S. Elizabeth Campbell

Territory tenure in song sparrows is related to song sharing with neighbours, but not to repertoire size

Song repertoires may be a product of sexual selection and several studies have reported correlations of repertoire size and reproductive success in male songbirds. This hypothesis and the reported correlations, however, are not sufficient to explain the observation that most species have small song repertoire sizes (usually fewer than 10, often fewer than five song types). We examined a second important aspect of a males song repertoire, the extent to which he shares songs with his neighbours. Song sharing has not been measured in previous studies and it may be partially confounded with repertoire size. We hypothesized that in song sparrows, Melospiza melodia, song sharing rather than repertoire size per se is crucial for male territorial success. Our longitudinal study of 45 song sparrows followed from their first year on territory showed that the number of songs a bird shares with his neighbourhood group is a better predictor of lifetime territory tenure than is his repertoire size. We also found that song sharing increases with repertoire size up to but not beyond eight to nine song types, which are the most common repertoire sizes in the population (range in our sample 5-13). This partial confound of song sharing and repertoire size may account for some earlier findings of territory tenure-repertoire size correlations in this species and other species having small- or medium-sized repertoires. Copyright 2000 The Association for the Study of Animal Behaviour.

Recognition of individual neighbors by song in the song sparrow, a species with song repertoires

SummaryPrevious theory and research have suggested that bird species with song repertoires in general, and song sparrows (Melospiza melodia) in particular, cannot readily discriminate between the songs of neighbors and strangers. In a recent study (Stoddard et al. 1991) we showed that song sparrows can in fact discriminate neighbors from strangers on the basis of song. In this study we sought to demonstrate that song sparrows can make the finer discrimination between individual neighbors and that they can do so on the basis of a single song type. We compared the response of territorial males to song playback of neighbors and strangers at three locations: the neighbors regular boundary, the opposite boundary, and the center of the territory. The birds showed strong neighbor-stranger discrimination at the regular boundary but not at the opposite boundary, nor in the center of the territory. The differences in song discrimination between different boundary locations indicate that song sparrows associate particular songs with particular territories, effectively discriminating between individual neighbors on the basis of song. Song repertoires themselves do not interfere with neighbor recognition to the extent originally postulated. As speakers are moved inside the territory from the border, however, the degree of discrimination diminishes. We believe that differences in speaker placement may have contributed to the variability in neighbor-stranger discrimination observed in previous studies of the song sparrow and perhaps other repertoire species as well. This interpretation is consistent with data from another song sparrow population showing that half the territory takeovers are by immediate neighbors.

MEMORY DOES NOT CONSTRAIN INDIVIDUAL RECOGNITION IN A BIRD WITH SONG REPERTOIRES

Summary We tested the hypothesis that memory or perceptual limitations imposed by song repertoires contrain the ability of song birds to recognize their neighbours by song. Using operant conditioning procedures, we trained male song sparrows (Melospiza melodia) (median repertoire size = 8) to discriminate between two artificial song sparrow repertoires of 32 songs each (64 songs total). Both song sparrows learned to discriminate concurrently between all 32 song pairs. The birds learned later songs as quickly as they learned earlier songs. These results suggest that song sparrows are capable of memorizing the full song repertoire of their neighbours under natural conditions. In a second experiment we found that song sparrows readily generalize from one exemplar of a song type to other variations of that song type. We conclude that the evolution of song repertoires of song sparrows have neither constrained nor been constrained by individual recognition of neighbours by song.

Good neighbour, bad neighbour: song sparrows retaliate against aggressive rivals

Many territorial animals, despite being in direct competition for resources such as space, food and mates, show reduced aggression towards their neighbours. This situation is called the Dear Enemy effect. One explanation of the Dear Enemy effect is that it is due to a conditional strategy like Tit for Tat where territory holders cooperate by reducing aggression towards neighbours that also show reduced aggression, but retaliate against aggressive neighbours. Previous research found evidence for such a conditional strategy in migratory species but not in species with long-term association between neighbours, suggesting that long-term neighbours might be engaged in more ‘forgiving’ strategies. We tested this hypothesis in male song sparrows, Melodia melospiza, which are resident year-round in our population (leading to long-term associations between neighbouring birds) and display the Dear Enemy effect. We found that following a simulated intrusion by a neighbour, song sparrow males responded more strongly to playback of this neighbour than to playback of a neutral neighbour from their respective boundaries, consistent with a conditional retaliation strategy. We suggest that the primary effect of an intrusion by a neighbour might be to increase the perceived risk of cuckoldry by the intruding male, and increased aggression and vigilance towards this neighbour might be a strategy to prevent cuckoldry.

Song sharing in two populations of song sparrows (Melospiza melodia)

Abstract Sharing song types with immediate neighbors is widespread in birds with song repertoires, and sharing songs may confer a selective advantage in some cases. Levels of song sharing vary between different geographical populations of several bird species, and ecological differences often correlate with differences in singing behavior; in particular, males in migratory subspecies often share fewer songs than males in resident subspecies. The song sparrow (Melospiza melodia) appears to fit this pattern: resident song sparrows in western North America generally share 20–40% of their repertoire (of about eight songs) with each neighbor, while migratory subspecies from eastern North America often share 10% or less. We compared song sharing in two populations within a single subspecies of song sparrow (M. m. morphna) in Washington State. These populations, separated by only 120 km, nonetheless differ in migratory tendencies and several other ecological and life history variables. We recorded complete song repertoires from 11 male song sparrows in a high-elevation, migrating population at Gold Creek in west-central Washington, and compared them to two samples (n = 15 and n = 36) from a coastal, resident population at Discovery Park, Seattle, Washington. Despite major differences in habitat, population density, and migratory tendencies, song sharing among Gold Creek males was as high as that among Discovery Park males. In both populations, sharing was highest between immediate neighbors, and declined with distance. We conclude that at the within-subspecies level, neither migration nor population density affect song sharing in song sparrows, a song repertoire species.

Juvenile sparrows preferentially eavesdrop on adult song interactions

Recent research has demonstrated that bird song learning is influenced by social factors, but so far has been unable to isolate the particular social variables central to the learning process. Here we test the hypothesis that eavesdropping on singing interactions of adults is a key social event in song learning by birds. In a field experiment, we compared the response of juvenile male song sparrows (Melospiza melodia) to simulated adult counter-singing versus simulated solo singing. We used radio telemetry to follow the movements of each focal bird and assess his response to each playback trial. Juveniles approached the playback speakers when exposed to simulated interactive singing of two song sparrows, but not when exposed to simulated solo singing of a single song sparrow, which in fact they treated similar to heterospecific singing. Although the young birds approached simulated counter-singing, neither did they approach closely, nor did they vocalize themselves, suggesting that the primary function of approach was to permit eavesdropping on these singing interactions. These results indicate that during the prime song-learning phase, juvenile song sparrows are attracted to singing interactions between adults but not to singing by a single bird and suggest that singing interactions may be particularly powerful song-tutoring events.

Indirect reciprocity: song sparrows distrust aggressive neighbours based on eavesdropping

The evolution of cooperation between unrelated individuals has been a central issue in evolutionary biology. The main problem in most theories of cooperation is how a cooperative player selects individuals to ‘trust’ so that he does not get exploited by noncooperators. While early models emphasized the role of direct experience with individuals in deciding who to trust, more recent work has shown that individuals can eavesdrop on interactions between other individuals to identify cooperators and noncooperators. This second route to cooperation is called indirect reciprocity. In spatially structured populations with repeated interactions between players, both sources of information (direct experience and observed reputation) are readily available. Most models and empirical studies to date, however, have considered indirect reciprocity only in one-shot interactions when direct experience is not available. We examined the role of indirect reciprocity in the maintenance of mutual restraint in aggression (Dear Enemy cooperation) between territorial male song sparrows, Melospiza melodia. We found that territory owners eavesdropped on simulated defections by a neighbour (intrusions onto a third bird’s territory) and subsequently retaliated against these defecting neighbours. Taken together with our previous results, these results suggest that both direct and indirect reciprocity can be at work in repeated-interaction scenarios, and together lead to emergence of cooperation.

Soft song is a reliable signal of aggressive intent in song sparrows

Animals frequently use signals to modulate aggressive interactions. Establishing that a signal is aggressive or threatening requires demonstrating that it is more commonly used in agonistic contexts, that it predicts subsequent aggressive behaviors by the sender, and that receivers respond differently to this signal. Like many birds, song sparrows (Melospiza melodia) produce a low-amplitude “soft song” vocalization that has been hypothesized to be an aggressive signal. Soft song meets the first two criteria, but previous research has failed to demonstrate that soft song provokes aggression or that receivers even perceive soft song differently from normal loud song. We used a playback experiment with taxidermic mount presentation to test whether territorial male song sparrows respond differently to loud and soft song playbacks. Subjects reacted more strongly to the soft song playback by approaching the mount more closely, increasing wing wave displays, and increasing the proportion of their own songs that were soft songs, with further trends toward increasing the number of flights and attacks. These results confirm that soft song is a conventional signal of aggression in song sparrows and that increased receiver retaliation maintains its reliability.

What is the Function of Song Learning in Songbirds

In this paper we approach the question of the function of song learning in songbirds by addressing the more particular question of why a bird chooses the particular songs he does from among the many songs he hears during his song learning period. Our generalizations about song learning are derived from observations of a sedentary (nonmigratory) population of song sparrows (Melospiza melodia). We examine this question in the field, rather than in the laboratory, because we believe that the variables controlling song selection are social and ecological. We proceed from the hypothesis that song learning is an adaptive strategy, and attempt to identify the specific design features and overall function of this strategy in our particular study species. From our studies, we have identified several design features which, taken together, serve to maximize the number of songs the young bird will share with his neighbors, especially his near neighbors, in his first breeding season. Why should it be advantageous for the bird to have songs he shares with his neighbors? We suggest four possible, non-mutually exclusive advantages, and discuss the evidence in support of the fourth one. First, shared songs may be attractive to females. Second, shared songs may provide a mechanism by which two neighbors might effectively codefend their territories against other birds: each bird would effectively be mimicking the other while repelling prospective intruders. Third, shared songs may function as a “badge” of familiarity among territorial males; shared songs are a reliable signal of familiarity since they must be learned in the local neighborhood. Fourth, at least in song sparrows, shared songs appear to facilitate communication among neighboring birds. In particular, a bird uses the songs he shares with a neighbor to direct his song to that bird. We conclude by noting the paradox that songbirds have the ability to improvise new songs (demonstrated in lab experiments), yet in the field birds of most species faithfully copy the songs of their older neighbors. We suggest that song researchers need to ponder this paradox, and figure out why it is so important for a songbird to have the same songs as his neighbors.

Individual differences affect honest signalling in a songbird

Research in the past decade has established the existence of consistent individual differences or ‘personality’ in animals and their important role in many aspects of animal behaviour. At the same time, research on honest signalling of aggression has revealed that while some of the putative aggression signals are reliable, they are only imperfectly so. This study asks whether a significant portion of the variance in the aggression-signal regression may be explained by individual differences in signalling strategies. Using the well-studied aggressive signalling system of song sparrows (Melospiza melodia), we carried out repeated assays to measure both aggressive behaviours and aggressive signalling of territorial males. Through these assays, we found that aggressive behaviours and aggressive signalling were both highly repeatable, and moreover that aggressive behaviours in 2009–2010 predicted whether the birds would attack a taxidermic mount over a year later. Most significantly, we found that residual variation in signalling behaviours, after controlling for aggressive behaviour, was individually consistent, suggesting there may be a second personality trait determining the level of aggressive signalling. We term this potential personality trait ‘communicativeness’ and discuss these results in the context of honest signalling theories and recent findings reporting prevalence of ‘under-signalling’.