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.     

Sex-specific timing of mate searching and territory prospecting in the nightingale: nocturnal life of females

Formal models have shown that diel variation in female mate searching is likely to have profound influence on daily signalling routines of males. In studies on acoustic communication, the temporal patterns of the receivers'' signal evaluation should thus be taken into account when investigating the functions of signalling. In bird species in which diel patterns of signalling differ between males singing to defend a territory or to attract a mate, the diel patterns of mate and territory prospecting are suggested to depend on the sex of the prospector. We simulated newly arriving female nightingales (Luscinia megarhynchos) by translocating radio-tagged females to our study site. The mate-searching females prospected the area mostly at night, visiting several singing males. The timing of female prospecting corresponded to the period of the night when the singing activity of unpaired males was higher than that of paired males. In contrast to females, territory searching males have been shown to prospect territories almost exclusively during the dawn chorus. At dawn, both paired and unpaired males sang at high rates, suggesting that in contrast to nocturnal singing, dawn singing is important to announce territory occupancy to prospecting males. In the nightingale, the sex-specific timing of prospecting corresponded to the differential signalling routines of paired and unpaired males. The temporal patterns in the behaviour of signallers and receivers thus appear to be mutually adapted.     

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.     

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 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.     

Examining the microclimate hypothesis in Amazonian birds: indirect tests of the ‘visual constraints’ mechanism

Proposed mechanisms for the decline of terrestrial and understory insectivorous birds in the tropics include a related subset that together has been termed the ‘microclimate hypothesis’. One prediction from this hypothesis is that sensitivity to bright light environments discourages birds of the dimly lit rainforest interior from using edges, gaps, or disturbed forest. Using a hierarchical Bayesian framework and capture data across time and space, we tested this by first determining vulnerability based on differences in within‐species capture rates between disturbed and undisturbed forest for 64 bird species at the Biological Dynamics of Forest Fragments Project in central Amazonian Brazil. We found that 35 species (55%) were vulnerable to anthropogenic habitat degradation, whereas only four (6%) were more commonly captured in degraded forest. To infer visual sensitivity, we then examined two different characters: eye size (maximum pupil diameter) relative to body mass and the initiation time of dawn song, which presumably reflects a species’ visual capacity under low light intensities. We predicted that species with large relative eye sizes and birds with earlier dawn songs would exhibit increased vulnerability in degraded habitats with bright light. Contrary to our predictions, however, vulnerability was positively correlated with the mean start time of dawn song. This indicates that species that wait to initiate dawn song are also more vulnerable to habitat degradation. After correcting for body size, there was no effect of eye size on vulnerability. Together, our results do not provide quantitative support for the light sensitivity mechanism of the microclimate hypothesis. More sensitive experimental tests, such as behavioral assays with controlled light environments, especially in a comparative framework, are needed to rigorously evaluate the role of light sensitivity as an aspect of the microclimate hypothesis among Neotropical birds.     

Singing in the moonlight: dawn song performance of a diurnal bird varies with lunar phase

It is well established that the lunar cycle can affect the behaviour of nocturnal animals, but its potential to have a similar influence on diurnal species has received less research attention. Here, we demonstrate that the dawn song of a cooperative songbird, the white-browed sparrow weaver (Plocepasser mahali), varies with moon phase. When the moon was above the horizon at dawn, males began singing on average 10 min earlier, if there was a full moon compared with a new moon, resulting in a 67% mean increase in performance period and greater total song output. The lack of a difference between full and new moon dawns when the moon was below the horizon suggests that the observed effects were driven by light intensity, rather than driven by other factors associated with moon phase. Effects of the lunar cycle on twilight signalling behaviour have implications for both pure and applied animal communication research.     

Sex ratio and male sexual characters in a population of blue tits, Parus caeruleus

Sex allocation theory proposes that parents should bias thesex ratio of their offspring if the reproductive value of onesex is greater than that of the other. In the monogamous bluetit (Parus caeruleus), males have a greater variance in reproductivesuccess than females, and high-quality males have higher reproductivesuccess than high-quality females due to extrapair paternity.Consequently, females mating with attractive males are expectedto produce broods biased toward sons, as sons benefit more thandaughters from inheriting their father's characteristics. Songand plumage color in birds are secondary sexual characters indicatingmale quality and involved in female choice. We used these malesexual traits in blue tits to investigate adaptive sex ratiomanipulation by females. We did not find any relationship betweenmale color ornamentation and brood sex ratio, contrary to previousstudies. On the other hand, the length of the strophe bout (i.e.,the mean number of strophes per strophe bout) of fathers waspositively related with the proportion of sons in their broods.The length of the strophe bout is supposed to reflect male qualityin terms of neuromuscular performance. We further showed thatsons produced in experimentally enlarged broods had shorterstrophe bouts than sons raised in reduced broods. These resultsare consistent with the hypothesis that females adjust the sexratio of their broods in response to the phenotype of theirmate.     

Light pollution alters the phenology of dawn and dusk singing in common European songbirds

Artificial night lighting is expanding globally, but its ecological consequences remain little understood. Animals often use changes in day length as a cue to time seasonal behaviour. Artificial night lighting may influence the perception of day length, and may thus affect both circadian and circannual rhythms. Over a 3.5 month period, from winter to breeding, we recorded daily singing activity of six common songbird species in 12 woodland sites, half of which were affected by street lighting. We previously reported on analyses suggesting that artificial night lighting affects the daily timing of singing in five species. The main aim of this study was to investigate whether the presence of artificial night lighting is also associated with the seasonal occurrence of dawn and dusk singing. We found that in four species dawn and dusk singing developed earlier in the year at sites exposed to light pollution. We also examined the effects of weather conditions and found that rain and low temperatures negatively affected the occurrence of dawn and dusk singing. Our results support the hypothesis that artificial night lighting alters natural seasonal rhythms, independently of other effects of urbanization. The fitness consequences of the observed changes in seasonal timing of behaviour remain unknown.     

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.     

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.     

Dawn singing of the Brownish‐flanked Bush Warbler influences dawn chorusing in a bird community

Despite numerous studies on the function of the avian dawn chorus, few studies have examined whether dawn singing may influence the singing of other species. Here, we built on our previous study which found male Brownish‐flanked Bush Warblers (Horornis fortipes) increase their dawn singing intensity after conspecific playback on the previous day. We reanalyzed those recordings to quantify the start of dawn singing in other nine sympatric songbird species. Ranking‐scaling analyses identified a distinctive sequential pattern of dawn singing among these bird species between the first and the second dawn chorus, and meta‐analysis showed a significant trend to singing earlier in the bird community accompanied by the increase in dawn singing intensity in Brownish‐flanked Bush Warbler. Species with songs most similar to that of the Brownish‐flanked Bush Warbler and species that were phylogenetically distantly related to the Brownish‐flanked Bush Warbler showed a greater shift in the onset of dawn singing. Our study is one of the few studies showing how bird song influences heterospecific singing, and this may influence the temporal organization of song activity in the community, and result in synchronization in singing activities among different species, such as singing in dawn and dusk chorus.