Eggs.—Mr. Harlow tells me that the Cape May warbler lays from 4 to 9 eggs to a set. The larger numbers must be very rare, but 6 or 7 seem to be the commonest numbers among my records, and sets of 4 seem to be uncommon. The eggs vary in shape from ovate to short ovate and are almost lusterless. They are creamy white, richly spotted and blotched with shades of reddish brown, such as “auburn,” “chestnut,” “sayal brown,” “bay,” or “snuff brown,” with an occasional scrawl of black. The undermarkings are of “fawn,” “light brownish drab,” “brownish drab,” or “light mouse gray.” The markings are more concentrated at the large end. The measurements of 50 eggs average 16.8 by 12.5 millimeters; the eggs showing the four extremes measure 18.4 by 12.3, 18.0 by 14.0, 15.0 by 12.0, and 16.0 by 11.5 millimeters (Harris).
Plumages.—Dr. Dwight (1900) describes the juvenal plumage, in which the sexes are alike, as “above, dark hair-brown, olive tinged on the back. Wings and tail black, edged chiefly with dull brownish olive-green, the coverts with drab and tipped with buffy white. The two outer rectrices with subterminal white spots. Below, including sides of head, mouse-gray with dusky mottling or streaking on the breast and sides; the abdomen and crissum dingy white faintly tinged with primrose-yellow.”
The partial postjuvenal molt, beginning early in July, involves the contour plumage and the wing coverts, but not the rest of the wings or the tail. This produces the first winter plumage, in which the sexes begin to differentiate. Dr. Dwight describes the first winter male as “above, dull olive-green, each feather centrally clove-brown veiled with olive-gray edgings; the rump canary-yellow, the feathers basally black. Below, including sides of neck, superciliary lines and spot under eye, canary-yellow, palest on abdomen and crissum, narrowly streaked on sides of chin, on the throat, breast and sides with black which is veiled by grayish edgings; auriculars mouse-gray.” The young female, he says, is “duller and browner above, and generally without yellow below, being dull white with gray streaking.”
The first nuptial plumage is acquired by a partial prenuptial molt beginning in late winter, “which involves much of the body plumage but not the wings nor the tail. The black crown, the streaks on the back, the chestnut ear-patches and the streaked yellow of the throat and breast are acquired,” in the male. The female in first nuptial plumage “shows a little yellow assumed by a limited prenuptial moult.” Both sexes are now in nearly fully adult plumage, except for the worn juvenal wings and tail.
Adults have a complete postnuptial molt in July and probably a partial prenuptial molt, as in the young bird, though there is not enough pertinent material available to prove the latter. Dr. Dwight (1900) says that the adult winter plumage of the male is "similar to first winter plumage but the head black, the back streaked and everywhere veiled with smoke-gray edgings. Below, whitish edgings obscure the black streaks, the chestnut ear-coverts and the bright lemon-yellow areas. The wings and tail are blacker than in first winter, the back is black, either streaked or spotted, and the yellow below is deeper.” Of the female, he says: “The adult winter plumage is similar to the male in first winter dress, the yellow below rather paler and with less heavy streakings.”
Food.—Throughout most of the year the Cape May warbler is insectivorous, and mainly beneficial, but for a short time on its fall migration it undoubtedly causes damage to ripe grapes by puncturing them to obtain the juice, often ruining a large percentage of the crop. Many complaints have been made and several have been published. Frank L. Burns (1915a) claimed that about 50 percent of his crop was destroyed at Berwyn, Pa., and says: “I believe that grape juice was the principal food of the Cape May Warbler during its lengthy visit in this neighborhood. It was present in countless numbers at Berwyn and vicinity as far as a mile south of the village, apparently by far the most abundant species for a period; the complaints of the the ‘little striped yellow bird’ were many, and so far as I am able to learn, all unbagged grapes were ruined; the loss must have been many tons worth several hundred dollars.” He sent ten stomachs to the Biological Survey for analysis and received the following reply:
Hymenoptera constituted on an average 57.5 percent of the contents of the stomachs. A third perhaps of this material was parasitic Hymenoptera and their destruction counts against the bird. The others were ants and small bees and are of neutral importance except perhaps the ants which may be injurious. Diptera made up 16.7 percent of the stomach contents and again a large proportion of them were parasitic species. Lepidoptera (small moths) constitute 16.7 per cent, beetles 7.8 percent and the remainder was made up of Hemiptera, spiders and miscellaneous insects. Except for the spiders the food was entirely composed of insects, and a large proportion of useful species were taken and no decidedly injurious ones. I should say that these Cape May Warblers did very little to pay for the destruction of grapes.
McAtee (1904), after investigating the damage done on grapes by this and the Tennessee warbler in Indiana, published the following report on the contents of a single stomach of a Cape May warbler:
8 Typlocyba comes, an especial pest of the grape, “an exceedingly abundant and destructive” jassid; 3 Aphodius inquinatus and one Carabid, kinds which may be considered neutral economically, but, in case of a departure from their ordinary diet, would on account of vegetarian tendencies become injurious; 1 Drasterias sp. (click-beetle), 1 tortoise-beetle, 1 flea-beetle (Haltica chalybea), all injurious beetles, the last of which is a particular enemy of the grape, which “appears on the vine in early spring and bores into and scoops out the unopened buds, sometimes so completely as to kill the vine to the roots,” and later in the season in both larval and adult stages feeds upon the foliage, and if abundant “leaves little but the larger veins”; 1 Notoxus sp., a weevil, with all the undesirability characteristic of the creatures bearing that name; 2 ants, harmful, if for no other reason than harboring plant lice; and a vespoidean hymenopteron (wasp) of neutral significance. * * *