Monday, February 29, 2016

Yellow-bellied Sapsuckers & Tree Damage

The yellow-bellied sapsucker (Sphyrapicus varius), also referred to as the red-throated, squealing, and whining sapsucker (McAtee 1911), is a species of woodpecker found across eastern North America. This migratory species breeds in the hardwood and coniferous forests of Canada, the upper Midwest, and the northeastern United States. In winter, yellow-bellied sapsuckers reside in the southeastern United States, Mexico and Central America, and the Caribbean.

Yellow-bellied Sapsucker (from Audubon)

Sapsuckers are known for their distinct feeding behavior: Excavating rows of holes in trees to feed on sap, cambium, and the insects attracted to these wells. Twenty to 100% of a sapsucker’s diet might be the sap itself, depending on the season (Varner et al 2006), and analysis of stomach contents suggest that the cambium itself may make up around 17% the sapsuckers diet (McAtee 1911). Observational data also shows that sapsuckers also rely heavily on these wells for insects for both adults and their young. After their young fledge the nest, adult sapsuckers will spend about 1 to 2 weeks teaching their young the art of sapsucking (Ehrlich et al 1988, p348).

While the sapsuckers’ work serves to feed them and their offspring, a “suite of animals” usurps these wells to feed on the sap themselves (Eberhardt 2000), including small mammals and other bird species (Varner et al. 2006). Ehrlich et al. (1988) notes that sapsuckers often guard their wells from both mammalian and avian thieves, particularly the ruby-throated hummingbird.

Moreover, there is method to the placement of these sap wells. According to Varner et al. (2006), yellow-bellied sapsuckers in their winter range tend to create wells on the north aspect of tree trunks of larger diameter trees. In addition, some research suggests that sapsuckers choose older, diseased trees within which to excavate wells, perhaps because of the increased amino acid content in the sap of weakened trees (Eberhardt 2000).

Sapsuckers have been documented to feed from at least 252 species of native trees and vines, and 31 species of non-native trees (Atwell 1911, p53). Mortality attributed to sapsucker damage has been documented among more than 10% of these species (Atwell 1911). In their breeding range, where sapsuckers feed on trees from April until October, tree mortality associated with sapsucker damage may only be 1 to 3% for some species (e.g., hemlock, red spruce), but can reach up to 40 to 67% for thin barked trees (e.g., red maple, paper birch, and gray birch) (Rushmore 1969). According to Atwell (1911), only tree species in the Mulberry family (Moraceae), such as mulberries and Osage orange, have no documented yellow-bellied sapsucker feeding activity.

The feeding behavior of sapsuckers can girdle and kill trees. The sapsuckers drill through the outer bark of the tree, past the bast (i.e., inner bark), into the cambium, sometimes penetrating the wood (McAtee 1911). This means, that a sapsucker – to access the sap of the tree – is actually puncturing the vascular tissues of the tree. The sapsucker is puncturing those straw-like vessels that move food and water, stopping some parts of the tree from receiving adequate water and nutrients.

Sapsucker damage can also provide entrĂ©e for pathogens and insects, making trees more susceptible to fungus in particular. For example, when the fungus Peridermium cerebrum infects a tree through sapsucker holes, it results in large knots protruding from the bark (McAtee 1911). In contrast, some trees will attempt to heal the wounds and grow thicker at the site of the sapsucker damage, resulting in “shelf-like” layers of woody growth (McAtee 1911, p19). This shelving is particularly pronounced in shagbark hickory (Carya ovata) (pers. obs., 10 Feb 2016).

Sometimes the damage done by sapsuckers can extend into the wood itself, leaving the wood “distorted and discolored in such a way as to destroy its commercial value” (McAtee 1911, p19). Anecdotal evidence has suggested that sapsuckers can affect commercial enterprises in other ways, including damage to apple and peach orchards, maples used for syrup production, and timber producing pines (McAtee 1911).

While the most noticeable sapsucker damage is on trees, woody vines may also sustain damage, including grape species (Vitus), Virginia creeper, and trumpet creeper. The news isn’t all bad: Poison Ivy is also affected.

Eberhardt, L. S. 2000. Use and selection of sap trees by yellow-bellied sapsuckers. The Auk 117(1): 41-51.

Ehrlich, P., D. Dobkin, and D. Wheye. 1988. The Birder's Handbook. New York: Simon & Schuster.

McAtee, W. L. 1911. Woodpeckers in relation to trees and wood products. Washington D.C.: U.S. Department of Agriculture, Biological Survey no. 39: September 26, 1911.

Rushmore, F. 1969. Sapsucker: Damage Varies with Tree Species and Seasons. USDA Northeastern Forest Experiment Station, Upper Darby, PA. USDA Forest Service Research Paper NE-136.

Varner, J. M., J. S. Kush, and R. S. Meldahl. 2006. Characteristics of sap trees used by overwintering Sphyrapicus varius (Yellow-Bellied Sapsuckers) in an old-growth pine forest. Southeastern Naturalist 5(1): 127-134.

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