Long-term influence of simulated territorial intrusions on dawn and dusk singing in the Winter Wren: spring versus autumn

Males of many songbird species have peaks of singing activity at dawn and dusk. Singing during those twilight periods can function in territory proclamation, and males are suggested to adjust song output to the level of intruder pressure. We used song playback during the breeding season to simulate intrusions into territories of male Winter Wrens (Troglodytes troglodytes) shortly after dawn. We then compared male singing behaviour during the dawn and dusk chorus before and 1 day after the simulated intrusion. One day after the playback, male Wrens increased their song output before sunrise, which confirms our results from a previous study on dawn singing in autumn territories. At dusk, on the evening following the playback, males slightly increased song output after sunset, but singing activity at dusk was generally very low. We found no significant changes of song output after sunrise, before sunset, and between 2 days of control without playback. These results are consistent with the hypothesis that dawn and dusk singing is important for territory defence in spring. Unlike in autumn, however, increased singing in spring at dawn and dusk could also serve to defend other resources such as fertile mates or to strengthen the pair bond after a territorial challenge. In comparison with the results on autumnal singing, male Wrens started singing earlier at dawn during the breeding season, and they generally sang more songs at dawn and immediately after playback. The increase in absolute numbers of songs sung in the morning after playback seemed greater in spring than in autumn; however, the proportional increase relative to overall song output was similar in both seasons.     

Conveying information with one song type: changes in dawn song performance correspond to different female breeding stages

Many bird species participate in dawn singing, a behaviour categorized by intensive singing at dawn; however, many of these species deliver only one song type at dawn. While there are many proximate and ultimate hypotheses for why birds sing at dawn, little is known about whether males are able to vary one simple song to convey different information. We used autonomous acoustic recorders to record dawn songs of field sparrows and quantified three parameters of singing performance: 1) bout length, 2) song rate, and 3) song complexity. We found that males sang the longest dawn bouts during their mate's fertile period, the highest song rates during the post-fertile period, and the most complex songs during the pre-fertile period. The change in dawn singing behaviour with their mate's breeding stage suggest the purpose of dawn song may be context dependent. Our results demonstrate that male field sparrows, while only having a single song type sung at dawn, may convey information for both intra- and intersexual purposes. While it is generally assumed that dawn song has a specific function, the variability in the duration, rate, and complexity of dawn song in field sparrows suggests that they are conveying different information and that dawn song likely has multiple functions.     

Patterns of dawn singing by Buff-breasted Flycatchers

ABSTRACT.   Many passerine species exhibit a "dawn chorus"—a bout of intense singing activity before or at dawn, but our understanding of this phenomenon is poor. Tyrant flycatchers (Tyrannidae) exhibit pronounced daily bouts of dawn singing. I documented this behavior in several populations of Buff-breasted Flycatchers ( Empidonax fulvifrons ) in Arizona, tape recording >30,000 songs of 23 individuals during dawn singing. Individual males sang a dawn bout each morning, even during breeding phases when daytime song was almost completely absent. Dawn bouts began 5–10 min before local civil twilight and continued for 25–30 min. Each male possessed two song types delivered at high rates during dawn singing (four times the rate during sustained daytime singing). Song rate varied significantly over the course of dawn bouts, increasing to 55 songs / min at mid-bout, then declining to the end of the bout. Type 2 songs comprised about 30% of songs during dawn singing, and decreased significantly in proportion during the final 10 min of the bout. Songs of the two types were delivered in a nonrandom fashion. Males sang at locations near territory boundaries and pairs of neighbors engaged in counter-singing from the same locations each morning. A number of dawn singing bouts ended with attempted or successful copulations. These observations are consistent with the social dynamics hypothesis for the functional significance of dawn singing in this species.     

Strophe Length in Spontaneous Songs Predicts Male Response to Playback in the Hoopoe Upupa epops

Hoopoe (Upupa epops, Coraciformes) males produce a very simple song during the breeding season in order to attract females and repel intruders. Strophes vary in length (i.e. number of elements) both within and between males, and previous studies have shown that this song cue is positively correlated with male condition and breeding success. In the present study we tested whether strophe length of males influences male behaviour during intra‐sexual contests, in a colour‐ringed population in southeast Spain. Paired males were presented with a recorded song with long strophes during the pre‐laying period, while they were near their mates, in order to provoke male mate‐defence behaviour. Most males responded to the playback, but the strategy of defence adopted depended on their own strophe length in spontaneous songs recorded before the experiments. While singing responses were common to most of the males, only those using long strophes adopted the most risky strategy of approaching the loudspeaker. However, the males that approached produced abnormal songs during playback, that were shorter and with fewer strophes than those of males that did not approach, and used shorter strophes in comparison with spontaneous songs before the experiment. These differences in quality of the song produced in response to the playback suggest that long‐strophe males were basing their response mainly on attacking rather than singing, while short‐strophe males tried to resolve the contest at a distance by means of their song. These results show that strophe length reflects some component of the competitive ability of males (either physical strength or aggressiveness) in the hoopoe, which together with previous results regarding its role for female choice, show that it is a sexual signal with dual function.     

The Dawn Song of the Blue Tit Parus caeruleus and its Role in Sexual Selection

Sexual selection arises when variance in male reproductive success is non-randomly related to phenotypic characters of males. Song can be considered as such a phenotypic character and several studies have shown that song complexity and/or song output are important in competition among males or in partner choice. In the blue tit Parus caeruleus a peak in male singing activity occurs at dawn during the female fertile period, i.e. after pair formation. The function of this dawn chorus is not well understood. In this study 20 male blue tits were recorded at dawn and song complexity and output were expressed as versatility, mean strophe length, mean percentage performance time and bouts with or without drift, i.e. with or without a systematic decline in percentage performance time. Females mated to males with a higher mean percentage performance time (output) and a higher versatility (complexity) started to lay eggs earlier, but the latter was not significant. Females mated to males that showed no drift in their song bouts laid significantly larger clutches. Our results thus suggest that in the blue tit, song output at dawn, rather than song complexity, might be a trait under sexual selection.     

Song strophe-length and reproductive success in a non-passerine bird, the Hoopoe Upupa epops

Hoopoe Upupa epops males produce a very simple song, with a repertoire size of one, in which the main difference between different strophes of a male and between the songs of different males is the number of elements they include (strophe-length). In several passerine species it has been shown that strophe-length is a sexually selected trait that reflects male quality and correlates with reproductive success. Here we analyse whether in a non-passerine, the Hoopoe, strophe-length of males is correlated with several variables of their reproductive success. Females paired with males singing long strophes laid their first clutch earlier, produced larger first clutches and laid second clutches after a successful first one more frequently than those paired with males singing short strophes. Moreover, males with long strophes produced more fledglings in their first clutches and in the whole season, partly because they brought more food for the brood than males with short strophes. The relationships found are not mediated by age effects. Previously we have shown that Hoopoe females in the early spring are attracted preferentially to songs with long strophes. Here we show that males singing long strophes also obtain postpairing benefits in terms of reproductive success, and that females paired with these males obtain direct benefits because these males provide greater feeding effort in the second half of the nestling period. These findings support the hypothesis that in the Hoopoe, strophe-length is a sexually selected cue under direct selection.     

Implications of conspecific background noise for features of blue tit, Cyanistes caeruleus , communication networks at dawn

Communication among animals often comprises several signallers and receivers within the signal’s transmission range. In such communication networks, individuals can extract information about differences in relative performance of conspecifics by eavesdropping on their signalling interactions. In songbirds, information can be encoded in the timing of signals, which either alternate or overlap, and both male and female receivers may utilise this information when engaging in territorial interactions or making reproductive decisions, respectively. We investigated how conspecific background noise at dawn may overlay and possibly constrain the perception of such singing patterns. We simulated a small communication network of blue tits, Cyanistes caeruleus, at dawn in spring. Two loudspeakers simulated a singing interaction which was recorded from four different receiver positions simulating potential eavesdroppers. During the recordings, resident blue tit males were vocalising and created natural conspecific background noise. Levels of conspecific background noise were high and varied among positions of potential eavesdroppers. We conclude that conspecific background vocalisations may potentially constrain the perception of singing patterns and may constitute costs for eavesdroppers. On the other hand, signallers may position themselves strategically and privatise their interactions.     

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.     

Differential effects of moving versus stationary territorial intruders on territory defence in a songbird

1.  In territorial contests, not only acoustic or other signals, but also the movements of a territorial intruder are likely to influence the response of a resident.
2.  We tested this movement hypothesis by simulating moving vs. stationary intruders into the territories of winter wrens Troglodytes troglodytes , using the same non-interactive song playbacks in both treatments.
3.  Male winter wrens showed a different long-term singing reaction in response to a moving than to a stationary intruder.
4.  One day after experiencing an intruder that was switching between three locations, residents started to sing earlier before sunrise, and they sang more and longer songs at dawn than before the intrusion.
5.  Residents receiving the same playback from one location only reacted by starting to sing later relative to sunrise, and by singing fewer and shorter songs than before the intrusion.
6.  We could not discriminate between the treatments when examining the short-term singing reactions during and immediately after the playbacks. However, our results clearly demonstrate an effect of the spatial behaviour of territorial intruders on the long-term territory defence of residents at dawn, about 24 h after an intrusion.
7.  We argue that spatial behaviour of territorial intruders should be an integral part of the study of animal territory defence behaviour. Investigating long-term changes in territory defence at dawn is a sensitive tool for discriminating between different types of intruders.     

Watch Your Tone: Social Conditions Modulate Singing Strategies

Bird song is typically depicted as a male singing a long‐distance signal to potentially unknown receivers to (1) deter males and (2) attract females. Nevertheless, many songbirds sing from close distances to a known receiver; males of these species may be under more intense selective pressure to modify their songs depending on the sex of the receiver in order to convey different motivational states (aggression versus courtship) to the different sexes. In a laboratory setting, we examined how receiver sex affected within‐song variation of the close‐range singing behavior in the brown‐headed cowbird (Molothrus ater). Although we know that cowbird song is influenced by flock composition, it is still unclear as to how the cowbird modifies his song based on social context. Using a cross‐correlation analysis of each male's different song types, we found that pairs of songs were significantly more dissimilar if they were directed to females compared with songs directed to males. We subsequently tested whether there were any consistent spectral or temporal patterns in the songs males gave to females versus to males. Our results lend support for the Motivational Structural Rules Hypothesis as songs directed toward males had higher entropy (i.e., harshness) than the same song type directed toward females. Our results suggest that cowbirds may have evolved the ability to alter multiple dimensions of their singing behavior based on receiver sex.     

An Interactive Playback Experiment Shows Song Bout Size Discrimination in the Suboscine Vermilion Flycatcher (Pyrocephalus rubinus)

Many aspects of the social behaviour of birds are mediated by vocal displays, and variation in song output or song structure conveys different information to receivers. After nest construction begins, when vermilion flycatcher (Pyrocephalus rubinus) females are potentially fertile, males increase their song rate during the dawn chorus. A previous study failed to give evidence that males discriminate among song rates. However, males sing in sequences of songs (song bouts), and an increase in song rate may be achieved by increasing the number of bouts, the number of songs in each bout (bout size) or both. Studying a vermilion flycatcher population in Mexico City, we evaluated whether dawn song rate is related to song bout size or to number of bouts. Bout size correlated with song rate and differed among males. We hypothesized that longer bouts are more threatening signals than shorter ones and predicted a stronger response by males towards the former. We exposed each male to three playback treatments: (1) Long song bout (Long), in which we replied to the male with twice the number of songs he sang in the bout, (2) Short song bout (Short), in which we played half the number of songs sung by the male and (3) Control, this was the same as the Long treatment but we used songs from a related species, the tropical kingbird (Tyrannus melancholicus). Males responded with a higher proportion of calls near the speaker when exposed to the Long treatment than during the Short or Control treatments, indicating that they discriminate among song bouts differing in size, that they may perceive longer bouts as more threatening and that they use calls rather than songs to address threatening situations. Our results suggest that song bout size is a relevant song attribute that conveys information during intrasexual interactions.     

Seasonal and diel budgets of song: a study of Savi’s warbler (Locustella luscinioides)

Seasonal and diel patterns of singing activity of Savis warblers (Locustella luscinioides) were studied in two areas of Central Europe 300 km apart, over a period of 18 years. We assess about 4,600 records of individuals singing. Males were found to exhibit similar singing activity in both study sites. They started to sing after arrival at the beginning of April and peaked from the end of April to the beginning of May. Thereafter, their singing activity was lower but more stable for a relatively long period from mid-May to mid-July. At the end of July, males sang only sporadically and singing activity ceased at the beginning of August. At the beginning and towards the end of the song-period males sang sporadically whereas in the period of the highest singing activity they sang over the entire 24-h period. During the whole song-period, there was a significant difference in singing activity between daylight and the dark (67.2 and 32.8%, respectively). However, the period of daylight was longer. Average singing activity showed similar levels in daylight and the dark with mean numbers of 5.9 and 6.6 males per hour, respectively. Major changes in singing activity were related to the twilight periods. There were distinctive dawn and dusk choruses. In the morning, Savis warblers exhibited similar levels of singing activity over 3 h of the dark before twilight, singing reached its highest level at twilight and 1 h after twilight. During the evening, singing activity reached its highest-level 1 h before twilight, while during twilight it was decreasing, with a considerable decline 1 h after nightfall.     

Song Types and their Use: Developmental Flexibility of the Male Blue-winged Warbler

How does an animal know the appropriate context in which to use its different vocalizations? I studied this problem with a songbird, the blue-winged warbler (Vermivora pinus). Mated males in nature tend to sing one song form (II) at dawn, under low light levels, and at a median rate of 15 songs/min. Their other song (I, the more familiar bee-buzz) is used later in the day, under higher light levels, and at a median rate of 6 songs/min. In the laboratory I tutored different groups of hand-reared males under normal circumstances (group 1: song II early in the morning, under low light, at a rapid rate; song I later, under high light, at a slow rate), under reversed circumstances (group 2: song I early, low light, rapid rate; etc.), and with no contextual cues (group 3: songs I and II both early and late, under both light levels, and at an intermediate rate). Few males developed entirely normal blue-wing song, but males of group 2 clearly reversed the use of song forms and components. The relationship between signal form and function in the songs of this warbler appears flexible and to some extent learned by the males.     

Singing by male and female Kloss gibbons (Hylobates klossii) in the Peleonan Forest, Siberut Island, Indonesia

Kloss gibbons (Hylobates klossii) are endemic to the Mentawai Islands in Indonesia and are one of only two gibbon species in which mated pairs do not sing duets. This is the first long-term study of the factors influencing the singing activity of Kloss gibbons within a northern Siberut Island population and follows two previous studies in central Siberut nearly 30 years ago. We collected data on the presence/absence of male and female singing within the study area on 198 days and within a focal group on 47 days. Rainfall during the time period in which they normally sing inhibits singing in both males and females. Our study supports the hypothesis that male and female songs function in intrasexual resource defence, as singing is associated with singing by same-sex neighbours, and same-sex choruses are more likely to occur after one or more days of silence (from that sex), suggesting there is pressure for individuals to communicate with same-sex neighbours regularly. Singing was not coordinated within a mated pair, suggesting that vocal coordination of the pair has been lost with the loss of the duet and that Kloss gibbon songs do not convey information to neighbours about the strength of the pair bond. On days when males sang predawn, females were more likely to sing after dawn and earlier in the morning. Additionally, the number of groups singing in female choruses was positively associated with the number of males that had sung in the predawn male chorus. We suggest that female songs have an intersexual territory defence as well as an intrasexual function.     

Honest advertisement and song output during the dawn chorus of black-capped chickadees

Costly signals can evolve under sexual selection, as only thosesignals that are difficult to produce and reflect the relativequality of individuals should be important in mate choice. Onesuch signal may be dawn singing behavior in birds. We assessedwhether the song output at dawn of breeding male black-cappedchickadees Parus atricapilhis honestly reflects quality, whererelative quality is assessed by relative dominance rank in winterflocks. Dawn choruses were recorded from 20 male chickadeesfrom 10 flocks during the fertile period of their mates in 1992,1994, and 1995. Dominance ranks of males were assessed by tabulatinginteractions at winter feeders from 1993 to 1995. A comparisonof the dawn singing behavior of the high-ranking and the low-rankingmales from each of the 10 flocks showed that high-ranking malesbegan singing earlier, sang longer, and sang at higher averageand maximum rates than low-ranking flockmates. Age of the maleshad less effect on song output at dawn than rank; older malestended to sing longer dawn choruses, but there was no differencein onset of singing, average song rate, or maximum song rateat dawn between hatch year and after-hatch year males. Our findingssuggest the dawn chorus can provide an accurate signal to femalesof the relative quality of their mate compared to neighboringmales     

The ecology of singing in Kloss gibbons (Hylobates klossii) on Siberut Island,Indonesia

Data are presented from a study of habituated Kloss gibbons on Siberut Island, Indonesia. Male Kloss gibbons can sing at any time from 0100 to 1300 hr, but the majority of songs is concentrated in the hour before dawn. Female Kloss gibbons sing only after dawn and the song bout includes a dramatic visual display. Neither countersinging nor coordinated chorusing has been proved in either sex. Males sing before dawn as often as possible but are inhibited by wet nights and by minimum temperatures below 21.5°C; postdawn songs of both sexes are inhibited by rain. The occurrence of any particular type of song bout is independent of the occurrence of the other types. Song trees used by males and those used by females do not differ in height. Song trees emerged from the neighboring canopy more than other available trees of similar height in the gibbons’ home range. Female song trees were most abundant on the slopes and where the trees were tallest. Almost all the male’s night trees could have been used for singing from had the weather been suitable. There was a greater likelihood of the male’s traveling a long way to the day’s first fruit source on mornings when he sang before dawn than on mornings when he did not. Considerations of sound transmission through tropical rain forest reveal that the times and frequencies used for singing by Kloss gibbons are optimal for communicating with neighboring groups.     

Social stimulation of dawn singing in Dusky Flycatchers: a serendipitous experiment

Hypotheses regarding dawn singing in birds remain largely unsupported by quantitative data, especially for suboscine passerines (suborder Tyranni). During a study of the singing behavior of a suboscine, the Dusky Flycatcher (Empidonax oberholseri), in Alberta, Canada, spring storms in 2002 caused the disappearance and presumed mortality of several territorial males and some of their replacements, creating a serendipitous experiment. Overall, 15 males disappeared and, as a result, the number of territorial males in our study area declined from 13 prior to the storms to six afterward. Only one male maintained the same territory throughout the breeding season. During the period of inclement weather and social instability (20 May to 9 June), males sang at high rates during the predawn period. From 10 to 19 June, following this period of unstable weather and turnover in territorial males, males began dawn singing later in the morning, and sang shorter bouts at lower rates. The proportion of males engaging in dawn singing also decreased between the two periods. In contrast, daytime singing activity of paired males was low during both time periods. Playback of dawn songs to territorial males between 20 June and 21 July caused a resumption of some dawn singing. Singing rates were higher on the day of playback than on the day before playback, and the times when dawn singing was initiated were earlier on each of 2 d after playback than on the day before playback. In addition, the proportion of males engaging in dawn singing increased between the day before and the day after playback. Both the decrease in dawn singing when population density was reduced and its partial restoration by playback suggest that dawn singing in this species functions in male–male interactions and support the social‐dynamics hypothesis as an explanation for the dawn chorus in this species.     

The blue tit's song is an inconsistent signal of male condition

Sexually selected traits are often hypothesized to signal malecondition or quality, though empirical evidence is mixed, anda number of alternative models of sexual selection do not requirecondition dependence. We examined the relationship between variousmeasures of condition and dawn songs in male blue tits (Cyanistescaeruleus). We detected 6 largely independent measures of variation(i.e., variables) in these songs. None of these variables wererelated to blue tits' ultraviolet–blue plumage, a demonstratedsexual signal, thus failing to support the redundant signalhypothesis. We found some evidence that the song variables wemeasured signaled male quality. There were correlations betweenbody size and certain song traits, though neither male age normale recapture in the subsequent breeding season (apparent localsurvival) predicted any song variation. We combined our resultswith published effect sizes comparing blue tit song with malequality variables using meta-analysis and found that a few songmeasures are correlates of male quality, though as in our fielddata, neither male age nor survival appeared related to song.Our relatively large sample sizes (>60), combined with ourmeta-analytical integration of 89 effect sizes, make the resultsregarding the signaling value of our measured components ofblue tit song robust. These results demonstrate that 1) onlycertain aspects of signal variation may be condition dependentand 2) even when components of a sexual signal appear correlatedwith condition in some studies, these signal components maybe unrelated or inconsistently related to a variety of conditionindices.     

Short records of marsh warbler ( Acrocephalus palustris) song provide indices that correlate with nesting success

The migrant Marsh Warbler (Acrocephalus palustris) has a complex song repertoire, but such complexity makes quantitative comparison of songs between individual males both time consuming and challenging. We investigate a streamlined method of song analysis that uses 2-min records of song to provide simpler relative indices of repertoire size, including the use of capture-recapture and species-richness models. For each male, three attributes of song were determined: the song complexity, the total repertoire elements and the estimated repertoire size based on the Burnham and Overton jackknife method. Males with higher song indices tend to have greater nesting success, suggesting that even short records of song can indicate male quality to prospective female mates. Why should male Marsh Warblers have long and sustained songs when only 2-min records correlate with nesting success? Assuming that song advertises the quality of the male, and that the quality of such advertisement is sustained throughout the males song period, we argue that the female may need to have only brief exposure initially to that song to assess its quality and hence the quality of the male. More continuous periods of song may reinforce that initial choice by the female, and allow males to remain conspicuous to transient females moving around the territories of potential male mates.     

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.