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To investigate adaptations for long-range acoustic communication in birds, I analyzed associations between broad categories of habitats and properties of territorial songs for eastern North American oscines. From published recordings. I obtained three frequency properties (maximal, minimal, and dominant) and three temporal properties of songs (presence of sidebands, presence of buzzes, minimal period of repeated elements). Sidebands and buzzes indicated rapid amplitude modulation of a carrier frequency. Habitats occupied by territorial males were classified into six categories (broad-leaved or mixed forest, coniferous forest, parkland or forest edge, shrubland, grassland, and marshes). Frequencies in songs correlated strongly with body size, which varied among habitats. Analysis of covariance and phylogenetic regression, after controlling for body size, revealed an association of maximal but not dominant or minimal frequencies with habitat. In contrast, the temporal properties of song were all strongly associated with habitat, even within phylogenetic groupings. These results suggest that the temporal properties of songs of many oscines have evolved to reduce the effects of reverberation in forested habitats. Exceptional species might have retained features of song subject to degradation to permit listeners to judge distances to singers. In addition, adaptations for acoustic communication in different habitats might include differences in the perception of songs.

When Darwin first proposed the possibility of sexual selection, he identified two mechanisms, male competition for mates and female choice of mates. Extending this classification, we distinguish two forms of mate choice, direct and redirect. This distinction clarifies the relationship between Darwin's two mechanisms and, furthermore, indicates that the potential scope for sexual selection is much wider than thus far realized. Direct mate choice, the focus of most research on sexual selection in recent decades, requires discrimination between attributes of individuals of the opposite sex. Indirect mate choice includes all other behavior or morphology that restricts an individual's set of potential mates. Possibilities for redirect mate choice include advertisement of fertility or copulation, evasive behavior, aggregation or synchronization with other individuals of the same sex, and preferences for mating in particular locations. In each of these cases, redirect mate choice sets the conditions for competition among individuals of the opposite sex and increases the chances of mating with a successful competitor. Like direct mate choice, indirect mate choice produces assortative mating. As a consequence, the genetic correlation between alleles affecting indirect choice and those affecting success in competition for mates can produce self-accelerating evolution of these complementary features of the sexes. The broad possibilities for redirect mate choice indicate that sexual selection has more pervasive influences on the coevolution of male and female characteristics than previously realized.

Evidence for reproductive character displacement (RCD) has accumulated more slowly than for ecological character displacement, perhaps because sampling scales and environmental covariates can obscure the role of RCD in speciation. We examined an early example of RCD in an anuran species group, the vocalizations of the sympatric cricket frogs Acris crepitans and A. gryllus . With a relatively fine spatial scale, we compared mixed-species choruses (syntopy), nearby locations where A. gryllus is recently extirpated (historic sympatry), and surrounding areas without secondary contact (allopatry). In each of these areas, we evaluated variation in dominant frequency, click rate, and mass of males. In addition, we determined the acoustic preferences of syntopic females. Temperature influenced dominant frequency of vocalizations in A. crepitans , but not in A. gryllus . Body size varied more and had a stronger influence on dominant frequency in A. crepitans than in A. gryllus . Consequently, the decrease in mass of A. crepitans in syntopy resulted in convergence of body size and divergence of dominant frequencies of the two species. In contrast, dominant frequency of A. crepitans did not differ between historic sympatry and allopatry. Females of both species used fine temporal structure to discriminate between conspecific and heterospecific vocalizations and showed no preferences for dominant frequency. Chorus noise limited the ability of A. gryllus females to detect and discriminate vocalizations, so convergence in mass might have resulted from RCD in dominant frequency to reduce heterospecific acoustic interference. However, influences other than RCD might have caused syntopic convergence in body size.

AbstractKentucky warblers (Oporornis formosus) each sing a single song pattern. To determine whether males could range (estimate the distance to) conspecific songs, we presented clean and reverberated versions of strangers' songs to 12 males in a factorial design. To assess differences between the playbacks and the subjects' own songs or neighbors' songs, we measured differences in minimal repetition periods between repeated acoustic elements in songs, features that could contribute to assessment of reverberation. Results indicated that Kentucky warblers can range conspecific songs and that similarity between playback songs and established neighbors' songs or a subject's own songs did not enhance this ability. Direct evidence that males misjudged the distance to reverberated playbacks excluded other interpretations of the results based on differences in the detectability or habituation of clean and reverberated songs. These results suggest further that assessment of reverberation is sufficient for ranging and that perceptual analysis of song is not necessarily linked to overt production. As a consequence, repertoires of songs do not necessarily promote interference between territorial neighbors.

Communication in noise differs in a fundamental way from communication without noise, because a receiver faces four possible outcomes every time it checks its input. These outcomes present inevitable trade-offs for a receiver in adjusting its threshold for response. A signaler also faces trade-offs, in this case between costs and benefits as the exaggeration of signals increases. Furthermore, a receiver's and signaler's performances are mutually interdependent. The utility of a receiver's threshold depends on the signaler's exaggeration (the level of the signal in relation to the level of noise), and the utility of a signaler's exaggeration depends on the receiver's threshold. Diminishing returns for both receiver and signaler suggest the possibility of a joint evolutionary equilibrium for a receiver's threshold and a signaler's exaggeration. The present analysis combines previous expressions for the utility of a receiver's threshold (Ur) and the utility of a signaler's exaggeration (Us) in order to explore the possibility of this joint equilibrium. Utilities for both parties are expressed as survival × fecundity, an approximate measure of the spread of genes associated with a phenotype. Thus, Ur and Us, as functions of both the receiver's threshold (t) and the signaler's exaggeration (e), represent the adaptive landscapes for each party, and the reciprocal partial derivatives of these utilities, ∂Ur/∂e and ∂Us/∂t, approximate the selection gradients for the receiver's threshold and the signaler's exaggeration. With parameters for both the receiver's and the signaler's performances set to plausible values for many cases of mate choice, the resulting analysis shows that there exists a joint optimum for the receiver's threshold and the signaler's exaggeration. This optimum is a Nash equilibrium at which neither party can do better by a unilateral change in behavior. In some conditions, the equilibrium for communication in mate choice occurs at a higher threshold and higher exaggeration than the equilibrium for communication with warning signals. In general, these results indicate that the normal situation for communication in noise is honesty with deception — honesty on average but with instances of disadvantageous outcomes for receivers or signalers. Furthermore, the relationship between honesty and costs is more complex than currently recognized. Most important, the joint optimum for receiver and signaler indicates that communication in noise cannot escape the problems created by noise. Noise is an inevitable component of communication, and perfection in communication is not expected in natural conditions.

Effective communication requires that the receiver not only detect the presence of a signal but also discriminate significant variations in signals. Consequently, both attenuation and degradation of the structure of acoustic signals during transmission will limit the range of communication. In this study we document two primary sources of degradation of acoustic signals during propagation through natural environments, irregular amplitude fluctuations and reverberations. Amplitude fluctuations arise especially from atmospheric turbulence, while reverberations also result from scattering surfaces, such as vegetation. Both primarily mask information coded in amplitude modulation of the signal and repetitive frequency modulation, like the trills in the songs of many passerine birds. Irregular amplitude fluctuations primarily mask low frequencies of amplitude modulation in signals. Atmospheric turbulence from wind is the primary determinant of the intensity of irregular amplitude fluctuations, although amplitude fluctuations also increase with carrier frequency and range. In contrast, reverberations depend primarily on carrier frequency and range. Reverberations are least at intermediate frequencies (2-8 kHz). At lower frequencies reverberations in sound transmission near the ground often take the form of discrete echoes, probably from canopy foliage or from the change in acoustic impedance between air in the canopy and overlying air masses. At higher frequencies reverberations usually consist of a steady decay in acoustic energy. Consequently, in contrast to irregular amplitude fluctuations, reverberations primarily mask high rates of amplitude modulation and repetitive frequency modulation in acoustic signals. Intermediate frequencies (2-8 kHz) are most suitable for long-range acoustic communication, because irregular amplitude fluctuations, reverberations, and attenuation increase with carrier frequency, while reverberations and attenuation from ground interference increase at low frequencies. The great majority of animals that engage in long-range acoustic communication use this middle range of frequencies. Perhaps because of the increase in reverberations at low carrier frequencies, the songs of rufous-sided towhees show a correlation between trill rate and the minimum frequency in trills. To minimize the effects of amplitude fluctuations and reverberations on long-range acoustic communication, signals should encode information either in frequency modulation or in repetitive amplitude modulation that allow enough redundancy or signal averaging to permit recognition of signals by receivers. Because reverberations are more severe in environments with many scattering surfaces, long-range acoustic communication in forests, as opposed to open environments, should avoid rapid amplitude modulation or repetitive frequency modulation. Among North Carolina passerine birds, species that breed in forests tend to avoid rapid repetition rates at any given frequency in their long-distance songs. The directionality of both the broadcast and reception of acoustic signals will influence the effects of scattering on reverberations and attenuation of acoustic signals. In scattering environments, the optimal directionality of sound production and reception will require compromises.