NORTHERN WHITE-HEADED WOODPECKER Xenopicus albolarvatus albolarvatus (Cassin)

Field characters.—Size somewhat under that of robin. Plumage wholly black, save for entirely white head (pl. 5g) and white area on wing, the latter showing best in flight. Flight course undulating, wing strokes intermittent. Voice: Usually a single syllabled high-pitched note, wiek; this note, or a similar one, repeated in short staccato series when bird is excited.

Occurrence.—Common resident in Transition and Canadian zones on west slope of Sierra Nevada. Observed from Sweetwater Creek (near Feliciana Mountain) and Smith Creek (6 miles east of Coulterville) east to Mono Meadow and near North Dome; not common in Yosemite Valley. Forages chiefly on living coniferous trees, but nests in dead stubs, usually less than 12 feet above the ground.

The Northern White-headed Woodpecker is a conspicuous member of the bird population at middle altitudes in the central Sierra Nevada. In certain places in the Yosemite region it is the commonest species of woodpecker. For example, near Chinquapin, on June 13, 1915, 12 of these birds were recorded during the taking of a seven hour census, while only 4 other woodpeckers were seen, each of which represented a different species.

The black body and white head of the White-headed Woodpecker as seen from behind are distinctive. (See pl. 5g). A side view of the bird when it is clinging to a tree shows a narrow white stripe along the folded wing. When the bird flies this stripe expands, forming an irregular white patch on the middle of the wing. Adult males have a narrow band of red across the back of the otherwise white head, but their mates lack this coloring. In juvenile males the hinder half of the head is more or less solidly red, and even in juvenile females there is usually a little red in the same place. In late summer the feathers on top of the head in adult birds may be so badly worn as to show the white much less clearly.

The White-headed Woodpecker is not so promiscuous in its foraging as are some other species, but gives most of its attention to live coniferous trees. This probably accounts for the fact that the feathers on its throat are almost always besmeared with pitch, whereas those of the Modoc Woodpecker, for instance, which lives in the same timber belt but forages mostly in dead trees, are relatively clean. So far as we could determine the foraging of this bird seems to be a rather haphazard proceeding. A female near Tamarack Flat was watched for a considerable period of time as she hunted over a number of trees. She would prospect one tree for a while, but when it had been gone over only partially, she would go on to inspect a second and then a third, with similar lack of thoroughness; often after examining portions of several trees the bird would return to one or more of those previously visited.

The usual call note of this woodpecker is a single wick, but when excited, the female calls cheep-eep-eep-eep, very fast, and repeats the call every few seconds. The male, under similar circumstances calls yip, yip, yip, yip, in a much shriller tone, but in slower time.

Because of the striking appearance of the Northern White-headed Woodpecker, and also because no special pains seem to be taken by the birds to conceal their nesting sites, it is an easy matter to locate the nests. In fact, nests of this species proved to be more easily located than those of any other woodpecker in the Yosemite region. For this reason we shall present some remarks upon the activities and ecologic relationships of woodpeckers in general while writing of this species. The matters here discussed may be readily confirmed as to accuracy by anyone who will watch a few pairs of these birds during the nesting season.

To locate occupied nests the enquirer has merely to tap the bases of stubs containing likely looking holes. If he merely passes by an occupied tree, no bird appears, but upon his tapping on the bole, a scratching sound within is followed shortly by the appearance of a white head framed in the entrance of a hole. If the observer remains, the bird flushes and quits the vicinity, but as it flies off one may usually see whether the back of its head bears the red fringe of the male or the plain white of the female. Our own experiences follow in some detail.

The behavior of a pair seen near LeConte Memorial Lodge in Yosemite Valley, on May 31, 1911, quickly led to the finding of a nest in a black oak close at hand. On June 24, 1915, a female bird, flushing at our approach to a dead stub near the east fork of Indian Cañon, disclosed the location of her nest. In May, 1919, when especial attention was paid to the nesting habits of birds, 9 occupied nests of this woodpecker were discovered. Three of these were found at Hazel Green on May 14 and 15, and 6 near Tamarack Flat May 24 to 26.

Of the 10 nests concerning which we have data, the lowest was located only 58 inches (measured) above ground and the highest, 15 feet (estimated). A nest of the Mountain Chickadee occupying what was evidently an old nest of this woodpecker was but 50 inches above the ground. Probably 7 feet would be the average height at which the nests of this species of woodpecker are placed. Dead stubs, either cut or broken off, seem to be the preferred sites for nest holes, for by far the greater number of nests were in such stubs.

No nest holes of this woodpecker were found in living conifers. Nor, on the other hand, do the birds seek what is commonly known as rotten wood, that is, wood too soft for the nest cavity to be maintained against the incessant wear involved in the birds' passage back and forth, incident to the rearing of a brood. The tree chosen must have been dead a sufficient length of time for the pitch to have hardened or to have descended to the base of the tree, and the outer shell of the tree must still be hard and firm, whereas the interior must have been softened to a moderate degree by decay. These conditions are not to be met with in every standing dead stub; hence the choice of a nest site becomes a matter of rather fine discrimination.

As evidence that considerable investigation by these birds precedes the excavation of the nesting hole finally occupied, we found many 'prospects,' varying from shallow pits, conical in form, where a woodpecker had begun excavation only to leave off without even penetrating the outer hard shell of the tree, to those where a bird had entirely completed the laterally directed tunnel into the softer wood within, but had not sunk the shaft which forms the nesting cavity proper. Often we found numerous fresh prospects on the same bole, and sometimes these were close to a newly completed and occupied nest cavity. We were led to conclude from all this that the White-headed Woodpecker is either notional or else very particular, in the selection of its home. Evidence points strongly to the birds excavating and occupying a new cavity each year, although one set of eggs was found in a hole which had been dug in earlier years.

Some stubs are literally riddled with holes, these probably recording successive years of occupancy. One stub had at least 5 fully excavated holes besides 11 or more prospects. Hence it will be seen that the activities of these and other woodpeckers contribute rather directly toward bringing down the standing dead timber. Drilling by woodpeckers results in an increase in the number of entrances through which insects may get at the heart wood of a tree and thus hasten its ultimate disintegration. Water, also, is thus afforded an easier entrance and this hastens decay. Eventually each and every tree must yield its place in the forest to seedlings. The woodpeckers hasten this process of replacement, once the tree is dead.

Many of the woods-inhabiting animals depend upon this woodpecker to furnish them convenient nest holes or retreats. We have found Mountain Chickadees and Slender-billed Nuthatches incubating their own eggs in holes drilled in earlier years by the White-headed Woodpecker; a Sierra Flying Squirrel was found occupying an old White-head's hole. Probably, tree-dwelling chipmunks and perhaps California Pigmy Owls also occupy holes of this woodpecker.

The nesting cavities of the White-headed Woodpecker are gourd-shaped, the entrance tunnel turning downward into a shaft which expands toward the bottom. The internal dimensions vary somewhat, but the size of the entrance hole is surprisingly constant. Also, the symmetry in the outline of the entrance is remarkable when it is recalled that an excavation just begun is often higher than wide and has an irregular margin. One hole, the first one listed below, measured 43 millimeters in four different directions—practically a perfect circle. The accompanying table shows the measurements (in millimeters) of four nests of this species which were studied.

Excavating is done by the removal of small chips or splinters, rarely over 25 millimeters in length by 3 or 4 millimeters in diameter. Because of the way in which chips are broken loose, the inside of the nest hole is usually irregular in surface finish. The small detached splinters of wood are, in the case of the White-headed Woodpecker, merely brought out to the entrance and dropped to the ground directly beneath. No attempt is made to keep the nest site concealed. An accumulation of such splinters below a hole is an infallible guide to a newly dug nest. In the case of the nest mentioned later as having been placed in a branch of a fallen black oak, the chips were strewn for a distance of 3 meters on the ground along side of the prostrate trunk.

Two of the nest cavities we found were in such unusual sites as to call forth comment. One at Hazel Green was in a slantingly 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 7-1/2 feet above the ground, and as with the other there were piles of chips immediately beneath it.

We were unable to conclude that the nest holes are located with any special regard to direction of exposure to the sun or weather. There seemed to be no rule as to their position with relation to the cardinal points. Probably the availability of appropriate surfaces in which to excavate is much more important in the choice of a site than is the possible protection from the elements.

In one stub was found a nest hole excavated in some previous year, but again occupied for the current season. This same barkless stub showed four weathered prospects. About the margin of the entrance to this nest were many claw marks, which aggregated into trapezoidal patterns, registering the positions of the toes, two up and two down. The tracks were most numerous immediately above the nest, showing that it was the custom of the woodpeckers, when going to the nest, to alight above the hole in the spot indicated and then back down to the entrance.

Eggs of the White-headed Woodpecker are typical of woodpeckers in general, in that they have a white, shiny surface entirely lacking any natural color markings—"immaculate" in the vernacular of the oologist. 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. The eggs always rest on a lining of fine chips or rotten wood, and the nest, even after the young are hatched, is maintained in a marked state of cleanliness.

A set of 4 fresh eggs collected at Hazel Green on May 4, 1919, weighted 20.5 grams, and a set of 5 fresh eggs taken the next day weighed 25.3 grams; while a set of 5, 4 of which were advanced in incubation, taken on the 14th, weighed only 20.7 grams. It is thus evident that these eggs lose considerably (15 to 20%) in weight as incubation advances.

We did not arrive within the range of the White-headed Woodpecker early enough in any year to ascertain whether or not both sexes assist in excavating the nest cavity. But after the eggs are laid, the male and female share alike in the duty of incubation. From the same nest hole we have in turn flushed the female and then the male; and in other instances birds of one sex or another were flushed, in about equal ration. Furthermore, the two sexes bear structural evidence of their equal share in the duty of incubation. During the nesting period the skin on the surface of the abdomen becomes thickened and underlain with a stratum of spongy, vascular tissue, the whole serving to transmit readily the necessary heat from the parent's body to the eggs beneath.

At Tamarack Flat, on May 26, 1919, a female White-headed Woodpecker was seen to flush from her nest about ten feet above the ground in a dead pine stub. Tapping by one of us on a nearby bole had caused her to leave, but she returned to the vicinity almost immediately. Then, for fully 25 minutes, while the observer remained within watching distance the bird foraged, preened, and flew about from one to another of the circle of 8 or 10 trees within a 50-foot radius of the nest, but always kept the nest tree in her sight. About every 5 minutes she would fly to the nest. In approaching it, she would swoop below its level and then glide up to the site with decreasing speed so as to end her flight with little or no momentum. Then, having gained claw-hold, she would poke the fore part of her body into the hole, withdraw it at once and repeat this performance four or five times before flying away again. Finally, after fully half an hour had elapsed, and her suspicions had been allayed, she went in, to remain. During this entire time the male kept out of sight and was heard only twice.

On another occasion, following the removal of a set of fresh eggs by chopping out the wall in front of the nest, the observer retired some distance, whereupon the male bird came almost at once to the site. He approached the hole from above and to one side, and upon arriving at the place poked his head into the space which marked the position of the entrance hole; but he did not enter the wide open cavity. He seemed to be puzzled at the sudden change in the configuration of his home. The next day he or his mate was seen busily digging a new nest in a stub about 50 yards distant from the first site.

Near the east fork of Indian Cañon, above Yosemite Valley, a nest containing young birds was discovered on June 24, 1915. The female parent flushed at our approach and lingered about the vicinity, uttering two kinds of notes at short intervals. The voices of the young could be heard within the nest tree, and upon investigation we found 5 of them, only a day or two old and still entirely naked. They lay on the nest lining of fine splinters and with them were the shells from which they had hatched, each of which had been cut around the middle.

A White-headed Woodpecker was watched on June 24, 1920, near the village in Yosemite Valley. It was intently searching the trunk of a large cottonwood and picking naked larvae out from under the bark scales of the living tree, now and then whacking off the edge of a bark plate in order better to explore the space underneath.

Stomachs of two adult birds, obtained at Merced Grove Big Trees on June 10, 1915, and at East Fork of Indian Cañon, June 24, 1915, both held ants, some of which were large carpenter ants. The stomach of one of the young birds from the nest mentioned above contained remains of 2 large spiders, a large ant, 2 boring beetles, and a whole fly larva.

Fig. 42. Feet of (a) Northern White-headed Woodpecker, and (b) Arctic Three-toed Woodpecker, showing greater size of latter, perhaps compensating for loss of fourth toe. Natural size.