ranges is determined by the presence of this woodpecker.

Sapsucker feeding behavior is the most distinctive feature of their ecology. Although sap itself makes up only about 20% of their overall diet, most of their foraging time is spent creating, maintaining, inspecting, and feeding from sap wells. Sap is consumed for its sugar, which varies in content by tree species and season. Sap wells are usually excavated into phloem tissues (which are closer to the surface) during early spring. These shallow wells begin as slits, are gradually widened into squares, and must be maintained continually for the sap to flow. Sapsuckers also drill through the phloem to reach the underlying xylem tissues which have higher sugar content (to fuel leaf growth). Xylem wells can be recognized as neat, round holes. Sapsuckers vigorously defend their wells against other birds, and even many insects that might be considered food under other circumstances. The remainder of their diet consists primarily of insects, especially ants, which are exposed by flaking off bark or collected directly from sap wells.

Like all woodpeckers, the Yellow-bellied Sapsucker drums to defend a territory and advertise its presence to a mate. This drumming is easily recognizable, thanks to the Morse code-like quality that noticeably slows down at the end. Listen for it this week at a woodlot near you.

Sightings of Yellow-bellied Sapsuckers were widespread this week, with reports from Brandon, Woodbury, Huntington, and Strafford. Other notable reports include a Sandhill Crane in South Burlington, a Snowy Egret at Bristol Pond, Fox Sparrows in Montpelier, Monkton and Brownington, a Tree Swallow in Burlington, and 4 Red-necked Grebes, 2 Common Loons, and a Barn Swallow on Grand Isle.

You can explore all the birds reported last week in Vermont and add your own sightings at Vermont eBird.