Last spring I was in the woods with a group of kids and stopped to point out rows of small, neat holes that had been drilled into the trunk of a paper birch, standing out dark against the white bark. “That’s from a sapsucker,” I told them. We’d already looked at the gaping cavities the Pileated Woodpeckers had excavated in the nearby cedars, but now I explained how Yellow-bellied Sapsuckers, another woodpecker species, make small, shallow holes like this and ate the sap that seeps out, rather than tearing the wood apart in search of insects.
“So is their favorite tree the sugar maple?” asked one of the kids.
What a good question – and not one I knew the answer to. We talked a bit about what trade-offs sapsuckers would face when selecting a tree, like the sugar content of the sap versus the hardness of the wood, and then we moved on to something else. The idea stuck with me, though. Do sapsuckers have a favorite tree species? What factors affect which trees they make their sap wells in?
This photo is of the trunk of a hemlock tree on the property where I live. There were also sapsucker holes in the trunk of the red oak next to it, and in addition to birch I’ve also noticed them on balsam fir and (yes) sugar maple, right alongside the holes left by humans tapping them to make syrup. So clearly these birds aren’t too picky, but I was curious, so I went searching to see if anyone had published research on sap well tree selection. The best paper I could find was “Use and Selection of Sap Trees by Yellow-belled Sapsuckers” by Laurie S. Eberhardt, which appeared in The Auk back in 2000. Her study site was in northern Michigan, which has a similar forest community to that here in northern Wisconsin. What she found can be summed up as follows.
- Sapsuckers do prefer some tree species over others. This study found that they most preferred paper birch, followed by red maple, juneberry, and bigtooth aspen, and it cited some previous papers that also found a preference for paper birch. That was my impression, just from where I’d noticed sap wells here, and it was nice to see it confirmed.
- Species isn’t the only factor that affects what trees they select. They would bypass trees of appropriate size and species that were close to their nest cavities in favor of similar trees that were further away, so something else was in play here.
- They’re not basing it on the trees’ microhabitat. The author checked to see whether the sapsuckers preferred trees growing in wetter spots, which can have higher-quality sap. They didn’t.
- They’re not basing it on characteristics of the individual trees’ bark, either. The author also checked for any correlation with the thickness and moisture content of the bark of sap well trees. Nothing.
Do you have sapsuckers in your woods? (They’re migratory, so ours will be leaving us soon.) What species of tree have you seen sapsucker wells in? What do you suppose is going on with their selection of individual trees if it’s not connected to the microhabitat or the bark characteristics? Let me know in the comments.
Laurie S. Eberhardt (2000). Use and selection of sap trees by Yellow-bellied Sapsuckers. The Auk, 117 (1), 41-51. doi: http://dx.doi.org/10.1642/0004-8038(2000)117[0041:UASOST]2.0.CO;2