Sapsuckers draw a crowd eager for extra drops

By Jeff Mitton

Tapping in a tree attracted my attention; earlier that morning I had watched a three-toed woodpecker feeding, and I hoped this was another.

But a red-naped sapsucker emerged from the trees to sit on a branch above me.

Sapsuckers are woodpeckers, but they are distinguished for their distinct mode of feeding. They tap horizontal lines of wells into the bark of aspen, willows, pines, spruces and larches to feed on the dripping sap. Measurements in the field indicate that willow sap is about 20 percent sugar by weight.

Sapsuckers tend their wells daily, pecking to make them deeper or larger and removing dried scar tissue that blocks sap flow.

Paul Ehrlich and Gretchen Daily, working at the Rocky Mountain Biological Lab in Gothic, made observations of red-naped sapsucker feeding stations in willows. Their observations clearly showed that the sapsucker wells were utilized by a variety of insects, birds and mammals.

Observations at Gothic, California, Michigan, Minnesota and in Canada all report that hummingbirds regularly attend sapsucker wells. In Gothic, broad-tailed and rufous hummingbirds were recorded during the majority of the observation sessions.Flowing resin attracts quite a few insects in a forest. Adult stoneflies and craneflies came to drink resin, and they were promptly eaten by sapsuckers. Several species of vespid wasps also fed on the sap, but they were vigilant and elusive; they disappeared as sapsuckers approached and returned after the sapsuckers left. But it was clear that the flowing sap was both a source of nutrition for sapsuckers and a lure for insects, which provide welcome addition of protein to their diet.

Orange-crowned and Wilson's warblers also attended the flowing wells to drink, but they did not enlarge the wells nor maintain them to keep them flowing. That task was left solely to sapsuckers. Sapsuckers chased the warblers from the wells, but warblers returned as soon as the wells were unattended.

Least chipmunks fed regularly at the wells, but they were skittish; they ran whenever warblers or sapsuckers arrived. Pine squirrels are larger and bolder than least chipmunks, and they ignored aggressive sapsuckers. The average feeding time for a chipmunk was 26 seconds, but the pine squirrels fed leisurely until they were full — their average feeding time was 10 to 20 minutes.

Observations at Gothic, California, Michigan, Minnesota and in Canada all report that hummingbirds regularly attend sapsucker wells. In Gothic, broad-tailed and rufous hummingbirds were recorded during the majority of the observation sessions. Typically, they would feed for about eight seconds, perch nearby for several minutes and then feed again before flying off.

Studies of yellow-bellied sapsuckers at the University of Michigan's Kellogg Biological station revealed that ruby-throated hummingbirds rely on sapsucker feeding stations in early spring. In that specific area, most of the feeding stations were drilled in paper birch. The birds would hover for about 10 seconds to sip nectar, perch briefly and then feed again. Birds would occasionally spend 10 to 20 minutes at a particular tree, cycling between feeding and perching. Nearly continuous observations of one female for two days revealed that she relied exclusively on paper birch sap.

Ruby-throated hummingbirds arrive in southern Canada in early spring, before flowers are in bloom. Spring observations of sapsuckers regularly noted that ruby-throated and rufous hummingbirds followed yellow-bellied sapsuckers through the forest and perched nearby as the sapsuckers drilled new holes or maintained established wells. Hummingbirds would feed from the wells after the sapsucker left and then visit the wells numerous times each day.

A number of species take advantage of sapsucker wells when they are available, but it seems that the northern portions of the distributions of ruby-throated and rufous hummingbirds are dependent on the sugars flowing from sapsucker wells.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera. 

October 2014