June 17, 1959, on the fifth day of the nestling period. The nest
contained three young. An adult flew to the nest; while standing on
its rim the bird dipped its head into the nest six times, afterward
appeared to be eating a fecal sac, than shifted position to the
unattached portion of the rim, gaped three times, thereupon spread its
wings, and sat motionless 35 minutes. In this attitude it formed an
effective shield sheltering the young from direct sunlight penetrating
the thin foliage of the honey locust in which the nest was situated.
The temperature at this time was 95 deg. F., but the sky was partly
cloudy. By 2:30 p.m. the sky had become overcast and the sun passed
behind a cloud. Although sunlight no longer fell directly upon the
nest, the bird remained in the shielding posture for another five
minutes before flying from its perch. Sun-shading was not observed at
either of the other nests containing young; dense overhead vegetation
protected those nests. Sun-shading has been noted in other species
where the nest was poorly protected from the sun. Lawrence (1953:62)
observed this behavior at two Red-eyed Vireo nests in conifers. The
"sun-shield" posture of the Bell Vireo does not correspond to any of
the sunning postures described by Hauser (1957).
_Feeding of the Nestlings_
Both sexes fed the young, and presumably began shortly after the first
nestling hatched. My data indicate that the female does more feeding
than the male (Table 10); in about eight hours of observation a total
of 67 morsels were brought, 43 by the female and 24 by the male, for
an average of once every 7.6 minutes. Nice (1929:17), however,
observed a male to bring food 53 times as compared to 21 visits by the
female. In five and one-half hours of watching, meals were brought
once every 4.9 minutes. Du Bois (_in_ Bent, 1950:257) recorded seven
trips in an hour and forty minutes, or one every fourteen minutes.
At three nests containing young the adults were sometimes silent and
sometimes vocal on their approach. The female often emitted a subdued
_chee_ which, coupled with the vibration of the nest caused by her
arrival, elicited begging behavior from the young. None of the males
was heard to utter such a call, but I have the impression that they
often did call although I failed to hear the sounds. The males did, on
occasion, sing several songs as they approached, even with food held
in their beaks. Such singing elicited begging from the nestlings. Once
the eyes of the young were open they often began begging when a silent
adult was within two or three feet of the nest; begging behavior
probably is elicited by tactile, auditory or visual stimuli in that
order, or, as the nestling period proceeds, by any combination of
these stimuli.
TABLE 10. FEEDING OF THE NESTLINGS.
====================================================
Day of | Length of | Adult involved
nestling period | observation +---------+---------
| | Male | Female
-----------------+--------------+---------+---------
1 | 30 min. | 3 | 5
2 | 60 min. | 1 | 4
3 | 60 min. | 2 | 5
4 | 30 min. | 1 | 4
7 | 60 min. | 4 | 7
2 | 60 min. | 3 | 3
6 | 60 min. | 3 | 6
7 | 30 min. | 3 | 3
9 | 60 min. | 4 | 6
+--------------+---------+---------
Totals | 510 min. | 24 | 43
-----------------+--------------+---------+---------
Not all trips made by parents resulted in successful feeding of young;
some visits seemed to be purely for inspecting the young. On other
occasions the adults experienced difficulty in transferring food to
the young, and, thus thwarted, would themselves eat the food. Nice
(1929:17) estimated that from five to twelve of a total of
seventy-five meals were eaten by adults.
_Nest Sanitation_
Both parents regularly removed fecal sacs from the nest, eating them
for the first five days and thereafter carrying them off and
presumably dropping them. It is doubtful that fecal sacs were actively
removed in the last two days of nestling life as the bottoms of nests
from which young flew away were invariably covered with excrement.
On several occasions a parent brought food to the nest and then
remained perched on the rim alternately peering into the nest and then
preening. Once bill swiping was observed and another time an adult
male sang once. The adult remained at the nest from twenty seconds to
a full minute.
_Fledging_
Eight young were fledged from the four nests in 1959. The nestling
period lasted from nine to twelve days. Human interference may have
been largely responsible for the fledging of the young at nine days.
Pitelka and Koestner (1942:100) found nestling life to last eleven
days. Nolan (1960:235) reports nestling periods varying from 10.5 to
12 days. The young Red-eyed Vireo is ready to leave the nest at ten
days but often remains an additional day before departing (Lawrence,
1953:68).
The oldest nestling at nest 2-a (1959) hopped out on June 17, 1959,
when I disturbed the parents. On this date the juvenal plumage was
only partly developed and the young bird was incapable of flight. By
the tenth day of nestling life the young in all the nests were
observed to hop to the rim, flutter their wings, hop back into the
nest and also to preen and scratch their heads. The young at fledging
are usually completely feathered, but have notably short tails and
relatively short, stubby wings. According to Ridgeway (1904:205) the
juvenal plumage is much like that of the adult.
_Nest Parasites_
Pitelka and Koestner (1942:103) found that incubating adults and later
the young suffered infestation of the northern fowl mite,
_Ornithonyseus sylviarum_. Nolan (1960:241) reports a heavy
infestation of this mite at four nests. Unidentified mites were noted
at four nests in my study area in 1959. Incubating adults were
observed to peck at their breasts and scapulars from the eleventh
through the fourteenth day of incubation. Serious infestations were
not noted at the nests until the ninth day of nestling life. At this
time the young were observed to scratch their heads and peck at their
breasts, scapulars, and the base of their tails. On the day of
fledging the nests were a seething mass of crawling mites; the mites
also extended well up the branches to which the nests were attached.
Nest 1-a (1959), which was discovered on June 18, 1959, presumably on
the day after fledging, was densely covered with mites. Some mites
were still crawling on this nest on June 20, 1959.
FLEDGLING LIFE
On June 20, 1959 I located one young 80 feet northeast of nest 2-a
(1959), about five hours after it had left the nest. One parent was
observed to feed it once. No young were seen thereafter from this or
any other nest. Extreme agitation on the part of one or both parents
on several occasions shortly thereafter, however, suggested the
proximity of the young. Search in the immediate vicinity on each of
these occasions proved fruitless. Three days after fledging their
young, pair 2 (1959) was primarily occupied with courtship activities.
Pair 1 (1959) was involved in courtship and nestbuilding one and
one-half days after the apparent fledging of their young. Nolan
(1960:238) indicates that the young remain within the territory and
perhaps are fed by the parents up until an age of about 40 days.
Sutton (1949:25) and Lawrence (1953:68) present contradictory reports
on fledgling-parent relationships in the Red-eyed Vireo. Sutton
concluded that the young quickly took leave of their parents whereas
Lawrence reported a young bird being fed 35 days after fledging.
_Second Broods_
The curve based on 66 nesting records of the Bell Vireo representing
the breeding activity in northeastern Kansas demonstrates a tendency
toward double-broodedness (fig. 6). The peak of the breeding season is
from May 20 to June 20. The large number (20) of replacement nests
built in late May of 1960 tends to distort the curve of the breeding
data; a second peak about 35 days after the first is evident.
I am of the opinion that the vast majority of vireos are
single-brooded solely by virtue of the limited success of early
nesting efforts, and that in "good" years most pairs would be
double-brooded. Each of the four pairs that successfully raised one
brood in my study area in 1959 renested within a day or two after the
fledging of the young. I do not know the fate of these nests. Nolan
(1960:237) reports at least one instance of a second brood in the
course of his study. Nolan (_op. cit._) notes that the literature, in
general, indicates that vireos are double-brooded, but that his
evidence, mentioned previously, is the only evidence based on banded
birds.
[Illustration: FIG. 6. Breeding season in northeastern Kansas
based on the number of completed clutches in each 10-day
period from May through July.]
REPRODUCTIVE SUCCESS
Only four nests were successful; all of these were observed in 1959.
The principal external factors responsible for nesting failure were
severe weather, predation, parasitism by Brown-headed Cowbirds
(_Molothrus ater_) and human interference (Table 11).
In late winter and early spring of 1960 heavy snow, continuously at a
depth of at least 10 inches, covered most of the Mid-west from
February 20 through March 20. Consequently, the growing season was
some two weeks behind that of 1959. Of all the species in the study
area, the Bell Vireo is the most dependent on dense foliage for cover
and concealment for its nests. Consequently the tardiness of the
season seemingly negatively influenced reproductive success of this
more than any other species of bird in the study area.
_Behavior_
Several aspects of the behavior of the Bell Vireo tend to contribute
to nesting failure. They include:
1. Nest-site. Nests are occasionally suspended from exposed branches.
Occurrences of this sort suggest that the dimensions of the fork are
more important in the choice of a site than availability of cover.
2. Song. The loud, continuous song of the male during nestbuilding
alerts cowbirds and predators to the presence of a nest. The
incongruous habits of the male of singing in the nest tree and while
sitting on the nest may facilitate location by some enemies,
particularly cowbirds.
TABLE 11. EGG MORTALITY IN BELL VIREOS.
======================================================
| | Eggs (N-29)| |
Mortality agents | N[H] | 1959 | N | 1960
| | Per cent | | Per cent
-----------------+------+------------+-----+----------
Predation | 4 | 13.8 | 5 | 10
Weather | 2 | 6.9 | 8 | 16
Cowbird | 14 | 48.3 | 37 | 74
+------+------------+-----+----------
Totals | 20 | 69[I] | 50 | 100
-----------------+------+------------+-----+----------
[H] Number of eggs out of the total number laid lost to
mortality agents.
[I] In 1959 nine eggs were successful (ultimately gave rise to
fledglings).
I am not fully convinced that song from the nest is simply a "foolish"
habit, since snakes, the principal predators with which this species
has to contend, are deaf. My own field observations and the
circumstances of the innumerable instances recorded in the literature
of male vireos singing from the nest suggest that this is a function
of the proximity of the observer. As mentioned elsewhere, vocal threat
is the initial as well as the primary means by which territory is
maintained. Song from the nest evoked by an enemy also serves to alert
the female to danger.
3. Flushing. The Bell Vireo normally relies upon cryptic behavior to
avoid detection at the nest. Most sitting birds, especially the
females, either flush silently when an enemy is about forty feet from
the nest or remain sitting upon the nest tenaciously, refusing to
flush even when touched or picked up. Some birds flushed at
intermediate distances of from three to fifteen feet. In so doing they
revealed the location of their nests. Since none of these
"intermediate flushers" enjoyed nesting success there is possibly some
correlation between these two factors.
_Predation_
Several complete clutches being incubated disappeared from nests that
were unharmed. Absence of eggshells in the vicinity suggests predation
by snakes.
On May 25, 1960, I found a _Peromyscus_ climbing toward nest 1-a
(1960). The mouse moved to within two inches of the nest whereupon I
removed the mouse. Such small rodents constitute another potential
source of predation.
_Cowbird Parasitism_
In this study the failure of 12 of 35 nests can be directly attributed
to cowbird interference. It is well established that the incidence of
cowbird parasitism of Bell Vireo nests is high (Friedmann, 1929:237;
Bent, 1950:260-261). Nolan (1960:240) found only one nest of eight
studied to be parasitized by cowbirds. He indicates that this is
surprising in view of the heavy molestation of the Prairie Warbler
(_Dendroica discolor_) in the same region. A possible explanation of
this phenomenon seems to lie in the much greater abundance of the
Prairie Warbler in comparison to that of the Bell Vireo. In my study
area the incidence of cowbird parasitism on Bell Vireos in 1959 and
1960 greatly exceeded that of all other nesting species that were
parasitized (Table 12).
As indicated previously, the female Bell Vireo leaves the nest
unoccupied several hours at a time in the transition period between
completion of the nest and the start of egglaying. Such behavior early
in the morning certainly would facilitate deposition of cowbird eggs.
Early in the nesting period the mere presence of a cowbird egg in the
nest prior to the laying of the host's first egg leads to abandonment
of the nest. This seems to be correlated with the relative strength of
the nesting tendency; anyhow cowbird eggs laid in later nests prior to
the appearance of the host's own eggs did not cause the nesting birds
to desert. The Bell Vireo does abandon the nest when all but one of
its own eggs have been removed by the cowbird. Mumford (1952:232)
records the removal of a cowbird egg by the host birds and I recorded
a similar instance involving nest 2-b (1960). On May 14, 1960, I
found one punctured cowbird egg on the ground about 10 feet west of
this nest. Occasionally a cowbird egg is buried beneath the lining of
a nest. Mumford (1952:23) observed this in mid-May in 1951 and I
observed pair 8 (1960) actively covering with building material a
cowbird egg on July 5, 1960. Covering a cowbird egg constitutes
effective removal. Since the egg cannot be turned, an adhesion
develops.
TABLE 12. INCIDENCE OF COWBIRD PARASITISM OF THE BELL VIREO
COMPARED WITH OTHER PASSERINES IN THE STUDY AREA IN 1959 AND
1960.
=========================================================
| Bell | Other
| Vireo |passerines
--------------------------------------+-------+----------
Total nests examined containing | |
at least one host egg | 35 | 43
Total nests parasitized | 24 | 14
Total number of cowbird eggs | 33 | 23
Per cent of nests parasitized | 68.6 | 32.6
Total number of cowbird eggs per nest | .94 | .54
--------------------------------------+-------+----------
The percentage of cowbird eggs hatched in relation to the number laid
is relatively low. For instance, Mumford (1952:231) has only one
record of a young cowbird successfully raised by a Bell Vireo. The
data available in Bent (1950:260-261) also indicate that the
percentage of cowbird eggs hatched is small. The Bell Vireo is less
tolerant of cowbird parasitism than are many of the species so
victimized, but is not so intolerant as the Robin, Catbird, and the
Yellow-breasted Chat (Friedmann, 1929:193).
SUMMARY
1. The behavior of a small population of Bell Vireos was studied in
the spring and summer of 1959 and again in 1960 in Douglas County,
Kansas, and results are compared with previous studies elsewhere.
2. The Bell Vireo sings more often daily and throughout the nesting
season than do the majority of its avian nesting associates. Six types
of vocalizations are readily distinguishable in the field: primary
song, courtship song, distress call, alarm note, specialized male call
note or _zip_, and the generalized call note or _chee_.
3. Territories are established in early May and occupied throughout
the breeding season and post-breeding season. The average size of the
territories in 1960 was 1.25 acres. Shifting of territorial boundaries
occasionally occurs after nesting attempts.
4. Territory is maintained primarily by song, but at least five
aggressive displays are manifest in the early phases of territorial
establishment. These include: (a) vocal threat, (b) head-forward
threat, (c) wing-flicking and sub-maximal tail-fanning, (d) ruffling
and maximum tail-fanning, and (e) supplanting attack.
5. The precise mechanism of pair-formation in the Bell Vireo is not
known. Early courtship activities are characteristically violent
affairs. Absence of sexual dimorphism suggests that behavioral
criteria are used by the birds in sex-recognition; the male is
dominant and the female is subordinate.
6. The principal displays associated with courtship include: greeting
ceremonies, "pouncing," "leap-flutter," pre- and post-copulatory
displays, and the posture, copulation. The marked similarity between
elements of courtship display and aggressive display suggests common
origin or the derivation of one from the other.
7. The nest-site probably is selected by the female. Nests are
suspended from lateral or terminal forks about 2 feet 3 inches high in
small trees and shrubs averaging 11 feet 2 inches in height.
8. Nestbuilding is intimately associated with courtship and is a
responsibility of both sexes. The male builds the suspension apparatus
and the female constructs and lines the bag. Both sexes participate in
adorning the exterior. Construction lasts from four and one-half to
five days.
9. The nest is compact, pendant, and composed of strips of bark and
strands of grasses that are interwoven and tightly bound with animal
silk. Nests built in May are bulkier than those constructed later in
the season.
10. Egglaying begins on the first or second day after the nest is
completed. The eggs are deposited early in the morning. The average
clutch-size of the Bell Vireo in Kansas is 3.39 eggs.
11. Both sexes sit on the eggs, but only the female truly incubates
because the male lacks a brood patch. Incubation lasts fourteen days.
12. The Bell Vireo is double-brooded in "good" years.
13. Nesting failure resulted from severe weather, predation,
parasitism by cowbirds, and human interference. Behavior that
contributes to nesting failure is selection of an unfavorable
nest-site, singing on and near the nest, and the tendency to flush
from the nest in view of potential enemies.
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1957. Some observations on sun-bathing in birds. Wilson Bull.,
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_Transmitted November 8, 1961._
29-1506
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