Thursday, January 22, 2015


North America has the distinction of being the only continent on which a group of birds has evolved the ability to tap into the sap of living trees. These are the four species of sapsuckers (Sphyrapicus): Yellow-bellied (S. varius), Red-naped (S. nuchalis), Red-breasted (S. ruber) and Williamson’s (S. thyroideus). The first three replace one another from east to west across North America, while Williamson’s occurs with both Red-naped and Red-breasted in the West.

Sap flows through the xylem and phloem of a tree with a function rather similar to that of blood in animals. It’s not important in respiration, as it is in us, but nutrients and other chemicals circulate in it. It is full of sugars, presumably an important component of its nutritive value. Phloem sap of deciduous trees can contain concentrations of greater than 25% sugars in summer.

Sapsuckers dig holes in the bark of trees to get at this sap. They dig phloem holes, usually square and shallow, and continue to enlarge and add to these holes, excavating new ones above the old in a vertical arrangement. They also dig xylem holes, smaller and usually circular and penetrating the cambium layer to reach the xylem. These are arranged in horizontal rows. Some trees show both types of holes, easily distinguished.

As soon as these holes are dug, sap begins to flow into them, and they can then be considered sap wells. They continue to flow while temperatures are sufficiently high, but when air temperatures drop below freezing, the sap freezes in the wells and is then inaccessible to the sapsuckers. This is why sapsuckers are the most highly migratory of woodpeckers. As temperatures drop during the winter, many Red-breasted Sapsuckers, normally nonmigratory, descend from the mountains to appear in Pacific Northwest lowlands in numbers.

Sapsuckers may spend over half of their foraging time constructing and maintaining their sap wells. They seem to prefer tree species with higher sucrose content in the sap rather than those in which flow rates are higher. However, their wells have been found in about 1,000 species of woody plants, native and introduced.

They also feed on insects attracted to or trapped in the sap as well as additional arthropods captured on the bark or in the air. They also take some fruit and leaf buds. Insects are captured to feed the young, but the adults often stop at sap wells and dip the insects in sap before taking them to the nest. This may be for added nutritional value or to acclimatize the young to feeding on sap.

Numerous other species find these sap wells nutritious, including both birds and mammals. In the Pacific Northwest, species that I have seen coming to the wells include Anna’s and Rufous Hummingbirds, Downy Woodpeckers, Black-capped Chickadees, Ruby-crowned Kinglets, and Eastern Gray Squirrels. The wells may be especially important to hummingbirds, some of which actually follow individual sapsuckers to note the distribution of their wells. Sapsuckers actively defend their wells against some of these species, but each sapsucker has so many wells that this isn’t a very productive tactic.

The fact that there aren’t any sapsucking woodpeckers in the temperate forests of Eurasia is an elegant example of the idea that not all niches are filled. But of course sapsucking insects of the order Hemiptera (true bugs, aphids and their relatives) are everywhere in the world.

Dennis Paulson


Larry Hubbell said...

Thank you for this very informative post. I have been watching the RBSS in the second photo for over a month and I have been wondering specifically about the difference between the two types of holes. If I understand correctly the Phloem holes are to get at the sugars in the sap (coming from photosynthesis) while the small Xylem holes are going deeper to get at the water that is flowing up from the roots. This means the high volume of fresh sap flowing down this tree is mostly of the Phloem variety, right?

Slater Museum of Natural History said...

Larry, here is how the Birds of North America account for Yellow-bellied Sapsucker put it: In early spring, sap wells made into xylem tissues to feed on sap moving upward; after deciduous species leaf out, sap wells tap phloem tissues. Just as you say. Dennis

Anonymous said...

This was a very, interesting and informative post! I knew that there were birds that utilized the sap of trees but I had no idea it was so unique and not found outside of North America. I was curious to know if the sap of other trees in say the temperate forests of Eurasia had as high of a sucrose content as the trees in the PNW and if that could be at all realated to why this niche is not filled in other places.


Dennis Paulson said...

Sarah, the main tree that the sapsuckers were feeding on was a conifer from Spain, and there are many records of sapsuckers feeding on trees from various parts of the world introduced into North America.

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