Use of the song repertoire in intersexual and intrasexual contexts by male red-winged blackbirds

Summary Live and stuffed male and female red-winged blackbirds (Agelaius phoeniceus) were presented to territorial males in order to study the repertoire organization of males during intersexual and intrasexual encounters. Territorial male red-winged blackbirds switched song types more frequently and sang more song types in response to presentation of a female stimulus than during control periods, but decreased their switching frequency when a male stimulus was presented. Switching frequency in response to the female stimulus was three times that in response to the male stimulus. These results support the hypothesis that song repertoires of male red-winged blackbirds evolved primarily in response to intersexual rather than intrasexual selection. They also suggest that male red-winged blackbirds have been selected to de-emphasize their song repertoires during encounters with conspecific males as a result of some as yet unidentified cost. Offprint requests to: W.A. Searcy     

Song repertoires and density assessment in red-winged blackbirds: further tests of the Beau Geste hypothesis

Summary The Beau Geste hypothesis proposes that song repertoires are advantageous in territory defense because nonterritorial males, who are prospecting for territories, use the number of song types they hear to assess the density of territorial males, and then avoid densely-settled areas. A territorial male can then inflate the apparent density of singers on his territory by singing several distinct song type. This hypothesis assumes (1) a positive correlation between the density of song types and the density of territorial males, and (2) a negative correlation between the rate of trespassing and the densities of both song types and territorial males. We studied the behavior of male red-winged blackbirds (Agelaius phoeniceus) and found support for the first, but not the second, assumption. Results showing a positive correlation between density of song types and density of territorial males show that the proposed method of density assessment is feasible in this species. However, we could find no support for the assumption that nonterritorial male red-winged blackbirds avoid densely settled areas. In contrast to the assumed negative correlation, rate of trespass per territory was not consistently correlated with male density, and trespass rate per unit area was positively correlated with male density. Further, these relationships were retained when we controlled statistically for the effects of territory quality. We conclude that prospecting male red-winged blackbirds to not attempt to avoid densely settled areas, and that, although they do avoid territories defended by many song types, they do not use song type density to assess the density of territorial males. It is thus unlikely that the Beau Geste hypothesis adequately accounts for the evolution of song repertoires in the red-winged blackbird.     

Male response to an aggressive visual signal, the wing wave display, in swamp sparrows

Signaling often involves complex suites of behaviors that incorporate different sensory modalities. Whatever modality is used to establish that a signal functions in communication researchers must demonstrate that receivers respond to it. The territory defense response of male swamp sparrows involves a variety of behaviors that includes both vocal and visual displays. One of these, the “wing wave” display, is a distinctive movement that predicts physical attack. Here, we use robotic taxidermic mounts paired with song to test the hypothesis that wing waving is a signal and, specifically, that male receivers respond to wing waving as a signal of aggressive intent. As predicted, subjects responded more aggressively to the mount during wing waving trials than during stationary trials. A second experiment demonstrated that this effect cannot be attributed simply to increased attention to movement. Less expectedly, subjects did not alter their own display behavior in response to wing waving as compared to a static mount. We conclude that the wing wave display in the context of singing is a signal that functions in male–male aggressive communication. Questions remain, including whether wing waving functions as a signal in the absence of singing and whether wing waving and song are redundant signals or communicate different information.     

Song-type sharing in song sparrows: implications for repertoire function and song learning

One hypothesis for the function of song repertoires is that males learn multiple song types so that they may share songs with neighbors, allowing them to match during territorial interactions. In at least one song sparrow population, in Washington, territorial males share a high proportion of song types with their neighbors and use these shared songs in matching. We recorded song sparrows in Pennsylvania and quantified sharing of whole songs and song segments. We found that song sharing is an order of magnitude less common in the Pennsylvania population. We found sharing of song segments to be significantly more common than the sharing of whole songs in three of the five fields we examined, while we found no significant differences between whole and partial song sharing in the remaining two fields. Finally, we found no evidence that sharing is greater between birds in the same field compared to birds in different fields. Taken with the data from Washington song sparrows, these results provide evidence for intraspecific geographic variation in the organization of song repertoires, and suggest that song sharing has not been a strong selective force in the evolution of song repertoires in song sparrows as a species. Furthermore, Washington and Pennsylvania song sparrows differ in how they learn song, in that Washington birds copy whole songs, while Pennsylvania birds appear to copy and recombine song segments, as has been found in laboratory studies of song learning. Thus both song learning and the function of song repertoires differ between populations of song sparrows. Such intraspecific geographic variation offers a unique opportunity to explore the ecological and historical factors which have influenced the evolution of song. Received: 30 June 1997 / Accepted after revision: 8 March 1998     

Aggression in female Red-Winged Blackbirds: A strategy to ensure male parental investment

Summary Female Red-winged Blackbirds (Agelaius phoeniceus) are often aggressive towards conspecific females during the breeding season. We hypothesize that the function of female-female aggression in this species is to guard the nonshareable portion of the male's parental investment.The investment-guarding hypothesis predicts that a female should be more aggressive toward another female evincing interest in mating with the territory-owning male than toward a female simply perching within the male's territory. Results of mount presentations to females with active nests confirmed this prediction. Nesting females attacked a stuffed conspecific female mounted in a precopulatory, soliciting posture significantly more often than a mount in a normal, perched posture.The male's nonshereable parental care consists of provisioning his young, and most of this care is invested in the brood of his primary (first-to-nest) female. It is therefore predicted that primary females should be more aggressive than secondary (later-nesting) females. Female mount presentations also confirmed this prediction. Primary females attacked the soliciting mount significantly more often than secondary females.     

Discrimination of vocal performance by male swamp sparrows

In aggressive communication, the interests of signalers and receivers are directly opposed, presenting a challenge to the maintenance of reliable signaling. Index signals, whose production is constrained by physical ability, offer one solution to the reliable signaling problem. Vocal performance, the ability to produce physically challenging songs, is likely such a signal in swamp sparrows. Maximum vocal performance varies between males and is correlated with aspects of quality. However, vocal performance can be modulated in aggressive contexts by increasing the frequency bandwidth and trill rate of songs. This study examines receiver response to (1) differences in performance of the same song types by different signalers and (2) individual modulation of performance between contexts. Results demonstrate that male receivers show differential response to between-male differences in song type performance, but do not show differential response to the smaller scale modulations of performance produced by individuals singing the same song type at different times. This pattern suggests that vocal performance cannot be effectively cheated and may therefore serve as a good example of an index signal.     

An experimental study of song type function in the bobolink (Dolichonyx oryzivorus)

Summary We tested the hypothesis that the alpha and beta songs of male bobolinks (Dolichonyx oryzivorus) have separate intrasexual and intersexual functions by presenting caged males and caged females on the territories of male bobolinks. Although all males in our study population share both alpha and beta songs, we found no evidence for a specialized intersexual function of alpha song or a specialized intrasexual function of beta song. Territorial males increased their singing rate of both alpha and beta songs during both the male presentations and the female presentations. The proportions of the two song types did not change between control and experimental periods for either male or female presentations. We found no evidence in support of specialized intrasexual and intersexual functions of note types. Only 10% of the vocalizations initiated by territorial males were complete alpha or beta songs, while 87% were song fragments, and the remaining 3 % were compound songs. We did find that territorial males responded to simulated male intruders by increasing the proportion of fragmented songs and decreasing the proportion of compound songs. Moreover, males included fewer notes per song initiated and sang a higher proportion of songs that included only introductory notes during male presentations. We conclude that the vocal response of territorial male bobolinks to conspecific male intruders is to sing short, simple sequences of either song type. We found no consistent changes in vocal behavior of territorial males in response to female presentations other than increases in song rates.     

Use of standardized methods to improve extinction-risk classification

Standardized classification methods based on quantifiable risk metrics are critical for evaluating extinction threats because they increase objectivity, consistency, and transparency of listing decisions. Yet, in the United States, neither federal nor state agencies use standardized methods for listing species for legal protection, which could put listing decisions at odds with the magnitude of the risk. We used a recently developed set of quantitative risk metrics for California herpetofauna as a case study to highlight discrepancies in listing decisions made without standardized methods. We also combined such quantitative metrics with classification tree analysis to attempt to increase the transparency of previous listing decisions by identifying the criteria that had inherently been given the most weight. Federally listed herpetofauna in California scored significantly higher on the risk-metric spectrum than those not federally listed, whereas state-listed species did not score any higher than species that were not state listed. Based on classification trees, state endemism was the most important predictor of listing status at the state level and distribution trend (decline in a species’ range size) and population trend (decline in a species’ abundance at localized sites) were the most important predictors at the federal level. Our results emphasize the need for governing bodies to adopt standardized methods for assessing conservation risk that are based on quantitative criteria. Such methods allow decision makers to identify criteria inherently given the most weight in determining listing status, thus increasing the transparency of previous listing decisions, and produce an unbiased comparison of conservation threat across all species to promote consistency, efficiency, and effectiveness of the listing process.