Marsha C. Salett
I was casually watching the chickadees, titmice, and juncos at my backyard feeders on the afternoon of February 12, 2021, when a flash of black and white wings at the edge of the woods caught my attention. I saw a Pileated Woodpecker (Dryocopus pileatus) fly into my yard, land about 10 feet up the trunk of an oak, and begin to probe the bark with its bill. It did not chisel into the wood with any speed or force but pried at the bark methodically for two to three minutes. There must have been bark beetle or ant larvae or other insects under the bark.
While the Pileated was feeding, a second woodpecker flew in and landed on the back of the tree a couple of feet below the Pileated. My first thought was that the newly arrived bird must be the Pileated’s mate, but it appeared all dark when it flew—no black and white wing pattern—and was not as large. When it moved to the front of the tree directly below the Pileated, I could see it was a Northern Flicker (Colaptes auratus). The Pileated worked the bark and the flicker was motionless. The birds paid no attention to each other—or so I thought.
The Pileated hopped a few feet farther up the trunk and began to probe and forage under the bark in a second spot for another few minutes. As soon as the Pileated left the first spot, the flicker moved up the trunk and began to forage there. When it stopped feeding, it remained still until the Pileated moved up the tree again and found a third spot to probe for grubs and insects. Then the flicker moved up to the second spot. This pattern continued four times as the woodpeckers made their way to the top of the tree, with the flicker following the Pileated and keeping the same distance—about two Pileated body lengths—between them. Eventually the flicker flew to the next tree and the Pileated flew back into the woods.
I had not seen this behavior before, so I described it to Wayne Petersen, who agreed that it was unusual behavior but wondered if it simply might be two different species attracted to the same food source. Stephen A. Shunk, in the Peterson Reference Guide to Woodpeckers of North America (2016), wrote that different species of woodpeckers may forage together in insect-infested trees and noted, “Extensive excavations often attract other species to forage. In fall and winter [Pileateds were] observed foraging on logs with Northern Flicker and Williamson’s Sapsucker.”
However, this Northern Flicker and this Pileated Woodpecker never foraged randomly in different parts of the tree. The flicker always followed the Pileated, watching and waiting until the Pileated finished feeding before it took the larger woodpecker’s place. It was obvious that the flicker fed only in the wake of the Pileated’s foraging, which had exposed food items. Wayne noted, “Perhaps the flicker noticed that the Pileated was successfully collecting some type of beetle or carpenter ant larvae and simply decided to follow the big boy and do the same thing,” which, he added, could be an example of commensalism—a relationship between two types of organisms where one species benefits from the second species, e.g., obtains food or shelter, while neither harming nor benefiting it.
This commensal feeding of the flicker and Pileated is an arboreal example of the beater effect, similar to when egrets follow tractors or cattle in fields and feed on the insects that are stirred up (W. E. Davis, Jr. personal communication). The beater effect is one of the more common commensal feeding interactions, particularly among shorebirds, waterbirds, and marsh birds (Ehrlich et al. 1988). William E. Davis, Jr. has written two Field Notes in Bird Observer on this topic: “Tricolored Herons and Great Egrets Use Double-crested Cormorants as Beaters While Foraging” (2000) and “Commensal Foraging of Brown Pelicans and Egrets with Double-crested Cormorants and White Ibises” (2007).
I did a brief search of the literature and could not find any examples of commensalism between Pileated Woodpeckers and Northern Flickers. Nor could I find how common this behavior is among flickers in general. Ehrlich et al. (1988) described commensal feeding in North American woodlands that involved Hairy Woodpeckers foraging on insects exposed after Pileateds stripped sections of outer bark from tree trunks. In the previous issue of Bird Observer, Larson and Carlson (2021) described how several passerine species feed on sap from wells drilled by Yellow-bellied Sapsuckers, another commensal association mentioned by Ehrlich.
If the Northern Flicker in my backyard learned to exploit a Pileated Woodpecker’s foraging behavior to find an easy food source when the ground it usually forages on is frozen and snow covered, then more power to that flicker.
References
- Davis, William E. Jr. 2000. Tricolored Herons and Great Egrets Use Double-crested Cormorants as Beaters While Foraging. Bird Observer 28 (6):383–385.
- Davis, William E. Jr. 2007. Commensal Foraging of Brown Pelicans and Egrets with Double-crested Cormorants and White Ibises. Bird Observer 35 (2):98–99.
- Ehrlich, Paul, David S. Dobkin, and Darryl Wheye. 1988. Commensal Feeding. The Birder’s Handbook. New York, New York: Simon and Schuster, Inc.
- Larson, David M. and Susan L. Carlson. 2021. Use of Sapsucker Wells by Passerines. Bird Observer 49 (1):56–59.
- Shunk, Stephen A. 2016. Peterson Reference Guide to Woodpeckers of North America. New York, New York: Houghton Mifflin Harcourt.