The Yellow-Bellied Sapsucker: A Well Traveled Woodpecker

A look at the yellow-bellied sapsucker.

The Yellow-Bellied Sapsucker: A Well Traveled Woodpecker

A yellow-bellied sapsucker.
A yellow-bellied sapsucker.

The yellow-bellied sapsucker (Sphyrapicus varius) is a woodpecker that has a wide range across North America. In summer, the sapsucker can be found as far north as Alaska, Minnesota, and Nova Scotia and in winter, the birds can be found as far south as much of Central America and the Caribbean. The yellow-bellied sapsucker is migratory — the birds spend the winters in the south, and their summers in the north. This is unusual for woodpeckers. Out of the twenty two woodpecker species found north of Mexico, fifteen of them don’t migrate at all, with many of these species only flying for short distances during their regular activities. This leaves us with seven woodpecker species that do migrate — and of those, the yellow-bellied sapsucker takes the crown for the largest migration range. Interestingly, the sapsuckers do their migratory flying at night.

Yellow-Bellied Sapsucker Habitat and Diet

The yellow-bellied sapsucker is found in forests. Maple forests, aspen forests, and mixed forests are all popular habitats for sapsuckers. Research has shown that maple forests have better outcomes overall for breeding sapsuckers than aspen forests. However, both maple and aspen forests allow sapsuckers to sustain their populations over time. Both of these forests have trees that are good for sap drilling — the yellow-birch in maple forests, and the paper birch in aspen forests. Sap is one of the primary food sources of the yellow-bellied sapsucker (which, given the name, isn’t very surprising). Sapsuckers have a variety of preferences for trees to tap for sap depending on their range. In urban areas of Southern Florida, for example, black olive and tropical almond trees are a preferred source of sap. Yellow-bellied sapsuckers also feed on arthropods (think of spiders, centipedes, insects, and the like), especially while nesting.

A tree with lines of sapsucker holes.
Pictured: Sapsucker holes in a tree. Drilling holes in straight lines is typical for them.
Why are woodpeckers so concussion proof?

One of the most interesting behaviors woodpeckers exhibit as a group is the tree drilling that they’re named for. A very common question related to this drilling is how do woodpeckers avoid rampant brain injuries due to ramming their heads against trees repeatedly, at high speeds, for ages. Woodpeckers routinely bash their heads into trees at far more than double the acceleration that would concuss humans. While humans are concussed at 300 g of acceleration, woodpeckers have been known to regularly drill into trees at accelerations on the order of 600–1500 g.

A variety of mechanisms have been proposed to explain the woodpeckers’ resilience to brain injury. There are three primary reasons woodpeckers aren’t all concussed all of the time. The first being their small size. The acorn woodpecker’s brain, for example, has a mass of 2.5 grams. A human brain has a mass of 1400 grams. The woodpecker brain is so much smaller than a human brain that they generate much less force when they smack into the skull. The second limiting factor that protects woodpeckers is the short duration of impact. One of the documented factors that increases the likelihood someone will be concussed is how long their brain is exposed to high acceleration. A woodpecker only drills against a tree for a millisecond or less each time, so their time of impact is very short.

The last quirk of woodpecker anatomy guarding them from brain injury is how their brain sits in their skull. Both human brains and woodpecker brains are hemispherical (if you cut a globe in half, you will have two hemispheres). If you cut the globe in half at the equator, your top hemisphere is oriented like a human brain is. If it were to go flying forward, only the small portion at the front will hit the skull. This concentrates the impact right around the forehead area. On the other hand, a woodpecker brain is much more like if you had cut your globe from the Arctic to Antarctica, and chose to look at the left section. The large flat side of a woodpecker’s brain rests against the front of their skull. When their brain goes smacking against their skull while they drill, the force is distributed among the largest possible region of their brain. This lessens the impact overall.

An image illustrating the difference in orientation of a human brain versus a woodpecker brain, as described in the paragraph above.
Notice the difference in alignment when comparing a human brain to a woodpecker brain.
A visualization of how the force is more distributed over the woodpecker's brain compared to a human brain.
Look at how much more relative space the force has to distribute itself on the woodpecker’s brain versus the human brain.
Ecological Status of the Yellow-Bellied Sapsucker

Ecologically speaking, the yellow-bellied sapsucker is in a good place to stick around. The International Union for Conservation of Nature (IUCN) has marked the yellow-bellied sapsucker as a species in the “least concern” of going extinct. Despite this, there are still some concerns for the sapsucker, especially as they’re being seen more and more often in urban and suburban areas. Being in areas with humans presents several new challenges for sapsuckers. Flying into windows is one of these concerns. Some research has been done into this phenomenon, known as “window strikes”. One study suggests that migrating birds are at a higher risk of striking windows than year-round resident birds, which poses a threat for migratory birds like the yellow-bellied sapsucker. Another threat to sapsuckers in areas with high human density are pets, domestic cats in particular. Outdoor cats have long been a threat to bird populations, and sapsuckers living in areas with them are no exception. Like many other species, the yellow-bellied sapsucker’s future will be strongly impacted by human activity.

References

Diamond, Joshua M, & Ross, Michael S. (2018). Tree selection and foraging height of wintering Yellow-bellied Sapsuckers (Sphyrapicus varius) in an urban environment. The Wilson Journal of Ornithology, 130(4), 932-939.

Gibson, L. J. (2006). Woodpecker pecking: How woodpeckers avoid brain injury. Journal of Zoology (1987), 270(3), 462-465.

Kerlinger, P. (2008, April). Sap hunter: why the Yellow-bellied Sapsucker does what most woodpeckers don’t: migrate. Birder’s World, 22(2), 52. https://link.gale.com/apps/doc/A175629258/ITOF?u=gainstoftech&sid=ITOF&xid=177e1123

Natola, Libby, & Burg, Theresa M. (2018). Population Genetics and Speciation of Yellow-Bellied, Red-Naped, and Red-Breasted Sapsuckers (Sphyrapicus varius, S. nuchalis, and S. ruber). The Journal of Heredity, 109(6), 663-674.

Sabo, Ann M, Hagemeyer, Natasha D G, Lahey, Ally S, & Walters, Eric L. (2016). Local avian density influences risk of mortality from window strikes. PeerJ (San Francisco, CA), 4, E2170.

Tozer, Douglas C, Nol, Erica, & Burke, Dawn M. (2011). Quality of mature aspen and maple forests for breeding Yellow-bellied Sapsuckers (Sphyrapicus varius). Canadian Journal of Zoology, 89(2), 148-160.

 

Cassandra is a third year Computational Media student at Georgia Tech. Her concentrations are People and Interaction Design, and she hopes to work in UX/UI design after graduating from Tech. Outside of school, she enjoys tabletop RPGs, crochet, and photography.