|
|
Issue 1, December 1998
Comparison of Nest Defense Between Male and Female Blue-Headed
Vireos
Elise Donnelly
Biology, College of William an Mary
Abstract
This study was conducted to determine which sex of the blue-headed
vireo(Vireo solitarius) shows greater nest defense during incubation
and nesting stages. Since both sexes in this species share parental
duties throughout the breeding cycle, both would be expected to face
equal costs of nest predation and benefits of nest defense. In order
to compare nest defense of males and females at early and late stages
of nesting, a stuffed, mounted blue jay was presented at nests in
which there were either eggs or nestlings. Latency to arrive, scold,
and dive, as well as number of dives, were compared for each sex at
each nesting stage. There was no significant difference between the
behavior of males and females with a Fisher exact test or analysis
of variance. A power analysis revealed that these tests had relatively
low power, ranging from 24 to 60 percent, to detect a difference had
one existed. In this study population, the sexes perform equivalently
in defense. More specifically, because the male and female blue-headed
vireo make similar investments, and therefore stand to gain equally,
they take equal risks in protecting their nest. These results support
the idea that evolutionary costs and benefits shape the behavior of
individuals.
Introduction
The objective of this study was to determine which sex of the blue-headed
vireo, a species in which parents play almost equal roles in care
of the young, is more defensive against potential predators during
incubation and nestling stages.
In order to successfully complete the many steps in the avian breeding
cycle and to raise healthy young, parents may have to protect their
nest from predators. The degree to which each parent defends the
nest is influenced by several factors, such as confidence of paternity,
relative parental investment, and re-nesting potential (Montgomerie
and Weatherhead 1988). Each parent has an overall reproductive investment
that involves not only gametes, but also its roles in raising hte
young, such as feeding and defense against predators (Trivers 1972).
Males and females each contribute gametes, with the female's egg
being more costly than the male's sperm. Depending on the species,
each parent will then contribute some amount of parental care. When
faced with a nest predator, an individual acts to maximize the benefits
of its current investment (Boucher 1977) without unduly compromising
its opportunities for future reproduction. If a bird has confidence
in its paternity and has made high investment, it will be more likely
to take higher risks in the defense of its nest.
Many
avian studies of nest defense have involved the presentation of a
predator model at nests. Those conducted with monogamous, biparental
bird species have found differences in defense intensities between
males and females. The dark-eyed junco male, which only helps by feeding
the young once they hatch, protects a nest with young to a lesser
degree than does the female (Cawthorn et al. 1997). A study
of willow tits, in which males play the same role as that in juncos,
found that males show less intense defense during incubation (Rytkonen
et al. 1993). Male willow tits are more protective than females
during the nestling stage, suggesting that relative defense levels
of each sex may vary with the stage of nesting. Both of these studies
involved bird species in which females invest more time and energy
in raising young than do males. The
blue-headed vireo (Vireo solitarius) is an insect-eating
passerine that breeds between April and August in the eastern United
States. These birds are socially and reproductively monogamous,
one male spending the breeding season with one female (Morton et
al., unpub. data). The pair cooperates to construct a pensile
nest, situated in the fork of two thin branches, where the female
lays from three to five eggs. She then begins incubation, a stage
which lasts approximately 12 days. The male and female alternate
incubation bouts throughout the day. Van Roo and coworkers have
found, through two summers of study, that the male and female diurnal
incubation bouts are roughly equal in duration. As expected in songbirds,
females spend nights on the nest. Once the eggs hatch, the male
and female brood for up to six days and then leave the nest, staying
close to guard and returning frequently to feed the nestlings. The
young fledge after 12 to 14 days, and each parent cares for one
or two of the fledglings, feeding them for two weeks and remaining
with them until autumn migration (Ross 1973).
The
blue-headed vireo presents an interesting case for the comparison
of nest defense between the two sexes. A parent that is less likely
to be the genetic parent of the young in the nest, usually the male,
may show less parental behavior than the female, which has a greater
probability of being the genetic parent (Westneat 1996). Morton
(unpub. data) found extremely low rates of extra-pair fertilizations,
EPFs, in blue-headed vireos. Evolutionarily, extensive parental
care by males could be favored, because they would benefit as much
as females in caring for young in the nest, their current reproductive
efforts. Re-nesting potential, on the other hand, relates to the
likelihood of successful future reproduction. A parent with higher
investment through its greater role in parental care, such as the
female junco, has lower re-nesting potential and therefore would
show greater parental care of its brood (Anderson 1994). In blue-headed
vireos, both sexes share parental duties. With equal confidence
in genetic parenthood and roughly equal opportunities for re-nesting
throughout the breeding cycle, males and females may face similar
costs and benefits with regard to offspring survival. Nest defense
should be equivalent for both sexes.
This
study compared nest defense of male and female blue-headed vireos
with predator presentations performed during two stages of the breeding
cycle to discern whether parents with roughly uniform parental duties
would show active defense with equal frequency and to an equal extent.
Methods
This
study was conducted at Mountain Lake Biological Station in Southwest,
Virginia, between June 15 and August 1, 1998. Trials were conducted
between 1400 and 1700 on days on which it was not raining.
A stuffed mount of the blue jay (Cyanocitta crystata), a
common nest predator of the blue-headed vireos, was presented at
eight nests in the incubation stage, once when there was a female
incubator and once more when the male was incubating. Nine nests
with nestlings, in each case where both parents were present close
to the nest, were also tested. Since a comparison of defense intensity
between the sexes and not a test of differences between stages was
performed, these same nests were used for both the incubation and
nestling trials except when they failed due to predation. At least
one parent from each nest was attracted using audio tape of conspecific
song, caught in a mist net, and banded with plastic color bands
and an aluminum silver band at some point prior to a trial. These
bands allowed for sex identification during predator presentations.
For
nests in the incubation stage, a mounted blue jay specimen was placed
2.5 m away from, and up to 0.5 m below the nest. The jay was harnessed
to a rope which ran the length of a 3.25-m aluminum pole and was
positioned horizontally, with folded wings, facing the nest. The
pole was inserted into the ground for presentations to nests no
higher than 5 m and elevated and secured to a tree trunk for higher
nests. The jay was covered with a piece of camouflaged cloth to
which a thin black twine was attached. An amplifier was placed under
the mount and was connected by a 5-m cord to a tape player with
a one-minute loop tape recording of a jay call.
The
researcher was concealed by the use of a green poncho and maintained
a position at least 3.25 m from the jay and 6.5 m from the nest.
Both target objects were highly visible from this position. A five
minute waiting period was designated prior to the removal of the
camouflaged cloth from the jay mount. A recording of a 15-second
jay call was then started and repeated each minute. Eight minutes
were allowed for a response and, once a bird began diving, the attack
was observed for two minutes. The recorded data included: 1)
latency to arrive or leave the nest, 2) latency to scold, 3) latency
to dive, and 4) number of dives. A dive was measured as any break
in horizontal flight directed at the predator mount. "No response"
was recorded if the bird did not scold.
The
same procedures were followed for nests with nestlings except that
both parents were always present and, therefore, only one trial
was obtained for each nest during this stage. Latency to arrive,
scold, dive, and number of dives were recorded for male and female
vireos.
With
a Fisher Exact test, a comparison of the presence or absence of
a response between the sexes for both incubation and nestling stages
was made. A Fisher Exact test was also used to determine if the
male and female of a given pair act independently of each other
in their likelihood to give an active response. To analyze the behavior
of those birds giving an active response, an analysis of variance
was performed. A two-way ANOVA was used to compare the latency to
scold and number of dives, after a square-root transformation, of
males and females in the incubation stage. In addition, a one-way
ANOVA was used in the nestling stage. Finally, the power of each
statistical test used in the data analysis was computed.
Results
Qualitative
Response
Using
a Fisher Exact test, it was established that one sex did not respond
more often than the other during incubation (Table 1: n=16, p=1.00
with female incubating; Table 2: n=16, p=.315 with male incubating)
or nestling stages (Table 3: n=18, p=.576). There was also evidence
that the sexes do not act independently of each other during a trial
(Table 4: n=13, p=.07).
Quantitative Response
A
two-way ANOVA on latency to scold for birds in the incubation stage
showed no significant difference due to sex (n=14, F=.04, p=0.85),
role (incubating or not) (n=16, F=0.33, p=0.58), or the interaction
between sex and role (n=14, F=0.01, p=0.91). A two-way ANOVA with
a square-root transformation examining the number of dives also
found no significant difference due to sex (n=14, F=0.49, p=0.50),
role (n=14, F=0.35, p=0.57), or the interaction between the two
(n=14, F=0.01, p=0.93).
A
one-way ANOVA on latency to scold for birds in the nestling stage
showed no significant difference due to sex (n=14, F=0.08, p=0.78).
An ANOVA on number of dives with a square-root transformation also
supported this lack of differentiation between the sexes (n=14,
F=0.02, p=0.88). Table 5 gives the means for latency to scold and
number of dives.
A
power analysis for each statistical test was performed. With a significance
criterion of a=0.05 and a large effect size, the probability that
a great difference could be detected was determined to be 60 for
all Fisher exact tests, 24 for the two-way ANOVAs performed on incubation
data, and 28 for the one-way ANOVAs done for nestling data. A large
effect size was chosen for this analysis on the basis of other nest
defense studies which found a difference between the sexes.
Discussion
Male
and female blue-headed vireo have equal investments of time and energy
in raising their young, and therefore, it was predicted that both
sexes would respond equally to a potential nest predator. Nest
defense studies conducted with monogamous bird species in which the
female has greater investment through her roles in parental care have
found that this sex was more protective of the nest than its counterpart
(Cawthorn et al. 1997, Rytkonen et al. 1993). Because
female and male blue-headed vireos have the same probability of being
the genetic parent, each sex should benefit uniformly from caring
for its current brood and, due to equal re-nesting potential, have
equivalent chances of completing a future brood. With shared parental
duties and similar investments at any given time in the nesting cycle,
similar defense levels would be expected and were detected by this
study. Although
no significant difference between male and female defense responses
was detected, a power analysis showed that these tests have low
power due to the small sample size. A larger sample size, cumulated
over several years of study, would allow for more robust support
of these conclusions.
Both
sexes of the blue-headed vireo appear to act in concert, but with
two strikingly distinct strategies for nest "defense." Some pairs
acted aggressively by scolding loudly and continuously and diving
at the jay mount. At other nests, the parents remained quiet, failing
to announce their presence or attack the potential predator. It
would be interesting to compare the success of nests faced with
actual predators to determine if the type of response was correlated
with nesting success.
This
study suggests that the male and female blue-headed vireo act equivalently
in nest defense. With equal investment in parental care and confidence
in parentage, they would be expected to take similar risks in protecting
their nest. This study supports the concept that animals will behave
in a manner that reflects their relative reproductive costs and
benefits.
Acknowledgements
I
would like thank my wonderful mentor, Brandi Van Roo of Indiana University.
Many thanks to Henry Wilbur, Eric Nagy, Ellen Ketterson, Dan Cristol,
Andy Taylor, and Diane Neudorf for their help and advice. I appreciate
the use of Mountain Lake Biological Station and the financial support
of NSF/REU site award DBI-9732155 to MLBS. Also, I wish to thank the
whole junco work crew for keeping an eye out for suspiciously parental
vireos.
References
Anderson,
M., 1994. Sexual Selection. Princeton: Princeton, NY.
Boucher,
1977. "On wasting parental investment." American Naturalist
111:786-788.
Cawthorn,
J. M, D. Morris, E. Ketterson, and V. Nolan, Jr., 1997. "The influence
of experimentally elevated Testosterone on nest defense in dark-eyed
juncos." Journal of Comparative Endocrinology 108 (1): 141-151.
Cohen,
J., 1988. Statistical Power Analysis for the Behavioral Sciences
(2nd Ed.). Lawrence Erlbaum: Hillsdale, NJ.
Montgomerie,
R. and P. J. Weatherhead., 1988. "Risks and rewards of nests defense
by parent birds." The Quarterly Review of Biology 63 (2):
167. Morton,
E., B. Stutchbury, W. Piper, and R. Fleisher. Unpublished data.
Ross,
J., 1973. "Ethological and ecological relationships of yellow-throated
and solitary vireos (Aves: Vireonidae) in Ontario." Ph.D. dissertation
at The University of
Toronto.
Rytkonen.
S., M.Orell, and K. Koivula., 1993. "Sex-role reversal in willow
tit nest defense." Behavioral Ecology and Sociobiology 33: 275-282.
Trivers,
R.L., 1972. "Parental investment and sexual selection." In B. Campbell,
ed.,
Sexual
Selection and the Descent of Man, 1871-1971, pp. 136-179. Heinimann:
London.
Westneat,
D. F. and R. C. Sargent., 1996. "Sex and parenting: the effects
of sexual conflict and parentage on parental strategies." Trends
in Evolution and Ecology 11 (2): A87-A91.
Journal of Young
Investigators. 1998. Volume One.
Copyright © 1998 by Elise Donnelly and JYI. All rights reserved.
|
|