During the summers of , we banded Brown-headed Cowbirds ( Molothrus ater) in La Plata County, Colorado. Of 11 birds banded during their hatch. breeding seasons, brood parasitism, brown-headed cowbirds, cost of parasitism, cowbird egg than in die Old World where songbirds are par- Should there be a positive or negative correlation . cess of song memes in indigo buntings. Jun 1, Brown-headed cowbirds are not what one would call nice. The black birds with brown heads are known as brood parasites for their habit of.
The foster parents then unknowingly raise the young cowbirds, usually at the expense of their own offspring.
Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. Cowbird nestlings also grow large very quickly. These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species. It is unknown whether they developed their breeding strategy because they had to move frequently to keep up with the bison herds, or whether they were able to follow the herds because their breeding strategy gave them the freedom to do so.
Expansion of agricultural areas and removal of forest cover have greatly benefited this species by providing more overall habitat and by giving cowbirds access to new host species that have not developed defensive strategies against nest parasitism. While it is clear that cowbirds have benefited from forest fragmentation, their role in population-level declines of many forest birds is less certain. A Compound Problem The cowbird does not depend exclusively on a single host species; it has been known to parasitize over different species of North American birds and therefore spreads its impact across many populations.
Because cowbirds are native to the U. However, unpermitted control of cowbirds is occasionally permissible under special circumstances outlined in the Migratory Bird Treaty Act. Some species, such as the Yellow Warbler, can recognize cowbird eggs and will reject them or build a new nest on top of them. Those species which accept cowbird eggs either do not notice the new eggs, or as new evidence suggests, accept them as a defense against total nest destruction.
Use feeders that are made for smaller birds, such as tube feeders that have short perches, smaller ports, and no catch basin on the bottom. Avoid platform trays, and do not spread food on the ground.
General Bird & Nest Info
To explain observed vocalizations at vireo nests, we compared seven models in each of three analyses including a null model, a model considering only nest stage, time of day, and their interaction, and additional analysis-specific models that considered either parasitism fate, depredation fate, or cowbird density. We ranked candidate models defined a priori using AICc AIC adjusted for small sample sizes; Akaike and assigned a relative probability to each model Burnham and Anderson After defining our best model for each analysis, we conducted post hoc tests of factor interactions to determine the relationship between nest stage, time of day, and measures of nest fate and cowbird density.
All analyses were conducted in R v. RESULTS We recorded vocalizations at 52 nests during the and seasons, including 23 nests recorded during building, 19 during laying, and 10 during early incubation Table 2. Four parasitized nests After comparing seven models relating vocalization rate to combinations of nest stage, time of day, and depredation fate, the best supported model included all main effects and two-way interactions Tables 3 and 4 ; there were no other competitive models Table 3.
Parasitism fate Near nest vocalizations by male vireos were associated with risk of nest parasitism within specific nest stages and periods of the day Fig. The best model of vocalization rate considering parasitism fate included all three main effects nest stage, time of day, and parasitism fate and all two-way interactions, and no other models were competitive Tables 5 and 6.
During the laying stage, however, male vireos were quieter in areas with relatively high cowbird densities laying: The best and only competitive model of near-nest vocalizations considering cowbird density included all three main effects nest stage, time of day, and level of cowbird density as well as all two-way interactions Tables 7 and 8. Importance of nest stage and time of day Nest stage, time of day, and their interaction were important in all analyses described above and better explained patterns in vireo vocalizations than either depredation fate, parasitism fate, or cowbird density alone Tables 3, 5, and 7.
In the midday, male song rate at nests was greatest in the incubation and building stages and relatively low during laying Table 2. During building and incubation, vocalization rates were lowest in the evening and highest in the midday. Song rates in the morning were moderate but significantly greater than in the evening and less than at midday. During the laying stage, near-nest vocalizations were most frequent in the morning.
For the endangered Black-capped Vireo, we found only limited, stage-specific evidence for both the predator-attraction and parasite-assessment hypotheses across landscapes that vary in parasitism risk. Thus, we suggest that neither hypothesis adequately captures the selection pressures that shape vireo vocalization patterns. Black-capped Vireos face a cruel bind in navigating the risks of predation and brood parasitism, a bind that they solve, in part, through temporal plasticity in vocalization frequency near the nest.
Predator-attraction hypothesis Patterns in Black-capped Vireo vocalizations near their nests do not support the predictions outlined by the predator-attraction hypothesis. Across all recordings, the relationships observed between song rate and depredation and parasitism fate were in contrast to the expectations of this hypothesis; namely, nests with more vocal males were less likely to be depredated, and there was no relationship between parasitism and song rate.
Additionally, although we found some temporal variation between vireo defense strategies, patterns relating vocalization frequency and depredation or parasitism fate were generally consistent, suggesting that vireos experience minimal trade-offs in mediating parasitism or predation risk through vocalizations.
We suspect that, during the early stages of the nesting cycle, selection for an optimal frequency of vocalizations at the nest might focus most on minimizing the threat of cowbird parasitism.
Although the predator-attraction hypothesis assumes that important nest predators cue in to parental behaviors such as vocalizations, this may not be true for vireos, particularly during the early nesting stages.
Cowbirds are most likely to depredate nests that are located late in the nesting cycle Conkling et al. Thus, had we observed vocalization behavior at vireo nests during nestling development, we might have garnered more support for the predator-attraction hypothesis. Parasite-assessment hypothesis In central Texas, the frequency of Black-capped Vireo vocalizations is strongly correlated with local cowbird density, although generally not in the direction predicted by the parasite-assessment hypothesis.
During the building and incubation stages, vireo vocalizations near the nest were most frequent at sites with higher cowbird densities. Thus, although cowbirds may be attracted to singing vireos, it does not appear that vireos adjust song to counter this threat. During the laying stage, however, vocalization rates were highest in areas where cowbird density is low, perhaps providing some stage-specific evidence for the parasite-assessment hypothesis.
Cowbirds typically parasitize nests near the end of the laying stage Daviesand vireos may vocalize less frequently in response to increases in perceived parasitism risk during this nest stage Forsman and Martin Alternatively, high cowbird densities may induce nest vigilance by vireos during the building or early incubation stages, although the mechanism for this phenomenon is not obvious. Temporally specific behavioral adaptations by vireos may alleviate some conflicts in nest defense strategy and ultimately optimize nest success.
Cowbirds often locate nests while hosts are building and then return to parasitize the nest during the host laying period DaviesBanks and Martin Vireos may minimize their risk of eventual nest parasitism by remaining quiet during the building stage to prevent cowbirds from finding the nest location.
Nests that are discovered by cowbirds may be abandoned before laying occurs because vireos are less invested in the particular nest location.
Once laying begins, however, vireos are more invested in their nest and benefit by remaining vigilant to ward off potential brood parasites.
Previous studies found that vireo nests suffer higher rates of predation during the nestling stage compared with incubation Stake and CimprichConkling et al. We assumed that nest predation rates would be highest during times of peak predator activity and, in general, avian predators of vireo nests on Fort Hood, including cowbirds, act diurnally Stake and Cimprich Stake and Cavanagh observed that cowbirds depredating vireo nests during the nestling stage most frequently acted during the midday between Scrub-jays and other corvids are generally active throughout the day, although the specific timing of predation events is unreported Luginbuhl et al.
Thus, vireos may minimize depredation threat by being relatively quiet during the middle of the day when avian predators are most active. Although patterns demonstrate that, in general, parasitism and predation risks do not require conflicting defense strategies during the early nesting stages, some differences in risk mediation suggest that, in cases of conflict, vireos minimize loss by optimizing behavior to defend against nest predation. During the building stage, for example, vigilant and more vocal vireos reduced their susceptibility to eventual nest depredation while increasing their risk of parasitism, even in the face of high cowbird densities.
In the evenings, vireos that remained vigilant did not increase their risk of nest parasitism but were able to reduce their probability of nest depredation. Although vireos demonstrate some flexibility in the timing of vocalizations, selection pressures for pair bonding and territory defense may limit plasticity in social communication.
In areas with high cowbird densities, vireos became more vigilant during the building stage, when cowbirds typically locate nests Banks and Martinand more secretive when they were laying, when brood parasitism generally occurs Davies These were not, however, effective defense strategies against parasitism; birds that vocalized more during building and less during the laying stage were also more often parasitized.
How cowbirds take advantage of other birds
Additionally, these patterns may amplify one another; if cowbirds are able to follow aural cues and locate a nest during the building stage DaviesBanks and Martinvireo reticence during laying may inadvertently aid cowbirds by allowing them more uninterrupted access to the nest.
Although vigilance during building may reflect a trade-off with predation risk, we did not observe evidence for such a trade-off during the laying stage. During laying, frequent vocalizations near the nest increased the likelihood of both parasitism and depredation.
Instead, vireo territory density and trade-offs with territory defense may help explain these apparent conflicts. High host density may favor selection for territory defense and increased vocalization rates. Indeed, previous studies have positively correlated vireo densities with parasitism rates Barber and Martin Thus, at sites with high cowbird densities, vocalization rates may reflect selection for territory defense rather than parasitism risk.
Alternatively, at high cowbird densities, building vireos may be more likely to become incidentally discovered by a cowbird, regardless of vocalization rate. Under those circumstances, mate communication may outweigh reticence as an effective defense against parasitism. In general, the apparent limits in Black-capped Vireo vocalization flexibility may in part explain why cowbird parasitism is so detrimental to this species and, without adaptation, vireo populations may remain reliant on intensive management programs that include cowbird control.
This contrast between two very similar species may suggest that, in some cases, parasitism is affected by behaviors not captured well by audio recordings, for example, early initiation of egg laying Boves et al. Alternatively, trade-offs between behavior and predation risk may differ across habitat types and predator guilds e.
Vocalization behavior and parasitism risk have an interactive relationship that is made increasing complex by the additional consideration of nest predation. Although, in many species, more vocal individuals suffer higher rates of brood parasitism Uyehara and NarinsClotfelterBanks and Martinadaptation of vocalization behaviors that minimize parasitism risk may be outweighed by selection for behaviors that enable individuals to maintain important conspecific bonds or minimize risk of nest predation.
Species that do not have nest predators cued by auditory signals, however, may be able to more readily adapt vocalization patterns that minimize parasitism risk. Without the complications of additional trade-offs with predation, some species may develop more temporal flexibility and avoid vocalizing during periods of the day or stages of the nesting cycle when they are most vulnerable to brood parasitism. If accepted for publication, your response will be hyperlinked to the article.
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