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.     

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.     

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.     

Avoiding predators at night: antipredator strategies in red-tailed sportive lemurs (Lepilemur ruficaudatus)

Although about one-third of all primate species are nocturnal, their antipredator behavior has rarely been studied directly. Crypsis and a solitary lifestyle have traditionally been considered to be the main adaptive antipredator strategies of nocturnal primates. However, a number of recent studies have revealed that nocturnal primates are not as cryptic and solitary as previously suggested. Thus, the antipredator strategies available for diurnal primates that rely on early detection and warning of approaching predators may also be available to nocturnal species. In order to shed additional light on the antipredator strategies of nocturnal primates, I studied pair-living red-tailed sportive lemurs (Lepilemur ruficaudatus) in Western Madagascar. In an experimental field study I exposed adult sportive lemurs that lived in pairs and had offspring to playbacks of vocalizations of their main aerial and terrestrial predators, as well as to their own mobbing calls (barks) given in response to disturbances at their tree holes. I documented the subjects' immediate behavioral responses, including alarm calls, during the first minute following a playback. The sportive lemurs did not give alarm calls in response to predator call playbacks or to playbacks with barks. Other behavioral responses, such as gaze and escape directions, corresponded to the hunting strategies of the two classes of predators, suggesting that the corresponding vocalizations were correctly categorized. In response to barks, they scanned the ground and fled. Because barks do not indicate any specific threats, they are presumably general alarm calls. Thus, sportive lemurs do not rely on early warning of acoustically simulated predators; rather, they show adaptive escape strategies and use general alarm calls that are primarily directed toward the predator but may also serve to warn kin and pair-partners.     

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     

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.     

The stomatal response to evaporative demand persists at night in Ricinus communis plants with high nocturnal conductance

Evidence is building that stomatal conductance to water vapour (g(s)) can be quite high in the dark in some species. However, it is unclear whether nocturnal opening reflects a mechanistic limitation (i.e. an inability to close at night) or an adaptive response (i.e. promoting water loss for reasons unrelated to carbon gain). Further, it is unclear if stomatal responses to leaf-air vapour pressure difference (D) persist in the dark. We investigated nocturnal stomatal behaviour in castor bean (Ricinus communis L.) by measuring gas exchange and stomatal responses to D in the light and in the dark. Results were compared among eight growth environments [two levels for each of three treatment variables: air saturation deficit (D(a)), light and water availability]. In most plants, stomata remained open and sensitive to D at night. g(s) was typically lower at night than in the day, whereas leaf osmotic pressure (Pi) was higher at night. In well-watered plants grown at low D(a), stomata were less sensitive to D in the dark than in the light, but the reverse was found for plants grown at high D(a). Stomata of droughted plants were less sensitive to D in the dark than in the light regardless of growth D(a). Drought also reduced g(s) and elevated Pi in both the light and the dark, but had variable effects on stomatal sensitivity to D. These results are interpreted with the aid of models of stomatal conductance.     

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.     

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.     

Loss of complex female song but not duetting in the ancestors of Carolina wrens

Female singing and coordinated male–female duetting are often but not always found in the same species. Both behaviors are more common in tropical than temperate songbirds, but few studies have differentiated between the factors selecting for each. Here we investigate the evolution of female vocal complexity and male–female vocal coordination in Carolina wrens (Thryothorus ludovicianus), one of the few non-tropical members of a songbird family (Troglodytidae) that is well known for producing coordinated male–female duets. Female Carolina wrens are not known to sing; rather, they produce relatively simple, sex-specific chatters, often during territorial encounters. We analyzed field recordings to show that females coordinate these chatters with male songs at rates similar to those observed in some tropical duetting wren species. We then used phylogenetic comparative methods to show that the evolutionary ancestors of Carolina wrens had female songs that were more acoustically complex than the vocalizations of current females, suggesting past selection against female vocal complexity. Levels of vocal coordination with males, in contrast, have changed relatively little from those of tropical ancestors. Our results suggest that these two aspects of female behavior, acoustic complexity and vocal coordination with males, have evolved independently and have different functions in communication.     

Signaller: receiver coordination and the timing of communication in Amazonian birds

The efficacy of communication relies on the detection of signals against background noise. Some species are known to alter the timing of vocalizations to avoid acoustic interference from similar signals of other species, but nothing is known about the possibility of coordinated adjustments in the timing of receivers' attention. I examined the possibility that co-occurring species might respond as well as vocalize at different times in a diverse tropical avifauna by presenting playbacks of recordings to territorial birds at typical and atypical times for singing during the dawn chorus. The results show that co-occurring species of birds in a diverse avifauna partition the timing of both production and response in a way that would reduce acoustic interference between species.     

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.     

The role of nocturnal vision in mate choice: females prefer conspicuous males in the European tree frog (Hyla arborea)

Nocturnal frog species rely extensively on vocalization for reproduction. But recent studies provide evidence for an important, though long overlooked, role of visual communication. In many species, calling males exhibit a conspicuous pulsing vocal sac, a signal bearing visually important dynamic components. Here, we investigate female preference for male vocal sac coloration—a question hitherto unexplored—and male colour pattern in the European tree frog (Hyla arborea). Under nocturnal conditions, we conducted two-choice experiments involving video playbacks of calling males with identical calls and showing various naturally encountered colour signals, differing in their chromatic and brightness components. We adjusted video colours to match the frogs'' visual perception, a crucial aspect not considered in previous experiments. Females prefer males with a colourful sac and a pronounced flank stripe. Both signals probably enhance male conspicuousness and facilitate detection and localization by females. This study provides the first experimental evidence of a preference for specific vocal sac spectral properties in a nocturnal anuran species. Vocal sac coloration is based on carotenoids and may convey information about male quality worthwhile for females to assess. The informative content of the flank stripe remains to be demonstrated.     

Dawn Singing Intensity of the Male Brownish‐Flanked Bush Warbler: Effects of Territorial Insertions and Number of Neighbors

The dawn chorus is a period of peak singing activity of many songbirds. Numerous studies have sought to understand this widespread phenomenon, and many hypotheses have been proposed to explain the dawn chorus. The social dynamics hypothesis proposes that dawn singing plays an important role in the announcement of territorial occupancy and the regulation of social relationships between males; it predicts that the dawn chorus vocal behavior varies with changes in social relationships. In this study, we tested the influence of territorial insertions and the number of neighbors, on the intensity of the brownish‐flanked bush warbler (Cettia fortipes)'s dawn singing. We found that simulated territorial insertions (playback) caused the males to increase their dawn singing significantly the next day, and males that had many neighbors exhibited more intense dawn singing than did males with few neighbors. Our study provides evidence that dawn singing plays an important role in the announcement of territorial occupancy and the regulation of the social relationships between the males.     

Conquering the night: understanding nocturnal migration in birds

Abstract

Migration is a biologically distinct and unique phenomenon that enables the birds to migrate twice-a-year between the breeding and wintering grounds. These movements are known as spring and autumn migration, respectively. Depending on their inherent programming, the migratory birds may fly during day or night or both. Different environmental factors such as, temperature, food, predator pressure and physiological demands of energy storage and expenditure, contribute to the pattern of migrations, day or nighttime. Since, most of them are nighttime migrants they have to make dramatic changes in their physiology and behavior to transform them from being diurnal to predominantly nocturnal. These changes result in different life history stages (LHSs) such as migration, reproduction and molt, in their annual cycle, which are regulated by endogenous circadian and circannual clocks. As a result, the birds start preparing well in advance for the approaching LHS. The present review focuses on behavioral strategies of a nocturnal migrant and understanding of the possible physiological responses to ensure successful migration.     

Detecting diel patterns in the songs of Chipping Sparrows using citizen-science data

Previous studies have revealed diel patterns in the songs of Chipping Sparrows (Spizella passerina), with songs shorter in duration before dawn than after. However, the extent to which this phenomenon generalizes to the full geographic range of these sparrows is unclear, as is the question of whether citizen-science data can be used to detect diel patterns in song. We analyzed all available songs of Chipping Sparrows from the Macaulay Library and xeno-canto databases and compared the distributions of song features of recordings made at different times of day. We show that, across their entire geographic range in North and Central America, Chipping Sparrows sing shorter songs before sunrise (dawn song) than after sunrise (day song). Furthermore, we show that Chipping Sparrows shorten their songs by singing fewer syllables, not by singing faster: the number of syllables per song accounts for the observed difference in duration, not the syllable nor the intersyllable duration. Our results demonstrate that recordings from public repositories can be used to determine whether daily song patterns exist in species even in the absence of prior fieldwork, and we further propose that citizen-science recordings can be used to inform cross-species hypotheses and facilitate future studies to determine whether diel patterns in song are associated with differences in social behavior.     

Temporal pattern changes in the calling song of the katydidMygalopsis marki Bailey in response to conspecific song (Orthoptera: Tettigoniidae)

Males of Mygalopsis markiBailey (Tettigoniidae: Orthoptera) alter the temporal structure of their song in response to other competing males. The song of males calling in aggregations has a high variance in the number of syllables per chirp, with short intervals between each chirp. In contrast, the temporal pattern of the song of isolated males is more evenly spaced, with an increase in length of the interchirp intervals and low variance in the number of syllables per chirp. In order to simulate a calling male moving closer to a male in an aggregation, a playback technique was adopted whereby the recorded calling song of a male was presented to itself via a loudspeaker in increments of 2dB. The change in song pattern of the resident male involved a reduction in the number of syllables per chirp and an increase in the interchirp interval, with the number of chirps per second remaining constant. This reduction in the output of the song, instead of not calling as a result of an acoustic contest, may still allow males to continue calling for females.