Valentin Amrhein

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.

Non-territorial nightingales prospect territories during the dawn chorus.

Male songbirds usually sing when they have occupied a territory, but the territory prospecting of non–territorial males is more elusive and has been rarely studied. Here, we simulated newly arriving, non–territorial males by translocating unmated male nightingales (Luscinia megarhynchos) to our study site. We show that territory prospecting of translocated males was largely confined to the hour before sunrise. The radio–tagged males made extensive excursions visiting several singing males at dawn, but after dawn they remained stationary outside occupied territories. As in many other songbird species, dawn was also the time when resident males sang the most. These results suggest that non–territorial male nightingales use the dawn chorus to assess singing residents or territory occupancy. For resident males, dawn singing may be important to announce territory occupancy to prospecting males and may thus play a role in territory maintenance.

Vocal interactions in nightingales, Luscinia megarhynchos: more aggressive males have higher pairing success

Song overlapping in birds is used and perceived as a signal of aggression, and evidence suggests that eavesdropping females base their extrapair mating decisions on the performance of males in vocal contests. In our study population of nightingales a large proportion of territorial males remain unpaired throughout the breeding season. A comparison between subsequently mated males and unpaired males may reveal whether females could use singing performance during vocal interactions in their choice of a social mate. We investigated how males that differed in their subsequent pairing status overlapped a noninteractive playback during the period of mate attraction, and how males used specific structural song components in response to playback. Subsequently mated males overlapped more playback songs than did males that remained unpaired throughout the breeding season. Males also adjusted the use of specific song components and decreased song rate during playback, suggesting that the flexible use of structural song components is more important in vocal contests than increasing song output. Because song overlapping is thought to be a signal of aggression, more aggressive males seem to have greater pairing success.

Dawn singing reflects past territorial challenges in the winter wren

Territory defence can be seen as a dynamic long-term process that involves some learning. For example, a resident may adjust its territory proclamation behaviour depending on its prior experience with territorial intruders. We investigated whether short territorial challenges could have long-lasting effects on the singing behaviour of male birds. We used song playbacks to simulate intrusions into autumn territories of male winter wrens, Troglodytes troglodytes, shortly after dawn and compared male singing behaviour immediately before and 1 day after the simulated intrusion. As in many other bird species, unchallenged male wrens tended to sing more songs before than after sunrise. One day after a simulated intrusion, however, this pattern was much more pronounced. Males significantly increased their song output before sunrise, but maintained or even reduced song output after sunrise. This result suggests that dawn singing before sunrise is particularly important for territory defence. On the day after the intrusion, the start of dawn singing varied less between males, although the average starting time remained the same. Our findings suggest that a territorial challenge can influence singing behaviour almost 24 h after the intrusion. The amount and timing of birdsong, as a preventive territorial proclamation, can thus be adjusted to past territorial challenges.

Seasonal variation in dawn song characteristics in the common nightingale

Many male temperate zone passerines show a marked peak of singing activity before sunrise. The two main functions of this so-called dawn chorus are mate attraction and territory defence. We examined how seasonal patterns of different dawn song characteristics were related to mating status and to the breeding cycle of females in the common nightingale, Luscinia megarhynchos. We investigated two measures of song output: song rate and percentage performance time. We also calculated the proportion of ‘whistle songs’, a song category that is thought to be important in female choice. We predicted that if the main function of dawn singing in nightingales is to attract a social mate, then mated males should change their dawn singing behaviour after pair formation. In contrast, if dawn singing is mainly used in territory defence, we expected no difference in song traits between mated and unmated males throughout the season. We found that song rate and the proportion of whistle songs were low at the beginning of the season and did not predict future mating status. After arrival of females, all measures of dawn song performance remained largely constant throughout the breeding season, and we did not find significant differences in the seasonal variation between mated and unmated males. These findings are consistent with the hypothesis that song at dawn is important to defend a territory throughout the breeding season.

Plants, Birds and Butterflies: Short-Term Responses of Species Communities to Climate Warming Vary by Taxon and with Altitude

As a consequence of climate warming, species usually shift their distribution towards higher latitudes or altitudes. Yet, it is unclear how different taxonomic groups may respond to climate warming over larger altitudinal ranges. Here, we used data from the national biodiversity monitoring program of Switzerland, collected over an altitudinal range of 2500 m. Within the short period of eight years (2003–2010), we found significant shifts in communities of vascular plants, butterflies and birds. At low altitudes, communities of all species groups changed towards warm-dwelling species, corresponding to an average uphill shift of 8 m, 38 m and 42 m in plant, butterfly and bird communities, respectively. However, rates of community changes decreased with altitude in plants and butterflies, while bird communities changed towards warm-dwelling species at all altitudes. We found no decrease in community variation with respect to temperature niches of species, suggesting that climate warming has not led to more homogenous communities. The different community changes depending on altitude could not be explained by different changes of air temperatures, since during the 16 years between 1995 and 2010, summer temperatures in Switzerland rose by about 0.07°C per year at all altitudes. We discuss that land-use changes or increased disturbances may have prevented alpine plant and butterfly communities from changing towards warm-dwelling species. However, the findings are also consistent with the hypothesis that unlike birds, many alpine plant species in a warming climate could find suitable habitats within just a few metres, due to the highly varied surface of alpine landscapes. Our results may thus support the idea that for plants and butterflies and on a short temporal scale, alpine landscapes are safer places than lowlands in a warming world.

Vocal interactions in common nightingales (Luscinia megarhynchos): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.

Responses to interactive playback predict future pairing success in nightingales

Birdsong is a sexually selected trait that serves in territory defence and mate choice. Individual song traits can be affected by the body condition of the male and thus may reflect his quality. Such relations between male quality and general singing performance raise the question whether differences in male quality also affect response strategies used in dyadic interactions. To address this question, we studied the relation between pairing success of male common nightingales, Luscinia megarhynchos, and their responses to rivals posing different levels of threat. Using interactive playback, we exposed males prior to mating to either aggressively or moderately singing rivals (by song overlapping and song alternating, respectively). Males that remained unpaired throughout the season (bachelors) interrupted their singing significantly more often after being overlapped than after alternating playback, whereas subsequently mated males kept the number of singing interruptions more constant across playback treatment. This suggests that subsequently paired males are less discriminative than are bachelors when challenged by rivals varying in aggressiveness. Regardless of playback treatment, males that later became paired responded significantly more strongly than did bachelor males. Thus, an increase in singing after a vocal interaction prior to mating predicted future mating success.

Acoustic features of song categories and their possible implications for communication in the common nightingale (Luscinia megarhynchos)

Summary In many passerine species, males sing more than one distinct song type. Commonly, songs are assigned to different song types or song categories based on phonological and syntactical dissimilarities. However, temporal aspects, such as song length and song rate, also need to be considered to understand the possible functions of different songs. Common nightingales (Luscinia megarhynchos) have large vocal repertoires of different song types but their songs additionally can be grouped into two distinct categories (particular groups of song types): whistle songs and nonwhistle songs. Whistle songs are hypothesised to be important to attract migrating females. We studied temporal properties of whistle songs and nonwhistle songs and examined the relationship between those song parameters and song output parameters, such as song rate and song length. To investigate how song parameters vary among males, we calculated the coefficients of variation for different song traits. We found that the variation in the proportion of whistle songs was significantly higher among males than variation in other song parameters. Furthermore, the proportion of whistle songs was negatively correlated with other song output patterns. These findings suggest that the production of whistle songs might be constrained and/or that whistle songs and their succeeding pauses may act as a functional unit in communication.

Communication Networks and Spatial Ecology in Nightingales

In most animals, communication plays a central role in a variety of contexts. In this chapter, we synthesize studies on vocal communication and spatial behavior in nightingales, Luscinia megarhynchos, with other research on songbirds to emphasize the need to integrate studies on communication with spatial and movement data to be able to understand communication in a dynamic social and communication network of individuals. By combing descriptive and experimental studies on singing, along with a communication network approach and studies on spatial behavior of males and females, the chapter takes a more ecological approach to animal communication which should be helpful to better understand the evolution and ecology of animal communication.