The female was more careful in brooding the young during the first few days. She would stop for intervals throughout the day, while feeding, and brood the young. Her way of completely covering the brood was to fluff out the under coverts against the rim of the nest and bring the wings down, just inside, so as to effectually close the nest.
* * * The female had different brooding attitudes for the varying circumstances. For protection against the cold of early morning she brooded in the manner described above, completely covering the young. Through the rains she brooded in much the same way as for cold, sheltering the young, so that after an unusually heavy downpour, the nest remained perfectly dry inside. During the heat of midday she usually stood in the nest with wings spread, shielding the young, but without shutting off the circulation of the air. On the contrary, at times she gently flapped her wings, as if fanning the young. During the strong winds she stood in the nest with wings outstretched, and leaned in the direction of the wind, so as to secure a delicate balance and at the same time keep the young in the nest.
Feeding of the nestlings was carried on about equally by both male and female parents for the first 7 days, after which the male was frightened away by a snake and did no more feeding, the female carrying on for the next 4 days. During observations covering nearly all of 10 full days and part of another there were 2373 feedings, 813 by the male and 1560 by the female, there being only 33 feedings during the whole of the last day. “During the first three or four days when the female was brooding, usually the male gave her the food, which she distributed to the nestlings.” Some of the food had to be broken up before it was given to the young; and sometimes it had to be thrust down their throats. There were 331 feedings of unrecognized food, and 553 of unidentified insects. The identified food consisted of 659 green worms, 326 fly worms, 162 other worms, 147 May flies, 103 moths, 75 millers, 65 mosquitoes, 26 larvae, 25 grasshoppers, 23 spiders, 18 ants, 14 grubs, 8 beetles, 4 damsel flies, 2 tree hoppers, and 1 bee. Feeding began at from 4:29 to 4:50 a. m., and ended at from 7:36 to 8:04 p. m., the average feeding period being 15 hours and 30 minutes per day. The parents were not seen to follow any system of rotation in feeding the young. “At no time while the nest was under observation did the parents feed by regurgitation,” though the parents on several irregularly occurring occasions were seen to insert an apparently empty bill into the mouth of a nestling, but it was long after hatching. The excreta were removed by both parents; they were eaten during about the first half of the nest life and carried away after that; the female did most of this. The parents were very watchful of the young, and were seen to drive away such birds as the cowbird, blue jay, wren, chickadee, brown thrasher, kingbird and blackbird, if they came too near the nest; the only bird that was not driven away was a catbird. The presence of a garter snake at the base of the bush caused great excitement; the snake was seen to climb up into the bush and carry off one of the young when it was about six days old; the young bird was dead before it could be rescued.
Schrantz (1943) writes: “The Yellow Warblers are hatched naked except for a scanty amount of down and are an interesting sight with their large bulging eyes and abdomen. It was observed that the eyes were commencing to open on the third day after hatching. By the fifth day the young can completely open their eyes, but in many cases would immediately close them when the nest was approached. At this age they would also duck down in the nest as if trying to hide. A slight tapping on the nests would cause a rapid outstretching of necks with open mouths.” Bigglestone (1913) found that almost any slight noise near the nest would produce the same results. Studies of weights by Schrantz showed that—
the young averaged, when hatched, 1.27 gms.; at one day old, 1.87 gms.; at two days old, 2.95 gms.; at three days old, 4.36 gms.; at four days old, 5.57 gms.; at five days old, 7.26 gms.; at six days old, 8.20 gms.; and at seven days old, 8.78 gms. * * *
Of the 168 eggs in forty-one nests, 119 eggs, representing 70.83%, hatched. Thirty-four eggs, representing 20.24%, disappeared due to wind, abandonment of nest, and unknown causes. Fifteen eggs, representing 8.93% were addled, two of which were buried with a Cowbird’s egg. Of the 119 nestlings, twenty-eight disappeared. This represents 16.66% of all eggs laid. Four of them were seen dead in the nests. The others disappeared from unknown causes. Therefore a total of 91 fledglings, representing 54.17% of the original 168 eggs, left the nest. * * *
After all the young left a nest, the parent birds could be found feeding them in the immediate vicinity of the nest for a period of about three days. After this time the birds became more dispersed from the nesting site, but could still be found in the vicinity for a week or ten days.
An unusual casualty is recorded in the following note sent to me by Dr. Harrison F. Lewis: “A nest of this species which I found in a sheep-pasture, was largely built of wool, presumably gathered from the neighboring bushes, where it had been left by the sheep. One of the young birds in this nest died as a result of having threads of the wool in the nest become entangled about its tongue and bill. Another member of this brood became entangled in a similar fashion, but I released it.”
Plumages.—Dr. Dwight (1900) calls the natal down “mouse-gray,” and describes the juvenal plumage, in which the sexes are alike, as “above, pale olive-brown. Wings clove-brown broadly edged with bright olive-yellow paling at tips of the quills, the edge of the outer primary bright lemon-yellow. Tail pale clove-brown, the inner webs of the rectrices lemon-yellow, the outer edged with olive-yellow. Below, pale sulphur-yellow, unstreaked.”