It’s a cool February morning in the San Francisco Bay Area’s Coastal Range. The air smells fresh and mulchy thanks to the recent rain. As you meander down the trail, a persistent tapping sound drifts toward you from deep in the woods. Tap-tap-tap-tap-tap. Quiet. And then again, tap-tap-tap-tap-tap. You recognize it as the sound of a feeding woodpecker, but it’s not the slow, more deliberate taps of an acorn or hairy. Their hammerings recall a carpenter methodically pounding nails. This sounds more like the repeated pulsing of a sewing machine.
You wander toward a grove of white alder and hear a soft descending whine call. A couple of the trees in front of you are decorated with strangely shaped scars. Some of them line the branches in long columns: small shallow squares and rectangles in the bark that appear to just reach the outer woody tissues of the tree. In late spring and summer, this phloem layer moves fluid and nutrients down the tree to support new growth.
But you also notice rings of small round holes circling the trunks. These appear to go deeper, into the xylem tissue, a system that moves fluid and nutrients up the tree to fuel the growth of its foliage. As you scan through the grove, you notice several trees with similar markings that look like old wounds, with swollen bark and dark edges. And then you see a ring of fresh holes oozing a glistening orangey sap. Of course! A sapsucker was hammering out these holes, and you just found its favorite stand of trees.
Sapsuckers are unique members of the woodpecker family, and as their name suggests, they eat the nutrient-rich sweetness exuding from xylem and phloem wells that they excavate in living trees. Their specialized tongues have tiny paintbrush-like tips with outward-pointing bristles that increase the surface area of the tongue tip, optimizing the volume of sap extracted with each swish.
Sapsuckers excavate sap wells for the sap, not specifically for the insects that are drawn to the rich liquid. But you can be sure that those holes will attract invertebrates with a vital source of food. Sapsucker carpentry also draws other bird species. In the Bay Area, Anna’s hummingbirds, yellow-rumped warblers, and ruby-crowned kinglets lap up the sugary drink. In western North America, rufous hummingbirds even time their spring migration to follow northbound red-naped sapsuckers, feeding from sap wells as they go. Sapsuckers fall in the special category of ecosystem engineers.
All woodpeckers serve as keystone species in the forests and woodlands they inhabit. The excavation of nest cavities place them in this lofty role. Woodpeckers create homes for numerous bird species, plus several mammals and innumerable invertebrates. In the Bay Area, chickadees, titmice, bluebirds, and small owls are a few of the birds that nest in old woodpecker cavities. These species can’t excavate on their own, and without the woodpeckers, these secondary cavity nesters would have no place to raise their young.
Removing the keystone from an arch would cause the arch to collapse; removing a keystone species from an ecosystem would cause detrimental harm to the ecosystem. And sapsuckers take this keystone thing one step further. They certainly excavate nest cavities, but they also provide an important food source for other forest organisms. This makes them a “double keystone species.”
I’ve been studying woodpeckers for more than 25 years, and sapsuckers are one of my favorite subjects. I would tell you not to get me started, but we’ve already gone this far! I’ve studied sapsuckers across the continent—in museum collections and in the wild—and they are indeed unique among woodpeckers. You may be wondering what species of sapsucker you found feeding in that alder grove. I’ll respond to your query, but the answer might surprise you. Sapsuckers have a story to tell, and we have to start at the beginning.
A long, long time ago: The first sapsucker
We know plenty about the modern natural history of sapsuckers, but their prehistory is a bit more of a mystery. Ornithologists have published several genetic studies on the phylogeny of sapsuckers—the story of how they came to be. My interpretation of this fascinating story begins with the original woodpeckers—of the family Picidae—that diverged more than 25 million years ago from their closest ancestors, an assemblage of birds that included the ancestral puffbirds, barbets, toucans, and honeyguides.
We generally believe that woodpeckers claim their origins in the Old World and possibly in southeastern Asia, which today supports the greatest diversity of woodpecker genera. Ancestral woodpeckers learned to climb trees in their search for food, and at some point they began digging into the trees for prey. By excavating into the wood of trees, they also found that they could lay their eggs and raise nestlings inside cavities of their own design. Those same prehistoric woodpeckers likely found that excavating into live trees was difficult and messy; sap could damage your feathers, and this became a poor survival strategy. So most of them focused on excavating in dead and dying trees. But one woodpecker lineage—the ancestral Sphyrapicus woodpeckers, commonly known as the sapsuckers—found that eating sap could sustain them through most of the year.
Before sapsuckers emerged, evolution led to a couple of crafty adaptations for all woodpeckers that perfectly suited their excavating behaviors, both for food and for nesting substrate. Hammering into trees surely led to some serious concussions and retinal detachments for many of the earliest woodpeckers. But as they continued to exploit this feeding niche, they evolved cranial and other structural forms that allowed them to withstand the forces of slamming their heads into trees. They developed spongy frontal and occipital bones in the skull that helped disperse the shock of forcefully contacting a hard surface; to withstand a deceleration force of up to 1,200 g, those bones had to become exceptionally robust! The cranial cavities also developed with very little cerebrospinal fluid, which held the brains tightly within the crania.
Further evolution dictated divergence. All woodpecker lineages kept the most general structural adaptations—including the amazing tongue structure, or “hyoid apparatus”—but they also began to specialize. Ancestral genera with less forceful feeding behaviors kept simpler structural features, and those that continued the heavy-duty hammering continued to develop a hardier anatomy. This led to a protruding frontal bone in the cranium, called the “frontal overhang,” which directed forces downward and away from the cranial cavity. A special jaw hinge evolved to work in concert with the frontal overhang. The vertebral and torso structures evolved to absorb those forces, minimizing impact to the eyes and brain. The bony parts of the bill grew wider and shorter to help distribute forces over a broader area.
Fast-forward several million years. The original sapsucker probably evolved on the North American continent about 10 to 11 million years ago, when the Melanerpes and Sphyrapicus genera diverged. And along with this genetic divergence, woodpeckers continued to specialize—right at the tips of their tongues.
Due to their penchant for constantly excavating endless rows of sap wells, the ancestral sapsucker retained the bulging frontal overhang and the other super-resilient structural features. But the “typical” woodpecker tongue tip wasn’t ideal for lapping up sap. Most of the heavy-duty excavators specialized in insect feeding, which led to tiny backward-pointing barbs at the tips of their tongues. These keratinous structures worked like fishhooks to help the woodpeckers extract their prey from deep bark furrows and larval galleries. The sapsuckers instead developed their paintbrush-tipped tongues.
Anatomy intact, what followed—about five to seven million years ago—was a divergence of ancestral sapsuckers that occupied different geographic regions. One lineage split off early and spread throughout the intermountain West: the Williamson’s sapsucker. And at the same time, the yellow-bellied sapsucker spread eastward, settling throughout the northern and eastern forests of the continent.
Both maintained their footholds, but starting about two and a half million years ago, their environment radically changed thanks to the Quaternary Ice Age. Between then and now, the ice surged and retreated several times. And while the ice repeatedly pushed the yellow-bellied sapsucker southward, yet another population of sapsuckers—very closely related to the yellow-bellied—began to pioneer the West. From the Rockies westward to the Pacific Coast, this cousin found its place in the western forests. It was the ancestral parent of the red-naped and red-breasted sapsuckers.
Starting about 75,000 years ago, the most recent ice age advance began. The Laurentide ice sheet forced the yellow-bellied sapsucker deeper into the eastern U.S., with a distinctive population setting up in the southern Appalachians. The western mountains became increasingly isolated, and eventually the Cascades and Sierra Nevada formed impassable barriers between the Pacific lowlands and the rest of the intermountain West, preventing contact between two established sapsucker populations. Over the next 50,000 years, this separation of the far West led to a rapid but subtle divergence in plumage characteristics, leading to two distinct lineages. The result? As the ice finally receded, the red-naped sapsucker remained in the Rocky Mountains and Great Basin ranges, and the red-breasted sapsucker had established itself in the Pacific forests.
Sapsucker hybridization
We typically think of evolution in terms of very broad time frames. However, the phenomenon continues today, and you can watch it in action daily by closely observing western sapsuckers. The recent divergence among the yellow-bellied (Sphyrapicus varius), red-naped (S. nuchalis), and red-breasted (S. ruber) sapsuckers has left us with the superspecies varius. A superspecies encompasses species that typically do not overlap in range but descended from one common ancestor and have only recently evolved to the species level. (Since the yellow-bellied is the most ancestral of the three species, the superspecies retains the name of its parental taxon, Sphyrapicus varius.) The three sapsuckers in the varius superspecies are so closely related that—where their ranges meet and in the absence of an individual of their own species—they will breed with one of their sister species. You may have learned once that speciation precludes hybridization. I’m here to tell you that it just ain’t so.
Biologists look at several factors when determining whether a given population indeed represents a distinct species. Some of these elements include: (1) vocalizations—are they different enough to render two populations unique? (2) plumages—are they divergent enough to clearly distinguish one species from another? (3) contact zones—are the zones where the two species meet narrow and limited in scope? (4) hybrid viability—can the offspring of hybrid pairs breed successfully? and (5) DNA—does the “genetic distance” indicate that the two have been separated long enough?
So how do you distinguish between the sapsuckers in the varius superspecies? The three are generally impossible to distinguish by voice or drumming patterns, indicating that the species are very closely related. The plumage differences can be subtle. The sapsucker contact zones vary considerably, but they are all very narrow and are limited to specific geographic regions, with a couple of exceptions where the red-naped and red-breasted interbreed over larger areas. These restricted contact zones work in favor of splitting the species. Hybrid viability, too, works in favor of defining the three species. Biologists believe that while first-generation hybrid individuals can breed successfully with “pure” individuals, the offspring of two hybrid birds may or may not be viable.
And finally, we have DNA evidence. Research clearly supports the yellow-bellied as a unique species. However, the genetic distance between the red-breasted and red-naped was once considered one of the closest among distinct species of birds. Their repeated physical separation during successive glacial advances further reinforced their genetic isolation. And when we combine plumage variation, limited contact zones, limited hybrid viability, isolation, and adequate genetic distance, we have three species of sapsuckers that are related closely enough that they make up a superspecies.
You can download our printable/zoomable PDF of the sapsucker guide above here.
Sapsuckers across the West
The Bay Area’s common sapsucker is the red-breasted. Its general range stretches from southeastern Alaska, down the Pacific Coast, and into California’s northern Coast Ranges. It reaches eastward in British Columbia and Washington to the northern Cascades, and south through the Sierra Nevada and the mountains of Southern California.
The red-breasted sapsucker comprises two subspecies. As the opposite of a superspecies, subspecies are too recently diverged to be distinct species, but different enough in plumage or distribution to warrant this lower taxonomic rank. Northern birds of the ruber subspecies tend to be deeper red, sometimes described as ruby-colored, and the red extends farther down the breast than it does in the southern birds of the daggetti subspecies. Northern individuals also typically show very little white in the head, and sometimes just a little nick of white near the base of the bill, often showing as the start of a malar, or mustache, stripe. The head patterns of southern birds can be quite variable, often closely resembling those of the red-naped sapsucker. The white often shows as a bold mustache stripe and even a narrow white supercilium, or eyebrow. They also show less red in the chest than the ruber individuals.
Limited numbers of red-breasted sapsuckers breed in the Bay Area mountains, and in the Santa Cruz Mountains in particular. These breeders may be the source of some of the region’s wintering sapsuckers, but there aren’t enough individuals breeding to account for all the sapsuckers you actually see each winter. So where is the source for the Bay Area’s wintering sapsuckers?
Sapsuckers in the Bay Area
Sapsuckers that nest in the Santa Cruz Mountains don’t likely move around too much outside of breeding season, and the nearest summer concentrations outside the region occur to the north and east. Numbers increase gradually in the northern coast ranges and into the Klamath Mountains, but the Sierra Nevada is loaded with breeding red-breasted sapsuckers.
In the winter, the Bay Area sapsucker population explodes, with hundreds of red-breasted sapsuckers reported annually. The precise origins of these birds is somewhat of a mystery, but they are likely a mix of birds from the breeding regions described above.
In general, sapsuckers of the northern red-breasted subspecies don’t migrate, although they may move downslope or coastward outside of the breeding season. Southern red-breasteds tend to leave the breeding grounds and move into lower mountains and valleys on their short migratory routes southward. Red-breasted sapsuckers in the Bay Area certainly represent the southern, or daggetti, subspecies. However—and that’s a big however—we return to the hybridization game. Let’s play together, shall we?!
Many of the hybrid sapsuckers that breed in British Columbia, Washington, and Oregon typically migrate southward, and it is very likely that many of those overwinter in California. These hybrids could represent any of the three hybrid combinations: red-breasted x red-naped, red-breasted x yellow-bellied, or red-naped x yellow-bellied. In some cases, when you’re confronted with an ambiguous sapsucker, it’s only possible to identify a bird to the superspecies level, calling it a “varius-type sapsucker.” Having lived in the heart of the largest red-breasted/red-naped contact zone—on the eastern slope of the Central Oregon Cascades—I became comfortable calling many birds “reddish” sapsuckers.
Keeping our eye on the proverbial ball, we also need to remember that all of the pure yellow-bellied and red-naped sapsuckers are migratory. The yellow-bellied, in fact, travels the greatest distance of the world’s woodpeckers each year, breeding as far north as eastern Alaska and overwintering as far south as Panama! Now, follow that ball with me. What species did you find in that alder grove? Well, it was almost certainly a red-breasted sapsucker, but any of the sapsucker forms we’ve discussed could hypothetically occur in the Bay Area in winter. They probably don’t all occur there annually, but you should at least be prepared for unusual visitors from the north. And pay attention to what they have to say. The sapsuckers of the West have a story to tell that’s written in the sap wells ringing your neighborhood trees.
Old Dominion University professor of biology Eric Walters contributed to this article.
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