Forbush (1927) gives the incubation period of the yellow-bellied sapsucker as “probably about 14 days.”
A. Dawes DuBois furnishes the following note: “Yellow-bellied sapsuckers were observed feeding young in a nest, in Hennepin County, Minn., on July 5, 1937. The nest was about 25 feet above ground in a partially dead tree at edge of willow-and-alder thicket adjoining woods. Both parents were bringing food. The squeaky note of the young was repeated with such regularity (about four times a second) as to indicate that only one nestling was uttering it. When the nestling was being fed at the entrance, by the poking method, these notes went up to a higher pitch, and were sometimes choked off almost to inaudibility.
“Two days later, the parents were still feeding very frequently. The male, who on the first day had been seen to bring a bright red berry about the size of a pea, again brought a bit of small red fruit. On one occasion, when the parents were away, the nestling put its head out of the hole; but it did not do so when being fed. In general, alarm calls of the parents had little if any effect upon the squeaking of the nestling, though at one time, July 7, the squeaking seemed to cease for a short interval when the parent gave the alarm notes. For the most part the series of squeaky notes is continuous. It was by hearing these sounds that this nest was discovered.”
Plumages.—[Author’s note: The young sapsucker is hatched naked, as is the case with other woodpeckers, but the juvenal plumage is acquired before the young bird leaves the nest. The sexes are alike in the juvenal plumage. A young bird, not fully grown and probably not long out of the nest, taken June 25, has the black crown largely concealed by the long brownish tips of the feathers, “ochraceous-tawny” to “buckthorn brown”; each of the black feathers of the back has a large terminal spot of grayish white, or yellowish white, producing a boldly spotted pattern; the nape and sides of the neck have smaller spots of the same color; the wings and tail are as in the adult-fall plumage; the chin and upper throat are dull white or pale buffy brownish; the lower throat and chest are pale brownish, broken by crescentic bars of dusky; and the center of the breast and the abdomen are pale yellow or yellowish white. Changes soon begin to take place, at irregular intervals, during which the sexes begin to differentiate. Young males may begin to show traces of red in the throat patch as early as July; and in August some may have the crown largely crimson; the black patch on the chest does not usually appear until much of the red has been assumed, but some birds show considerable of both red and black before the end of August. Other young males may not acquire much red before the end of September. Progress toward maturity continues all through fall, winter, and early spring by protracted partial molts; probably most individuals acquire the fully adult plumage by early spring, but I have seen birds that had not fully completed this prenuptial molt by the end of April.
Young females follow the same sequence of molts but are somewhat later in developing the red crown, which apparently is not acquired until October or later. The adult body plumage of both sexes is acquired during winter and early spring. Adults have a partial prenuptial molt about the head and throat early in spring and a complete molt late in summer and fall. In fresh fall plumage, the lighter markings are more or less suffused with yellowish or buffy tints, and the belly is deeper yellow.]
Food.—W. L. McAtee (1911) learned by stomach examinations that the yellow-bellied sapsucker consumed cambium and bast averaging 16.71 percent of its diet. He continues:
It must be noted also that cambium is a very delicate, perishable material, at certain times no more than a jelly, and thus never receives a percentage valuation in examinations of long-preserved stomachs corresponding to its bulk when first swallowed. Neither do we get any record of the sap consumed by these birds [the three species of sapsucker] and they are inordinate tipplers. Hence the value of the percentages cited lies not so much in their accuracy as to the quantity of cambium eaten as in the fact that they indicate a steady consumption of this important substance. There is no doubt that cambium, bast, and sap are depended upon by sapsuckers as stable diet.
We may get some idea of the amount of sap consumed by the bird from Frank Bolles’ (1892) record of his three young captive sapsuckers. He says: “Ordinarily they disposed of eight teaspoonfuls [of diluted syrup] each during the twenty-four hours. Part of this evaporated, and part was probably secured by black ants which visited the cage by night.”
Bolles (1891), describing the method of feeding of birds in the wild, says: “The dipping was done regularly and rather quickly, often two or three times in each hole. The sap glistened on the bill as it was withdrawn. I could sometimes see the tongue move. The bill was directed towards the lower, inner part of the drill, which, as I found by examination, was cut so as to hold the sap.”
This is the common method of feeding, but sometimes, when two or more holes have coalesced into a vertical groove, the bird will run its bill upward along the edge of the wound, sipping the sap much as we might, with our finger, wipe off a drop running down from a pitcher’s lip.