Variation in Vocal Performance in the Songs of a Wood-Warbler: Evidence for the Function of Distinct Singing Modes

Male North American wood‐warblers (family Parulidae) subdivide their song repertoires into two different categories, or modes, of singing (first and second category songs). These two modes are thought to be specialized for interacting with females and males, although the data are inconclusive. I conducted an acoustic analysis of the song types used by yellow warblers (Dendroica petechia) for type I (first category) and type II (second category) singing to ask whether there are consistent structural differences between them which could provide insight into how they might function as separate signals. I found that type I songs are performed closer to the upper boundary of a song performance limit, measured in terms of the difficulty of production, compared with type II songs. By contrast, the performance of specific song types did not depend on whether they were used for type I singing vs. type II singing by different males. In addition, type I songs had a greater amplitude increase across the first two syllables compared with type II songs. There was no relationship between the performance of type I or type II songs and male condition. These results suggest that wood‐warblers might subdivide their song repertoire into distinct categories to highlight the relative vocal performance of their songs.     

Rufous‐capped Warblers Basileuterus rufifrons show seasonal,temporal and annual variation in song use

In the majority of songbird species, males have repertoires of multiple song types used for mate attraction and territory defence. The wood‐warblers (family Parulidae) are a diverse family of songbirds in which males of many migratory species use different song types or patterns of song delivery (known as ‘singing modes’) depending on context. The vocal behaviour of most tropical resident warblers remains undescribed, although these species differ ecologically and behaviourally from migratory species, and may therefore differ in their vocal behaviour. We test whether male Rufous‐capped Warblers Basileuterus rufifrons use distinct singing modes by examining song structure and context‐dependent variation in their songs. We recorded multiple song bouts from 50 male Warblers in a Costa Rican population over 3 years to describe seasonal, diel and annual variation in song structure and vocal behaviour. We found that Rufous‐capped Warbler songs are complex, with many syllable types shared both within and between males’ repertoires. Males varied their song output depending on context: they sang long songs at a high rate at dawn and during the breeding season, and shortened songs in the presence of a vocalizing female mate. Unlike many migratory species, Rufous‐capped Warblers do not appear to have different singing modes; they did not change the song variants used or the pattern of song delivery according to time of day, season or female vocal activity. Our research provides the first detailed vocal analysis of any Basileuterus warbler species, and enhances our understanding of the evolution of repertoire specialization in tropical resident songbirds.     

Song rate as a signal of male aggressiveness during territorial contests in the wood warbler

Aggressive signaling is an important component in animal communication, as it provides an efficient mechanism for settling conflicts over resources between competitors. In songbirds, a number of singing behaviors have been proposed to be aggressive signals used in territory defense, including song rate. Although aggressive signaling in songbirds has received considerable research attention, adequate evidence for most putative aggressive signals is not available. In this study, we experimentally investigated whether the song rate of male wood warblers Phylloscopus sibilatrix is a signal of their aggressive intent in male–male interactions. We found that males responded differentially to simulated territorial intrusions depending on the song rate of an intruder. Moreover, males that continued to sing during territorial contests increased their song rates, and this behavior predicted the strength of aggressive escalation by the signaler. These results suggest that song rate is an aggressive signal during male–male interactions in the wood warbler. We also found high intra‐individual repeatability in the strength of aggressive response to simulated intrusions, likely reflecting differences in personality (aggressiveness) or quality of male wood warblers. We conclude that changes in singing rate may be an efficient mechanism of signaling immediate shifts in motivation of signalers during territorial contests, especially in species that lack large repertoires or have simple songs.     

A novel song parameter correlates with extra-pair paternity and reflects male longevity

Although elaborate bird song provides one of the prime examples of a trait that evolved under sexual selection, it is still unclear whether females judge the quality of males by attributes of their song and whether these song features honestly signal a male's genetic quality. We measured the ability of male dusky warblers Phylloscopus fuscatus to maintain a high sound amplitude during singing, which probably reflects an individual's physiological limitations. This new measure of singing performance was correlated with male longevity and with extra-pair paternity, indicating that females who copulated with better singers obtained 'good genes' for their offspring. Our findings are consistent with the idea that females assess male quality by subtle differences in their performance during the production of notes, rather than by the quantity or versatility of song. In addition, observations on territorial conflicts indicate that attractive males invest less in competition over territories because they can reproduce via extra-pair paternity.     

Physically Challenging Song Traits,Male Quality,and Reproductive Success in House Wrens

Physically challenging signals are likely to honestly indicate signaler quality. In trilled bird song two physically challenging parameters are vocal deviation (the speed of sound frequency modulation) and trill consistency (how precisely syllables are repeated). As predicted, in several species, they correlate with male quality, are preferred by females, and/or function in male-male signaling. Species may experience different selective pressures on their songs, however; for instance, there may be opposing selection between song complexity and song performance difficulty, such that in species where song complexity is strongly selected, there may not be strong selection on performance-based traits. I tested whether vocal deviation and trill consistency are signals of male quality in house wrens (Troglodytes aedon), a species with complex song structure. Males’ singing ability did not correlate with male quality, except that older males sang with higher trill consistency, and males with more consistent trills responded more aggressively to playback (although a previous study found no effect of stimulus trill consistency on males’ responses to playback). Males singing more challenging songs did not gain in polygyny, extra-pair paternity, or annual reproductive success. Moreover, none of the standard male quality measures I investigated correlated with mating or reproductive success. I conclude that vocal deviation and trill consistency do not signal male quality in this species.     

The Influence of Song Mode on Responses of Male American Redstarts

Male American redstarts (Setophaga ruticilla) exhibit two modes of singing: repeat and serial mode. Repeat mode consists of repetitions of a single song type, usually one possessing an accented terminal note. Serial mode is constituted by a series of different songs with unaccented terminal notes, sung in a versatile sequence. Different authors have observed that males discriminate between these two modes. In the experiments reported in this paper, we tested whether males use accented terminal notes, song versatility or both to discriminate between the two song modes. We observed that males responded more intensively to the accented song. The accented note seems a very important song characteristic for evoking male American redstart responses. In contrast, males did not show any difference in their response to single and varied serial songs.     

Seasonal changes in the nocturnal singing routines of Common Nightingales Luscinia megarhynchos

Male Common Nightingales Luscinia megarhynchos famously sing at night. There are two distinct types of nocturnal singing routine (the dusk-to-dawn pattern of variation in song rate): (1) dusk and dawn choruses, with little or no song during the middle of the night; (2) a rapid increase in song rate after dusk, reaching a broad peak in the middle of the night, declining towards dawn, and followed by a dawn chorus. Males sing different nocturnal singing routines at different times in the breeding season. Earlier in the breeding season, most males sing Type 2 routines. Later in the breeding season, most males sing Type 1 routines, as do birds on the wintering grounds. At least some individuals may also sing Type 1 routines during the first few days after their arrival on their breeding territories, before the arrival of females. The main function of nocturnal song appears to be mate attraction. The patterns of variation in song rate over the course of the night are qualitatively similar to those predicted by stochastic dynamic programming (SDP) models of daily singing and foraging routines, for birds that do not forage at night, in circumstances when birds can pair at night (Type 2 routines), and when they cannot (Type 1 routines). The observed seasonal changes in the types of routine sung are also consistent with the predictions of these models.     

The song structure and seasonal patterns of vocal behavior of male and female bellbirds (Anthornis melanura)

The bellbird (Anthornis melanura) is a honeyeater endemic to New Zealand, which uses song to defend breeding territories and/or food resources year round. Both sexes sing and the song structure and singing behavior have not yet been quantified. The number of song types, spectral structure, repertoire size, and singing behavior of male and female bellbirds was investigated for a large island population. Song types differed between the sexes with males singing a number of structurally distinct song types and females producing song types that overlapped in structure. Singing behavior also differed between the sexes; males often sung long series of songs while females sung each song at relatively long and variable intervals. Singing by both sexes occurred year round but the frequency of male and female singing bouts showed contrasting seasonal patterns. The frequency of female singing bouts increased as the breeding season progressed, whereas male singing bouts decreased. In contrast to almost all studied passerines, female bellbirds exhibited significant singing behavior and sung songs of complex structure and variety that parallel male song. These results provide a quantitative foundation for further research of song in bellbirds and in particular the function of female vocal behavior.     

Temporal Aspects of Singing Interactions among Territorial Ovenbirds (Seiurus aurocapillus)

The temporal aspects of singing interactions among birds have received relatively little attention. To determine if the song delivery of one individual is affected by that of its territorial neighbor, I recorded singing interactions between territorial ovenbirds (Seiurus aurocapillus). Ovenbirds appeared to adopt one of two roles during singing interactions, Type I or Type II singers. Type II singers placed more of their songs immediately after the song of their neighbor than expected. The singing pattern of Type I singers could not be distinguished from a random pattern with respect to their neighbor's songs. In each observed pair of interacting birds, one individual was a Type I singer and one was a Type II singer. Although there was some intra-individual variation, most birds maintained the same role with each of their recorded territorial neighbors. Variation occurred between the two study sites in the extent that individuals overlapped the songs of their neighbors. Song overlap was common at one site, but occurred as, or less, often than expected at the other site.     

Do Hosts Discriminate between Sexually Dichromatic Male and Female Brown‐headed Cowbirds?

Female brood parasites are recognized as threats to reproductive success by many host species. Male brood parasites may accompany females while they search for nests to parasitize and males depredate nests throughout the nesting cycle. Hence, selection may also favour recognition of males. We examined whether two common host species perceive male brown-headed cowbirds ( Molothrus ater ) as brood parasites, as nest predators, or neither. We quantified visits of male cowbirds to nests of yellow warblers ( Dendroica petechia ) and red-winged blackbirds ( Ageliaus phoeniceus ) to assess the frequency with which these host species interact with male cowbirds. Males were observed near nests during hosts' laying and incubating stages, although less frequently than female cowbirds. No visits by cowbirds occurred while parents cared for nestlings. We then presented models of male and female cowbirds plus a non-threatening control to yellow warblers and red-winged blackbirds during laying and nestling periods. If hosts perceive males and females similarly, they should respond more intensely to the cowbird models during the laying period, when nests are most likely to be parasitized. Both species responded similarly to male and female cowbird models during laying, which suggests that hosts view cowbirds of both sexes as threats. The responses of yellow warblers with nestlings to male cowbirds were strongly influenced by the order of model presentation. Warblers first presented with the male cowbird gave much reduced anti-parasite responses than those that first interacted with the female then the male cowbird. These results suggest that yellow warblers recognized male vs. female cowbirds, but that discrimination was not expressed during laying. By contrast, red-winged blackbirds did not discriminate between male and female cowbirds at either nesting stage.     

The multiple functions of male song within the humpback whale (Megaptera novaeangliae) mating system: review,evaluation, and synthesis

Humpback whales (Megaptera novaeangliae) are seasonal breeders, annually migrating from high‐latitude summer feeding grounds to low‐latitude winter breeding grounds. The social matrix on the winter grounds is a loose network of interacting individuals and groups and notably includes lone males that produce long bouts of complex song that collectively yield an asynchronous chorus. Occasionally, a male will sing while accompanying other whales. Despite a wealth of knowledge about the social matrix, the full characterization of the mating system remains unresolved, without any firm consensus, as does the function of song within that system. Here, I consider and critically analyse three proposed functions of song that have received the most attention in the literature: female attraction to individual singers, determining or facilitating male–male interactions, and attracting females to a male aggregation within the context of a lekking system. Female attraction suggests that humpback song is an advertisement and invitation to females, but field observations and song playback studies reveal that female visits to individual singers are virtually absent. Other observations suggest instead that females might convey their presence to singers (or to other males) through the percussive sounds of flipper or tail slapping or possibly through vocalizations. There is some evidence for male–male interactions, both dominance and affiliative: visits to singers are almost always other lone males not singing at that time. The joiner may be seeking a coalition with the singer to engage cooperatively in attempts to obtain females, or may be seeking to disrupt the song or to affirm his dominance. Some observations support one or the other intent. However, other observations, in part based on the brevity of most pairings, suggest that the joiner is prospecting, seeking to determine whether the singer is accompanying a female, and if not soon departs. In the lekking hypothesis, the aggregation of vocalizing males on a winter ground and the visits there by non‐maternal females apparently for mating meet the fundamental definition of a lekking system and its role though communal display in attracting females to the aggregation, although not to an individual singer. Communal singing is viewed as a form of by‐product mutualism in which individuals benefit one another as incidental consequences of their own selfish actions. Possibly, communal singing may also act to stimulate female receptivity. Thus, there are both limitations and merit in all three proposals. Full consideration of song as serving multiple functions is therefore necessary to understand its role in the mating system and the forces acting on the evolution of song. I suggest that song may be the prime vector recruiting colonists to new winter grounds pioneered by vagrant males as population pressures increase or as former winter grounds become unavailable or undesirable, with such instances documented relatively recently. Speculatively, song may have evolved historically as an aggregating call during the dynamic ocean conditions and resulting habitat uncertainties in the late Miocene–early Pliocene epochs when Megaptera began to proliferate. Early song may have been comprised of simpler precursor sounds that through natural selection and ritualization evolved into complex song.     

The Effect of Male Competition on the Courtship Song of Drosophila melanogaster

The courtship song of Drosophila has been extensively used as a model system for studies of sexual selection and species recognition. Traditionally, the courtship song has been recorded from males placed individually with a female. However, under natural conditions females are exposed to multiple courting males, and the effect of their joint signal on mate recognition by the female is not yet understood. Here, we recorded the courtship behavior of D. melanogaster males singing either individually to a female lpar;1:1) or in the presence of an additional male lpar;2:1). We compared the structure of the male song in the two experimental designs. Our results show that when two males courted a female their songs could overlap each other. Males produced a significantly different signal in the presence of competition; the duration of each song component was significantly shorter and the rate of singing was markedly lower. The present study demonstrates that male competition can dramatically alter the acoustic signal detected by the female.     

Nocturnal and diurnal singing activity in the nightingale: correlations with mating status and breeding cycle

This study on the nightingale, Luscinia megarhynchos, is the first to examine both nocturnal and diurnal singing activity of mated and unmated males throughout a species' entire breeding cycle. Nocturnal song was sung mostly by unmated males. After pair formation, males ceased nocturnal singing and resumed it if their mate deserted. These results strongly suggest that nocturnal song of unmated males functions to attract a mate. Diurnal singing activity before females settled was low and did not predict future mating status. However, unmated males showed a continuous increase in diurnal singing activity until the end of the breeding cycle, but diurnal singing activity of mated males decreased after the egg-laying period. Mated males resumed nocturnal song for, on average, 3 nights during egg laying by their mates. This second period of nocturnal song coincided with the peak of diurnal singing activity. Such a high male singing effort during egg laying might allow the female to adjust her reproductive effort to male quality, deter rival males (e.g. through honest announcement of the female's fertility) or attract females for extrapair copulations. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Do female great tits (Parus major) assess males by eavesdropping? A field study using interactive song playback

Male singing behaviour correlates with extra-pair success in several passerine birds. Singing interactions during territorial contests provide relative information on the males involved. Such information may be important in female extra-pair behaviour and eavesdropping on singing interactions among males may allow females to make such relative assessments. We used interactive playback to instigate singing contests with male great tits during the peak fertile period of their mate in an attempt to alter females'' assessment of mates'' quality relative to neighbours (potential extra-pair partners). We escalated a contest to one male (by overlapping his songs) and then subsequently de-escalated a contest (by alternating) to a neighbour. Intrusions onto neighbouring territories by females mated to either treatment male were then monitored. Females mated to escalation treatment males were more likely to intrude following playbacks than females mated to de-escalation treatment males. Although the absolute song output of males did not differ between treatments, males produced more song relative to playback in de-escalation treatments and relative song output was positively correlated with female intrusions. Therefore, female great tits eavesdrop on singing interactions and change their visitation rates to neighbouring territories according to their mate''s singing performance relative to neighbours.     

Nonlocal male mountain white-crowned sparrows have lower paternity and higher parasite loads than males singing local dialect

Bird song often varies geographically, and when this geographicvariation has distinct boundaries, the shared song types arereferred to as song dialects. We investigated the role of songdialect in male mating success in a wild breeding populationof mountain white-crowned sparrows (Zonotrichia leucophrysoriantha). In 2 of 3 years, males singing unusual songs ("nonlocal"males) had lower total fertilization success (measured by microsatellitepaternity analysis) than did males singing the local dialect ("local" males). Similarly, females produced disproportionately more young with local than with nonlocal males. However, dialectwas not a significant predictor of male mating success whencontrolling for other factors that might affect paternity.Instead, the low mating success of nonlocal males was apparentlydue to an interaction between song dialect and parasite load.Nonlocal males were more severely infected by bloodborne Haemoproteusthan were local males, although they did not differ in anyother measured aspect of quality. Immigrant birds may be immunologically disadvantaged, possibly due to a lack of previous experiencewith the local parasite fauna, resulting in low mating success.     

Effects of Rapid Broadband Trills on Responses to Song Overlapping in Nightingales

In communication, animals often use complex signals with different traits carrying different information. In the song of some songbirds, both trills and song overlapping signal arousal or the readiness to escalate a contest in male‐male interactions, yet they also differ inherently from each other. Song overlapping is restricted to interactions and has a clear directive function as the songs are timed specifically to the songs of a counterpart. Trills, however, can be used without opponents actively singing and do not have such a directional character unless when combined with directed traits. This difference raises the question whether trills can enhance the agonistic function of song overlapping when being used simultaneously. Here, we exposed male nightingales (Luscinia megarhynchos) prior to pairing to overlapping playback treatments differing in the presence or absence of rapid broadband trills. Males responded differently to the two playback treatments suggesting that song overlapping and rapid broadband trills have some synergistic effects. Consequently, the separate or simultaneous use of trills and of song overlapping may allow males to adjust information encoded in their singing on a fine scale. Furthermore, males that remained unpaired throughout the breeding season responded differently to the playbacks than did subsequently paired males, emphasizing the implications of differences in territory defence behaviour on males subsequent pairing success.     

Do females invest more into eggs when males sing more attractively? Postmating sexual selection strategies in a monogamous reed passerine

Maternal investment can play an important role for offspring fitness, especially in birds, as females have to provide their eggs with all the necessary nutrients for the development of the embryo. It is known that this type of maternal investment can be influenced by the quality of the male partner. In this study, we first verify that male song is important in the mate choice of female Eurasian reed warblers, as males mate faster when their singing is more complex. Furthermore, female egg investment varies in relation to male song characteristics. Interestingly, clutch size, egg weight, or size, which can be considered as an high‐cost investment, is not influenced by male song characteristics, whereas comparably low‐cost investment types like investment into diverse egg components are adjusted to male song characteristics. In line with this, our results suggest that female allocation rules depend on investment type as well as song characteristics. For example, egg white lysozyme is positively correlated with male song complexity. In contrast, a negative correlation exists between‐song speed and syllable repetitiveness and egg yolk weight as well as egg yolk testosterone concentration. Thus, our results suggest that female egg investment is related to male song performance in several aspects, but female investment patterns regarding various egg compounds are not simply correlated.     

Song and Male Quality in Prairie Warblers

To determine if the songs of male prairie warblers could potentially reveal to female listeners information about the quality of singers, we compared various aspects of prairie warbler song structure and performance to attributes that might reflect a male singer's potential to enhance the fitness of his mate. We found that all the tested male attributes—arrival date, age, body size, annual survival, and fledging success—were associated with singing, most with multiple aspects of singing. Several of the song traits that were associated with potential indicators of male quality had also been found previously to be good predictors of female social mate choice. In particular, longer songs with longer elements, performed at lower frequency and with greater consistency, were associated with both female mate choice and potential indicators of quality. Thus, female prairie warblers may assess potential mates with the help of a set of song characteristics that collectively reveal an array of attributes that together indicate overall male quality.     

Extrapair paternity in chestnut-sided warblers is correlated with consistent vocal performance

The elaborateness of many bird songs is commonly presumed tohave evolved under the influence of sexual selection by femalemate choice. Thus, aspects of acoustic diversity, such as songrepertoire size, are seen as likely targets of female choice.In many songbird species with song repertoires, however, therepertoires are small. In such species, female choice mightbe based on song features other than, or in addition to, songdiversity. To investigate this conjecture, I assessed singingand paternity in a population of chestnut-sided warblers (Dendroicapensylvanica), a species in which song repertoires are of modestsize. Twenty-two song traits were evaluated to determine whichones best predicted male extrapair reproductive success. Thecandidate traits encompassed measures of song diversity (e.g.,song repertoire size), gross-scale song performance (e.g., singingrate), and fine-scale song performance (e.g., variability amongsongs in a bout). Regression analysis revealed that the bestpredictor of extrapair success was singing with little variability.In particular, the most successful males sang with consistentpitch and timing, as well as high pitch. The greater extrapairsuccess of males with more consistent vocal performance maybe due to female preference for such performance, which couldbe an indicator of male quality.     

Experimental Tests of the Function and Flexibility of Song Consistency in a Wild Bird

Measures of bird song that capture aspects of motor performance, such as consistency, have become a major focus in understanding sexual selection on song. Despite accumulating evidence that consistency is related to reproductive success in many species, the relative importance of male–male interactions and female–male interactions is still unclear. We studied the function and flexibility of song consistency and song rate in common yellowthroat warblers (Geothlypis trichas). A previous study of this population found that song consistency—measured as the amount of variability within a bout of songs—was positively correlated with the likelihood of siring extrapair young. In this study, we conducted two experiments aimed at testing (1) the role of song consistency and rate in mediating male–male and male–female interactions and (2) whether song effort is flexibly adjusted to changes in social context. In the first experiment, we simulated a male territorial intrusion with song playbacks that varied in consistency and rate; focal males responded aggressively to playbacks, but their response did not differ with playback consistency or rate. In the second experiment, we presented focal males with a taxidermic female mount and female vocalizations; focal males approached the speaker, but continued to sing and did not perform the aggressive rattle vocalization observed during male encounters. Immediately after the simulated female encounter, focal males increased in song consistency. Taken together, our results are most consistent with the hypothesis that song consistency in common yellowthroats is primarily a female‐directed signal that is actively adjusted in response to rapidly changing social conditions.