Comparison of Nest Defense Between Male and Female Blue-Headed Vireos
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.
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 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 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.
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.
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.
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.
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