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 (494 minutes) 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 delay by
the female in beginning to build is puzzling; because all evidence
indicates that she helps select the nest-site, I would expect her to
help with the initial building. There seems to be no clear advantage
in her delay in beginning to build.) 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 (1960), 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 (1960), in constructing nest
1-e (1960). 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.
_Gathering of Nesting Material_
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.
_Length and Hours of Nestbuilding_
As indicated by Nolan (1960:230), 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 (Nice, 1929:16; Pitelka and Koestner, 1942:99;
Hensley, 1950:242; Nolan, 1960:230).
One instance of protracted building was recorded. Nest 6-d (1960) was
begun on May 29, 1960, and not completed until nine days later on June
6, 1960. In contrast nest 1-g (1960) 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.
_Abortive Nestbuilding Efforts_
Eight of 38 nests started in 1960 were never completed (Table 6). 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 (1960), 2-c (1960) and 6-e (1960) 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_
Renesting after desertion or successful fledging occurs within two to
thirty-six hours. Young were fledged from 1-a (1959) on June 19, 1959,
and nest 1-b (1959) 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.
_The Nest_
Several authors have described various aspects of the nest of the Bell
Vireo, notably Goss (1891:535); Simmons (_in_ Bent, 1950:256), Nice
(1929:13) and Nolan (1960:230-231). 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 (1960) had
been packed with small pieces of soil bearing moss prior to lining.
TABLE 6. ABORTIVE NESTING ATTEMPTS IN MAY AND JUNE OF 1960.
==================================================
Nest | Length | Cause of abandonment
| of time |
| worked on |
-----+-----------+--------------------------------
1-a | 1 day | Heavy rain
1-h | 2 days | Female failed to build
2-a | 1/2 day | Female failed to build
2-c | 1 day | Protracted territorial dispute
4-a | 1 day | Female failed to build
5-a | 1 day | Female failed to build
6-c | 1 day | Heavy rain
7-a | 2 days | Female failed to build
-----+-----------+--------------------------------
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
(Table 7). 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 (1960) AND JUNE (1960).
========================================================
Measurements | May (N 10) | June (N 8)
------------------------+---------------+---------------
External depth | 61.6 mm. | 59.3 mm.
Depth of cup | 45.5 mm. | 46.3 mm.
Outside diameter | 57.3/55.5 mm. | 54.3/53.5 mm.
Inside diameter | 43.4/42.2 mm. | 45.5/43.9 mm.
Thickness of forward | |
wall 1 inch below rim | 13.8 mm. | 7.6 mm.
Thickness of bottom | 11.3 mm. | 4.6 mm.
------------------------+---------------+---------------
EGGLAYING AND INCUBATION
_Egglaying_
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 (_eh-eH-EH_) 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 (1960:232). 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.
_Clutch-size_
The average clutch-size of the Bell Vireo in Kansas, based on
thirty-three records, is 3.39 eggs (Table 8). Seasonally, the largest
average clutches are produced in the middle of the breeding season,
that is, in June. Lack (1947:308-309) 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 (NUMBER OF RECORDS
IN PARENTHESES)[F].
========================================================
| | | | Mean
Year | May | June | July | annual
| | | | clutch-size
----------+---------+----------+---------+--------------
1959 | 3.0 (7) | 3.2 (12) | 3.0 (1) | 3.06
1960 | 3.3 (6) | 3.83 (5) | 4.0 (2) | 3.72
----------+---------+----------+---------+--------------
1959-1960 | 3.17 | 3.52 | 3.5 | 3.39
----------+---------+----------+---------+--------------
[F] 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.
_Incubation_
Nice (1954:173) 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 (1929:99) 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
(1942:99) 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 (1960:234) marked hatched in approximately
fourteen days. Eight eggs artificially incubated by Graber (1955:103)
required an average of 15.01 days to hatch. As Van Tyne and Berger
(1959:293) 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.
_The Roles of the Sexes in Incubation_
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 (1952:128), the male Warbling Vireo that sits on
the eggs lacks a brood patch.
Bailey (1952:128) 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.
[Illustration: FIG. 4. Comparison of periods of incubation
by both sexes in cold (54 deg. F.) rainy weather (A) and in warm
(82 deg. F.) sunny weather (B).]
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 (1960) sat on the eggs
longer than did the female (fig. 4). The temperature during this hour
and a half of incubation was 54 deg. F. One solution to this problem is
supplied by Skutch (1957:74). 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 (1960:232)
suggests similar reasons for incubating by the male and adds the
"conservation of heat supplied to the eggs by the female."
[Illustration: FIG. 5. Daily participation in incubation as
indicated by the sex of the adult on the nest upon approach of
the observer.]
My data, based on incubation beginning with the second egg, indicate
that the female incubates more often daily than the male (fig. 5). The
male sits on the eggs only occasionally in the morning, but almost as
often as the female in the afternoon. Nolan (1960:233) 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 (20) from 7:00 a.m. to 9:00 a.m. to
indicate that the males sit on the eggs infrequently (3 of 20
instances) 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: (1) 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; (2) 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.
_Relief of Partners in Incubation_
Relief of partners involves some ceremony. When the female is
incubating, the male sings several times as he approaches the nest
tree; the female responds with several _chees_, but otherwise remains
immobile. The male sings several more times upon alighting in the nest
tree whereupon the female _chees_ again and flies directly from the
nest. A few seconds later the male appears at the edge of the nest
and, after inspecting the eggs, hops in and settles upon them. When
the male is sitting he is notably anxious prior to an exchange with
the female, often arising and craning his neck as he surveys the
surrounding vegetation, seemingly searching for his mate. The singing
of the male and the calling of the female serve as signals,
coordinating the exchange.
NESTLING PERIOD
_Hatching Sequence_
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 (1959) the first young hatched on June
8, 1959, the second on June 10. In 3-b (1959) one young hatched each
day from the 12th through the 14th of June. In 5-a (1959) 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 (1959) 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.
_Development of the Nestlings_
Young are pinkish at hatching and devoid of visible natal down. Du
Bois (_in_ Wetherbee, 1957:380), inspected day-old nestlings by means
of a magnifying glass and was unable to detect any down. Nolan
(1960:236) 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
(_Vireo huttoni_) is essentially naked at birth, save for sparse
hairlike down on the head and back (Wetherbee, 1953:380). The Red-eyed
Vireo, according to Lawrence (1953:67) 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 (Lawrence, 1953:67).
From the first day the young are able to squeak. Poking a young bird
was sufficient to elicit this sound, phonetically a nasal _peek_. The
only other vocalization noted throughout the nestling period was an
abbreviated _chee_.
For the first three days tapping the nest or even movement of it
caused by wind would elicit begging. By the fifth day at nest 2-a
(1959) only vigorous agitation of the branch to which the nest was
attached evoked any response. At this nest on June 16, 1959, one young
begged while the other cowered. Cowering is correlated with opening of
the eyes, as the young bird that begged had its eyes only partly open.
Both young cowered on June 19, 1959. Table 9 summarizes the maturation
of the nestling Bell Vireos.
TABLE 9. MATURATION OF NESTLING BELL VIREOS. THE FIRST DAY
THAT AN ACTIVITY WAS OBSERVED IS SHOWN.
==================================================================
| Day of nestling life
+---+---+---+---+---+---+---+---+---+----+----
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
--------------------+---+---+---+---+---+---+---+---+---+----+----
| | | | | | | | | | |
Eyes open | | | | x | | | | | | |
Feathers erupt | | | | | x | | | | | |
Sound: Squeak | x | | | | | | | | | |
_Chee_ | | | | x | | | | | | |
Begging | x | | | | | | | | | |
Cowering | | | | | | | | x | | |
Head scratching and | | | | | | | | | | |
Preening | | | | | | | | | x | |
Hopping to rim of | | | | | | | | | | |
nest | | | | | | | | | x | |
Fledging | | | | | | | | | | |x[G]
--------------------+---+---+---+---+---+---+---+---+---+----+----
[G] This is the commonest fledging day.
_Parental Behavior_
No eggshells were found in nests on the days of hatching. Presumably
they had been removed by the parents. Nolan (1960:234) indicates
immediate disposition of the eggshell after hatching. Lawrence
(1953:62) suggests that conspicuous removal of eggshells by the female
Red-eyed Vireo informs the male that the young have hatched.
Both sexes brood and the exchange of partners resembles that described
for the incubation period. Decrease in brooding in the daytime begins
about the sixth day of nestling life. Nolan (1960:235) reports a sharp
decrease in brooding when the oldest nestlings are seven days old.
Brooding decreases notably on the sixth day of nestling life in the
Red-eyed Vireo (Lawrence, 1953:62). Nice (1929:17), Hensley
(1950:244), and Nolan (1960:235) report that the female Bell Vireo
assumes a slightly greater role in brooding than the male.