Several readers have sent pictures of trees with neatly arranged rows of round or square holes encircling the trunks and oozing sap.
No, it’s not an alien from Mars or a bothersome beetle that’s boring the holes. It’s a woodpecker with the almost pejorative name of yellow-bellied sapsucker. The bird uses its beak to drill the holes as food wells for sap, as its name implies.
But the bird doesn’t suck up the sap; it licks it up with a brushlike tongue. And to keep the tree sap from gumming up, the sapsucker returns to its sap wells frequently to prime the flow.
How the bird keeps the sap flowing is not fully understood. The sap of a tree, like the blood of a human, normally coagulates around a puncture wound. But a sapsucker somehow keeps the tree sap from coagulating, which leads some scientists to speculate that the bird could have an anticoagulant in its saliva.
Anyway, the bird goes about its business of drilling holes into trees as diverse as oak, maple, hickory, pecan and sweetgum. The holes penetrate the tree’s vascular cambium, the plant tissue just beneath the bark that funnels nutrients throughout the tree. Round holes generally go deeper into the tree than do square or rectangular holes and require less maintenance from the sapsucker for sap flow.
Insects scurrying around trees often get trapped in the sticky sap, thereby providing another food source for the clever sapsucker. Even an insect merely crawling around the bark or foliage is a food target for a sapsucker, as are flying insects, which the bird can capture in midair.
The sapsucker is a winter bird for us in Houston. It migrates here from breeding grounds in Alaska down through Canada and the northern U.S. Some birds go as far south as Panama for the winter.
But whether here on wintering grounds or up north on breeding grounds, the bird’s sap wells are a rich source of nectar and a trap for insects that furnish food for other birds such as kinglets, warblers, orioles and hummingbirds. As a result, we call the sapsucker a “keystone” species.
It’s somewhat like the keystone in a stone archway that supports all the other stones in the structure. Likewise, a sapsucker’s sap wells are a cornerstone food supply for other birds, particularly hummingbirds.
Accordingly, ruby-throated hummingbirds may time their migration to coincide with the migration of yellow-bellied sapsuckers. And on breeding grounds, a female ruby-throated hummer may construct her nest near the sap wells of a sapsucker and even follow the woodpecker on its route to various sap well