Grinnell and Storer (1924) made some studies of the feeding habits of the willow woodpecker in the Yosemite region, of which they say:
A pair of Willow Woodpeckers proved to be regular tenants of Curry’s apple orchard on the floor of the Yosemite Valley. They, or their ancestors, had evidently worked there for some years, with the result that most of the 150 trees in the orchard showed marks of their attention, and many of the trunks were fairly riddled with drillings somewhat like those of the sapsucker. * * *
However destructive this drilling may seem to be, it does not seriously affect the vitality of the trees; the pits are but 4 to 5 mm. deep, penetrating only those outer layers of the bark which after a time scale off. We should judge that all evidence of this woodpecker’s work is thus removed through natural process within about three years. The heartwood of the tree therefore seems not to be damaged at all by the woodpecker’s work; it is damaged, however, by the work of the true sapsucker. Our inference from these facts is that the willow woodpecker feeds on the inner layers of bark, which the bird exposes through the perforations described above. We watched a bird at work; moreover, bits of inner bark-fibers were found adhering to the bristles around the bill of a bird shot.
Evidently this observation and report started the same old controversy that arose in connection with the eastern bird, which has been referred to under that subspecies. Charles W. Michael intimated, in course of conversation with Dr. Grinnell, that they were mistaken in ascribing these drillings to willow woodpeckers rather than to red-breasted sapsuckers. This led to the publication, by Dr. Grinnell (1928a), of the evidence produced by Mr. Michael and himself, to which the reader is referred. In spite of some evidence, and more supposed evidence, to the contrary, it now seems to be generally conceded that the downy woodpeckers seldom, if ever, drill these holes for themselves, but that they often feed from holes drilled by sapsuckers. The small amount of drilling done by the downy woodpeckers seems to do the trees no great harm.
Behavior.—Grinnell and Storer (1924) write:
The quietness of the willow woodpecker, as compared with most other species in its family, is noteworthy. We heard no single call note from it, and only at long intervals did we hear the indescribable short trill characteristic of this bird. Individuals are much restricted in range, foraging along a relatively short line of cottonwoods or willows day after day. Once a bird is located, it can usually be found in the same place regularly. When foraging it moves about with very little commotion, and even when drilling for insects works so quietly that only a keen auditor can detect its presence. No matter what the season of the year, a pair of these birds is to be found usually within hearing of each other. The bird’s close adherence to deciduous trees makes it more conspicuous and easier to observe in late fall and winter than in the summertime when the trees are fully leaved out; but even in winter, our experience with the willow woodpecker led us to consider it about the most elusive of all the diurnal birds of the Yosemite region.
We had always supposed that the rapid series of notes uttered by this species were given only by the adult male and hence constituted a sort of song. But on June 24, 1920, in Yosemite Valley a juvenile male was found, with his head out of a nest hole eight feet above the ground in a dead branch of a live willow, giving every few moments this very series of notes. The large crown patch of red on this bird established its age and sex clearly. There was every indication that the notes were being given as a food call.
M. P. Skinner contributes the following note:
One seen in Sequoia National Park in August was drilling at the bases of willow shoots near a river. It perched lengthwise of the stems. It managed to keep well hidden, but worked industriously and did not change its position much during the short time that I could see it. Later, I caught a glimpse occasionally of the woodpecker’s red head, although the bird kept hidden most of the time. This reminded me that I had often wondered why red usually marked a woodpecker’s head. Certainly it makes a wonderful recognition mark. In that way it might well be that red on the constantly moving head of the woodpecker would be of value to the race.