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Within the scope of my research it was difficult to detect the over-all sequence of epigamic displays that result in synchronization of the physiological states of the sexes throughout the period of courtship. Possibly all displays, except the post-copulatory one, occur in no particular order in the courtship period. However, each ritualized display seemingly strengthens the pair-bond.

In situations where attacking and fleeing are the two conflicting tendencies, wing-flicking and tail-flicking are incorporated into threat display, but do not lose all of their original function, for they facilitate attack. Tail-fanning, as a display element, increases the awesome aspect of the threatening bird and in courtship presumably makes the sexes more attractive to one another.

Courtship feeding has not been recorded for the Bell Vireo. In general, it is unknown in North American vireos, with the exception of the red-eye . It would serve no "practical" purpose in the Bell Vireo since the male regularly relieves the female during incubation, thus allowing her ample opportunity to forage. In the Red-eyed Vireo, only the female regularly incubates, and courtship feeding is definitely functional. Nolan described a brief pecking or pulling with their bills between pairing birds. This may be incipient "symbolic" courtship feeding, or perhaps mutual preening.

SELECTION OF NEST-SITE AND NESTBUILDING

As far as can be determined, the nest-site is selected by the female. Typically, the pair makes short, low-level flights from tree to tree with the female invariably in the lead. The birds usually forage within each tree; the female interrupts this activity to inspect small forks of low, pendant branches and the male occasionally pauses to sing. The singing is loud but not particularly regular, as it is later when the male accompanies the female during actual nestbuilding. Method of selection of site resembles that described by Lawrence for the Red-eyed Vireo.

Nests are suspended from lateral or terminal forks about 27 inches high in bushes and small trees that, in the study area, averaged 11 feet, four inches in height . The height above ground of the nests does not vary appreciably as the season progresses as is the case with nests of Red-eyed Vireos, for which Lawrence noted that late nests were placed higher than those built earlier in the season.

Most nests are so situated that they are protected and concealed by the dense foliage of trees. Where nests are placed in low bushes, as coralberry or dogwood, the bush is invariably overhung by the foliage of a much taller shrub or tree.

The nest tree or shrub was in every instance situated at the edge of a thicket or isolated from adjacent trees by several feet. Preference for open situations is characteristic of the species. In contrast, the nest of the White-eyed Vireo is placed toward the center of thickets.

In the choice of sites in the study area, the Bell Vireos were almost unopposed by other avian species, owing to the size of the fork utilized and the fact that the nests are located peripherally, rather than centrally, in the bush or tree. This lack of competition for a nest-site provides a Bell Vireo with an ample supply of nest-sites within any one territory.

TABLE 5. NEST-SITES UTILIZED IN 1960.

This particular site is of further interest because it is the same one utilized for nest 1-a . In all, four instances of utilization of a nest-site a second time were recorded. Two-a and 2-d were built in the same fork; 1-c and 1-h were in the same tree, but not the same fork. It should be mentioned that 1-a and 2-a were abortive attempts that did not progress beyond the suspension apparatus. Nice recorded a similar instance of the re-use of a nest tree, but different forks were used.

Re-use of an exact nest-site would ordinarily be impossible if the initial attempt were not abortive, because the presence of a completed nest would pose problems in construction with which the birds would probably be unable to cope. Since nests are used only once there would be no tendency to adopt the old nest. However, abortive nests, usually little more than a few strands of nesting material secured to the fork, might stimulate the birds to continue building. Re-use of a single nest-site in 15.8 per cent of 38 nests built in 1960 seems to be more than fortuitous circumstance. This re-use may have physiological benefits in conjunction with apportionment of energy for other nesting activities, because rapid location of a nest-site would mean that energy normally expended in searching and selecting could be rechanneled for actual construction. In each of the instances of rebuilding, the new nest was begun on the same day that the previous nest was abandoned.

The re-nesting of pair 9 is worthy of note. These birds were established in the elm thicket on Clark land. Elm was by far the most abundant tree, with dogwood, Osage orange and honey locust also relatively common. There were only six boxelders in the territory and yet the four nests built by this pair were placed in them. This is the only instance of seeming preference.

Nestbuilding by Bell Vireos can be best discussed in terms of the phases of construction described for the Red-eyed Vireo, Lawrence , which are: construction of the suspension apparatus, construction of the bag, lining of the bag and smoothing and polishing of the exterior, and adornment of the exterior. Red-eyes may continue adornment far into the period of incubation. Both the male and female Bell Vireo have been observed to add spider egg sacs and other silk to the exterior of the nest as late as the sixth day of incubation.

Nice recorded only the female Bell Vireo building, but she did recall, from previous studies, having seen males aiding somewhat. Pitelka and Koestner wrongly concluded that the female Bell Vireo builds unaided, but Hensley observed that both sexes participated in nestbuilding, and Mumford reported two instances of building by both adults. His description of the activities viewed in mid-May suggest that they were of the transitional period between the first and second phases. On the second occasion he recorded both adults building during the second phase. Since no details accompany this second observation I assume that it pertained to activity not necessarily typical of this phase of construction. Whereas both sexes of the Bell Vireo cooperate in building the nest, only the female Red-eyed Vireo builds according to Lawrence . But Common saw both Red-eyed Vireos building a nest.

The suspension apparatus is constructed by only the male on the first day. He punctuates each trip to the nest with song. The single song phrase is given from three to eight times when the male, carrying nesting material in his bill, arrives in the tree. Typically, he alights on several perches within the nest tree before flying to the nest. He often interrupts his work with several songs; when he has finished adding a load of material he sings from several perches within the nest tree before departing. The male periodically stops building to court the female.

In eight hours of observing the first phase of construction at five different nests, I saw the female come to the nest 28 times; the male made 95 trips. The female came alone only once, and brought nesting material ten times, but did not build; on the other 18 occasions her visits were brief and she usually confined her activities to an inspection of the nest. Twenty of the visits by the female were made late in the first phase, marking a gradual transition to her assumption of building responsibility. The courtship and building activities of the male plus the presence of a partly completed nest seem to stimulate the female to commence building. Her visits become more frequent as construction of the suspension apparatus nears completion. At a time early in the second day the transition has taken place, and the female becomes the sole worker.

On May 7, 1960, male 2 , at the time unmated, was observed as he came upon a nest of the previous year. The nest, after a year's weathering, suggested in appearance perhaps an early second-day nest. The bird flew to the nest and tugged and wove loose strands of grass for three minutes. Before leaving the site, the bird sang twice from different perches. This observation suggests that a partly constructed nest can elicit nestbuilding behavior, even in an unmated male.

The techniques of building by the male consist primarily of laying pieces of grass or bark across the fork, or along one of its branches, and then fastening them in place with pieces of animal silk. Once a "racket" has been formed, spider egg cases and plant down are emplaced among the fibers. The male employs weaving, twisting, and pecking motions of the head to emplace material.

As previously indicated, the female is the principal worker in the second and third phases of construction. The male infrequently visits the nest, but regularly visits the nest tree. The molding of the bag is accomplished by piling leaves, grasses and plant down onto the suspension apparatus. This material is also bound in with animal silk. As the amount of material accumulates, the female begins to trample it and gradually the bag takes shape. When trampling is first attempted, the nest often fails to support the female and she falls through the bottom of the nest. Such an occurrence was observed on May 23, 1960, on three consecutive trips by female 1 , in constructing nest 1-e . As the bag deepens, additional strands of grass are added to the wall and woven into place.

The male is extremely attentive during this and the following phase. He follows the female as she gathers nest-material accompanying both this activity and her building with rapid song; he may give an average of seven song phrases per minute. The male brings to the nest a strand of grass, or some other material, about every twentieth trip. He frequently inspects the nest and the activities of the female from perches near the nest. Construction of the bag is ordinarily completed in the third day.

The third phase, the lining of the interior and the smoothing of the exterior, involves an additional one and one-half to two days. Smoothing of the exterior refers to tightening of the grasses woven into the bag and addition of more animal silk. In lining the nest, the female stands on one of the branches of the fork and emplaces one end of a long, thin strand of some relatively stiff piece of grass or strip of bark. She then jumps into the bag and, while slowly turning around, pecks it into place, thus coiling the strand neatly around the interior of the bag.

As previously mentioned, the fourth phase overlaps the periods of lining, smoothing, egglaying, and incubation. The principal activity is the addition of white spider egg sacs to the exterior. The trips are infrequent; but, occasionally, birds will interrupt an hour of incubation with three or four minutes of active adornment, during which several trips may be made. Both sexes participate in this phase.

Nesting materials were gathered anywhere within the territory. Occasionally materials were collected from within the nest tree, but usually they were obtained 20 to 200 feet from the nest-site. On several occasions I observed birds inspecting stems or branches where bark was frayed. Loose ends are grasped in the beak and torn free with an upward jerk of the head. Possibly the notch near the distal end of the upper mandible aids in grasping these strands. Plant down is first extracted and then rolled into a ball by means of the beak while held with the feet before being transported to the nest.

As indicated by Nolan , accurate determination of the length of nestbuilding is difficult because of continued adornment and polishing after the nest is functionally complete. Most of the early nests for which I have records took from four and one-half to five days to construct. A four-to five-day period of building is reported by other observers .

One instance of protracted building was recorded. Nest 6-d was begun on May 29, 1960, and not completed until nine days later on June 6, 1960. In contrast nest 1-g begun on May 31, 1960, was finished three days later on June 2, 1960. Nestbuilding occurs between the hours of 6:00 a.m. and 5:30 p.m. Heavy rain in the early morning may delay building.

Eight of 38 nests started in 1960 were never completed . Six of these abortive attempts were abandoned during, or shortly after, the completion of the suspension apparatus. Five of these nests were abandoned because the female did not begin building following the end of work by the male. The early abandonment of the other three nests 1-a , 2-c and 6-e was attributable to the interruption of building by the male because of heavy rain and protracted territorial conflicts. The occurrence of these abortive nests at any time within the nesting efforts of a single pair indicates that such attempts are not examples of "false nestbuilding."

Renesting after desertion or successful fledging occurs within two to thirty-six hours. Young were fledged from 1-a on June 19, 1959, and nest 1-b was discovered when late in the second phase of construction on June 22. If the nest was started on June 20, then renesting took place within 15 hours after fledging.

Several authors have described various aspects of the nest of the Bell Vireo, notably Goss ; Simmons , Nice and Nolan . I can add but little to these descriptions.

The nest itself is a compact structure composed of strips of bark and strands of grasses that are interwoven and tightly bound with animal silk. The floor of the cup is first lined with a layer of small leaves and then the entire interior is lined with fine stems or strips of bark. Feathers are occasionally used to pad the bottom prior to lining, as are pieces of wool and milkweed down. Nest 2-e had been packed with small pieces of soil bearing moss prior to lining.

TABLE 6. ABORTIVE NESTING ATTEMPTS IN MAY AND JUNE OF 1960.

Early nests tend to be bulkier, having thicker walls and bottoms than later efforts. However, nests in May were found to have 16 per cent thicker bottoms and 41 per cent thicker walls than nests in June . Standard nest measurements do not show this to be so, for the exterior and interior diameters at the rim are governed by the angle between the two branches of the fork.

TABLE 7. DIMENSIONS OF NESTS IN MAY AND JUNE .

EGGLAYING AND INCUBATION

Egglaying begins the first or second day after completion of the nest. The female sits in the nest occasionally for periods of five to twenty-five minutes on the day the nest is completed. This is interrupted by periods of nest-adornment and foraging; such activities sometimes keep the female off the nest for several hours. Prior to the laying of the first egg, only the female is seen on the nest, although the male is often seen sitting quietly within the nest tree a few feet from the female. The infrequency of the "congested" song and the alarm after the inception of "broodiness" indicates the waning of courtship behavior. As later in incubation only the "normal" song and the scold are heard.

Eggs are laid early in the morning prior to 5:30 a. m. according to Nolan . The nest is usually left unoccupied for considerable periods after the first egg is laid, but, on the first day of laying, both sexes have been observed sitting for brief periods averaging ten minutes in length. Eggs are laid at one-day intervals until completion of the clutch. I found incubation to begin with the second egg.

The average clutch-size of the Bell Vireo in Kansas, based on thirty-three records, is 3.39 eggs . Seasonally, the largest average clutches are produced in the middle of the breeding season, that is, in June. Lack indicates that in European passerines the highest seasonal average clutch-sizes likewise occur in June. The largest average clutch-size in the Bell Vireo is presumably related to some aspect of the availability of food.

TABLE 8. AVERAGE NUMBERS OF EGGS PER NEST .

These data have been supplemented from the literature pertinent to Kansas.

Caution is necessary in determining mean clutch-size in the Bell Vireo. Eggs occasionally disappear from the nest prior to or during incubation, without subsequent addition of cowbird eggs. Unfamiliarity with the history of such a nest on the part of the observer would lead to an inaccurate determination of clutch-size.

Complete clutches are not replaced with the same regularity as are nests. I have recorded intervals of six to thirty days between successive clutches. Successful replacement of clutches is determined by a number of factors: nest-site, completion of a nest, weather, predation, and parasitism by the cowbird. The difference between the number of renesting attempts and the successful replacement of clutches seems to indicate that different physiological processes are responsible for these two phenomena and that there is lack of synchrony between them. The development of the ovarian follicle requires a specific number of days that is not always coincident with the building of replacement nests. If, in the Bell Vireo, replacing a nest were solely a responsibility of the female, instead of involving the male to a considerable extent, it would seem likely that replacement of nests and the replacement of clutches would be more closely coordinated.

Nice considers the incubation period to be the elapsed time between the laying of the last egg in a clutch and the hatching of that egg, when all eggs hatch. My data indicate that, normally, intensive incubation begins when the second egg is laid and lasts fourteen days in the Bell Vireo. Nice also considered the incubation period in this species to be fourteen days but believed it to commence when the third egg was laid. Pitelka and Koestner noted that the first and second eggs hatched fourteen days after laying of the second egg. However, they thought incubation began with the first egg. This would mean a fifteen-day period for this egg. All the eggs that Nolan marked hatched in approximately fourteen days. Eight eggs artificially incubated by Graber required an average of 15.01 days to hatch. As Van Tyne and Berger indicate, periods of sitting on the nest, even all night, do not necessarily mean that incubation has begun, for it has been demonstrated in several species that birds may sit on an egg without actually applying heat. My own observations demonstrate that the first egg may be left unattended for several hours at a time on the day that it is laid.

Both the male and female sit on the eggs in the daytime. My study of histological sections of ventral epidermis indicates that the male does not possess a brood patch; the increased vascularization typical of the brood patch in females is not evident in males. But, the male loses most of the down feathers of the ventral apterium. Also, according to Bailey , the male Warbling Vireo that sits on the eggs lacks a brood patch.

Bailey suggests that male passerines lacking brood patches that habitually sit on eggs do not heat the eggs. Thus it cannot be considered true incubation since no increase of temperature in the eggs is effected by such means. He further notes that it is at night when eggs are likely to experience a drop in temperature that embryonic development will be impaired. I have no data directly pertaining to which sex sits at night, but it is presumably the female, because she is always seen on the nest early in the morning and late in the evening.

If a highly-vascularized brood patch is essential for true incubation, then it is surprising that males take regular turns on the nest in cold, rainy weather. On May 20, 1960, male 3 sat on the eggs longer than did the female . The temperature during this hour and a half of incubation was 54? F. One solution to this problem is supplied by Skutch . He indicates that, "the type of the incubation is determined largely by innate factors, so that it persists through fairly wide fluctuations in weather, although it may break down in extreme conditions." Obviously then, in the example described above, the weather conditions do not qualify as "extreme." Sitting by the male is certainly functional to some extent for it relieves the female to forage; furthermore, the eggs are sheltered from inclement weather and protected from predators. Nolan suggests similar reasons for incubating by the male and adds the "conservation of heat supplied to the eggs by the female."

My data, based on incubation beginning with the second egg, indicate that the female incubates more often daily than the male . The male sits on the eggs only occasionally in the morning, but almost as often as the female in the afternoon. Nolan found that 95.5 per cent of the male's time on the nest and only 40 per cent of the female's time were attributable to the early hours of the day. Although I lack data on the critical hours of 5:00 a.m. to 6:59 a.m., I have enough observations from 7:00 a.m. to 9:00 a.m. to indicate that the males sit on the eggs infrequently in those hours. The discrepancy in the two sets of data, which may be merely an artifact of sampling techniques, does suggest two possible alternatives: the male sits on the eggs in the morning and gives the female, who sits on the eggs throughout the night, an extended rest and an opportunity to forage; the female continues to sit throughout the morning, especially during the early hours of daylight, a time of day when the temperature may still be low enough to impair development of the embryo.

NESTLING PERIOD

As indicated earlier, hatching normally occurs fourteen days after the second egg is laid. Hatching of the young was staggered at three nests under observation. In nest 2-b the first young hatched on June 8, 1959, the second on June 10. In 3-b one young hatched each day from the 12th through the 14th of June. In 5-a two young hatched on June 15, the third on June 16, and the fourth on June 17. Size of the young differed notably for about three days as a result of staggered hatching, but after that day the younger birds tended to catch up in size with their older brood-mates. The fourth young in nest 5-a grew steadily weaker and was missing from the nest on June 23, 1959. Staggered hatching is usually thought to be related to the availability of food that will insure survival of at least some of the nestlings when a shortage of food exists. It is doubtful that staggered hatching has adaptive significance in the Bell Vireo, since there seems to be no shortage of food for the young. In small passerines such as the Bell Vireo the principal problem is to insure fledging as quickly as possible because of the danger from predators.

Young are pinkish at hatching and devoid of visible natal down. Du Bois , inspected day-old nestlings by means of a magnifying glass and was unable to detect any down. Nolan also indicates that the young are naked at birth and that the "body color is between flesh and rufous except where folds of the straw yellow skin obscure the underlying colors." The Hutton Vireo is essentially naked at birth, save for sparse hairlike down on the head and back . The Red-eyed Vireo, according to Lawrence is naked at birth save for a sparse covering of greyish natal down, on the head, shoulders, and back.

In the Bell Vireo the pterylae darken slightly on the second day and the color becomes more intense daily until the quills of the dorsal tracts, the wings, and the tail break from their sheaths on the sixth day. In Red-eyed Vireos the pterylae darken by the end of the first day and the quills break through the skin on the fifth day, erupting from the sheaths by the seventh day .

TABLE 9. MATURATION OF NESTLING BELL VIREOS. THE FIRST DAY THAT AN ACTIVITY WAS OBSERVED IS SHOWN.

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