They made a number of careful measurements of four nests, at heights varying from about 5 feet to about 10 feet above ground; the internal dimensions varied somewhat, but the size of the entrance hole was “surprisingly constant”; in one case this hole was a perfect circle, 43 by 43 millimeters, and in another 37 by 37 millimeters; in the other two cases the entrance hole measured 47 millimeters in height and 42 in width; translated into inches this shows a variation in the two dimensions of from 1.45 to 1.85 inches, which does not seem to be “surprisingly constant.” The total depth of the cavity varied from 275 to 400 millimeters, or from about 10 to 15 inches.
They say further: “Two of the nest cavities we found were in such unusual sites as to call forth comment. One at Hazel Green was in a slanting upright limb on a prostrate dead black oak trunk lying in a grassy meadow, fully 150 feet from the margin of the forest. The hole was excavated on the lower side of the stub. The other nest was at Tamarack Flat, in the butt end of an old log, lifted above the ground when the tree fell over a granite outcrop. This hole was about 7½ feet above the ground, and as with the other there were piles of chips immediately beneath it.”
Grinnell, Dixon, and Linsdale (1930) mention a nest they found in the Lassen Peak region that was “four meters up in the trunk of a dead-topped aspen.” Bendire (1895) mentions a nest found near Camp Harney, Oreg., that was about 25 feet from the ground in a dead limb of a pine; this nest seems to be at about the limit as to height above ground. A set in my collection was taken from a nest 10 feet up in a dead aspen.
Eggs.—The white-headed woodpecker lays three to seven eggs, four being the commonest number, and five rather often. These vary in shape from ovate to short-ovate. They are pure white and moderately or quite glossy. Grinnell and Storer (1924) say: “The eggs in one set had a wrinkled appearance at the smaller end as though that end had been compressed before the shells had hardened. Eggs which are advanced in incubation are apt to be soiled by pitch; this is doubtless brought in by the parent birds on their bills, feet, or plumage.” Sometimes the eggs show tiny black dots, or are profusely smeared with black from the same cause. The measurements of 50 eggs average 24.26 by 18.11 millimeters; the eggs showing the four extremes measure 26.40 by 18.29, 25.40 by 19.50, 21.84 by 17.78, and 22.86 by 16.76 millimeters.
Young.—Incubation is said to last for 14 days and to be shared by both sexes. Both parents also assist in the care and feeding of the young. Clarence F. Smith tells me that “the female at one nest made trips about twice as frequently as the male; her visits were about two minutes apart, while the visits of the male were about five minutes.” Grinnell, Dixon, and Linsdale (1930) write:
On July 1, the young woodpeckers, by this time half-grown, were being fed by the parents, mostly by the female. Food was brought at intervals averaging fifteen minutes each. The birds foraged at distances up to a quarter of a mile away from the nest. The female carried away the feces.
On July 11 the female seemed to be coaxing the young from this nest. When the young woodpeckers stuck their heads out of the cavity, the parent would move away from the entrance and call, although it remained on the tree trunk. When a person shook the stub two of the young birds flew out and went thirty meters before coming to the ground. When placed on a tree trunk the birds could move freely upward or downward. Within a few minutes one of the young birds could fly so well that it successfully evaded capture by the observer.
Plumages.—As with other woodpeckers, the young are hatched naked and blind, but the juvenal plumage is acquired before the young bird leaves the nest. The juvenal plumage is much like that of the adult but duller, and the bill is shorter and weaker; the contour plumage is softer and looser; the lower parts are brownish black instead of clear black, and the back is only a little darker; the white in the primaries is more restricted. In the young male, the posterior half of the crown is largely “vermilion” or “salmon orange”; these reddish colors are much reduced or entirely absent in the young female. Ridgway (1914) says that the feathers of the hind neck and underparts are sometimes, perhaps on younger birds than I have seen, “indistinctly and narrowly margined at tip with grayish, and the hindneck sometimes indistinctly spotted with whitish.” By the middle of September this juvenal plumage, including the wings and tail, has been replaced by the first winter plumage, which is like that of the adult, except for somewhat less white in the primaries. Adults have a complete annual molt, which begins in July and is generally completed before the end of September.
Food.—The white-headed woodpecker forages for its food mainly, if not entirely, on the trunks and branches of coniferous trees, living or dead. Mr. Skinner writes to me that he has seen it feeding on the trunks of sequoias, sugar pines, and Douglas firs, searching most diligently and thoroughly in the crevices in the bark for insects and their eggs; it generally begins low down on the tree and progresses upward, working pretty well up to the top of the tree before flying off; occasionally, one has worked horizontally around a tree trunk, but not downward. Dr. J. C. Merrill (1888) describes its method of feeding very well, as follows:
So far as I have observed, and during the winter I watched it carefully, its principal supply of food is obtained in the bark, most of the pines having a very rough bark, scaly and deeply fissured. The bird uses its bill as a crowbar rather than as a hammer or chisel, prying off the successive scales and layers of bark in a very characteristic way. This explains the fact of its being such a quiet worker, and as would be expected it is most often seen near the base of the tree where the bark is thickest and roughest.