Territorial Songbirds Research Articles

In a songbird, the black redstart, parents use acoustic cues to discriminate between their different fledglings

Several studies on parental investment in territorial songbirds have reported the existence of brood division. This is a type of postfledging care in which each parent has long-term feeding preferences for different young within a brood, creating two family units. Recent theoretical work indicates that conflicts between individuals should select for brood division. However, little is known about the mechanisms involved in the onset and maintenance of this behavioural strategy. Given the high rate of fledglings' begging calls, we hypothesized that acoustic discrimination could explain the stability of feeding preferences at a proximate level. In a 3-year field study, we recorded the responses of parent black redstarts, Phoenicurus ochruros, a territorial songbird, to playback of the begging calls of fledglings fed by the male and by the female. Parents responded more to the calls of the fledglings that they preferentially fed. A principal component analysis of the calls suggested that parents may recognize individual offspring. To our knowledge, this study provides the first evidence that a bird can acoustically discriminate between two categories of its own offspring: those that it preferentially feeds and those fed by the other parent.

Territorial responses of male blue tits to simulated dynamic intrusions: effects of song overlap and intruder location

Territorial songbirds frequently challenge each other with intrusions and close-range singing interactions. The timing of songs in such interactions may signal aggressive intentions and experiments have shown that song overlapping may indicate a higher willingness to escalate an interaction than song alternating. A territory owner should therefore perceive an intruder overlapping its songs and staying inside its territory as a greater threat than one alternating and exiting the territory quickly. To test this hypothesis we used playback to interact with territorial male blue tits, Parus caeruleus, by varying the amount of overlap of a subject's song; we also used multiple loudspeakers to simulate an intruder changing song posts. The territory owners approached and followed simulated intruders that changed song post inside their territories more slowly when more of their songs had been overlapped at the first loudspeaker, and they did not follow an intruder outside their territory. Nevertheless their responses remained high throughout playback and interaction terms between proportion of songs overlapped and loudspeaker position inside or outside the territory indicated that male blue tits combined and used information from both singing and intrusion patterns.

Strategies that facilitate or counter eavesdropping on vocal interactions in songbirds.

Most territorial songbirds live in communication networks where eavesdropping on vocal interactions may constitute an important option for gathering information regarding the relative dominance, condition and quality of individuals being eavesdropped upon. The relatively low costs and risks of eavesdropping, together with the obvious advantages of gaining such comparative information about other individuals, predict eavesdropping to be a widespread phenomenon even though it is not necessarily advantageous for the participants to be eavesdropped upon. Special eavesdropper strategies that facilitate eavesdropping (how best to eavesdrop) may therefore have evolved together with strategies for interacting that either co-facilitate (public signaling) or counter eavesdropping directly (private signaling) or indirectly by preventing any subsequent negative consequences of being eavesdropped upon (anonymity). This paper reviews the predictions for the strategies and also gives examples supporting their use by territorial songbirds in connection with vocal interactions.

Conspecific Attraction and the Conservation of Territorial Songbirds

Abstract: Conspecific attraction, the tendency for individuals of a species to settle near one another, is well described in colonial species, especially birds. Although this behavior may occur in territorial birds, evidence has been lacking. If territorial birds do exhibit this behavior, it would have major conservation implications. Birds could potentially be attracted to specific sites with artificial stimuli, making conservation of those species more efficient. In 2001 and 2002, we tested whether conspecific attraction occurs in an endangered, territorial songbird, the Black‐capped Vireo (Vireo atricapilla) by playing vireo vocalizations in unoccupied habitats at Fort Hood, Texas. We were successful in attracting 73 birds to five experimental sites in 2001 and 75 birds to seven experimental sites in 2002. No birds settled on comparable control sites. Many birds attracted to the vocalizations paired and bred. At most research sites the primary threat to the species, the brood‐parasitic Brown‐headed Cowbird (Molothrus ater), was controlled, allowing vireos to achieve high nesting success relative to a nearby, unmanipulated population. Second‐year birds were more responsive to conspecific vocalizations than older birds, as they were more common on experimental sites than in the established population. In 2002 birds recolonized experimental sites from 2001 where vocalizations were not played in 2002, indicating that 1 year of playbacks may be sufficient to establish a population. Our results provide the first experimental evidence that territorial songbirds use the presence of conspecifics when deciding where to settle and suggest that conspecific attraction may provide a valuable conservation tool.

The effect of vegetation density on male mate guarding and extra-territorial forays in the yellow-breasted chat (Icteria virens).

Extra-pair paternity is widely documented in birds, but the behaviors that lead to extra-pair copulations (EPCs) have been less well studied, particularly in territorial songbirds. We studied the behaviors associated with extra-territorial forays (ETFs) and male mate guarding in a socially monogamous, but genetically promiscuous, neotropical migrant passerine, the yellow-breasted chat (Icteria virens). Focal observations and radiotelemetry revealed that both males and females engaged in ETFs. 65% of the females in our study engaged in at least one foray onto a neighboring territory. 50% of males also were observed engaging in ETFs, but males were much more conspicuous during these intrusions compared to females. Females preferred to remain close to the ground in areas where vegetation was the densest. Female behavior was difficult to observe during ETFs but females sometimes interacted with neighboring extra-pair males. Males attempted to guard their mates by remaining close to them and following them during extra-territorial intrusions. We observed instances in which a male attacked his mate and appeared to herd her back to his territory. However a male's ability to maintain close proximity to his mate was significantly and negatively correlated with vegetation density. Our results suggest that the behaviors which lead to extra-pair encounters are influenced by the behavior of all participants and are modified by the characteristics of the habitat.

Male–male vocal interactions and the adjustment of song amplitude in a territorial bird

Recent studies have shown that territorial songbirds do not maximize vocal amplitude. Instead, song intensity appears to be a flexible trait that is individually regulated. Given the benefits of singing loudly with regard to signal transmission in the context of territory defence and mate attraction, we investigated whether songbirds adjust the sound level of their territorial songs depending on social influences. While interacting with the playback of a simulated conspecific rival, seven male nightingales, Luscinia megarhynchos, increased the sound level of their songs, on average, by more than 5 dB. The nightingales increased their song level to significantly lower values (about 1 dB) in response to control songs of other species, so the revealed increase in vocal intensity during interactions cannot be fully explained by the acoustical masking of the interfering conspecific songs. Thus, the birds improved signal transmission to a higher degree during conspecific male–male interactions.

Countersinging as a signal of aggression in a territorial songbird

Neighbouring territorial songbirds often interact through countersinging, where birds sing in response to the singing of neighbours such that their song bouts are temporally related. Complex forms of countersinging such as song type matching or song overlapping appear to be correlated with aggressiveness and readiness to escalate confrontations. Less attention has been paid to the importance of simpler forms of countersinging, where matched song types are not used and where individual songs do not temporally overlap. I examined countersinging behaviour in male Carolina wrens, Thryothorus ludovicianus, which countersing regularly. Why they countersing and how countersinging is perceived by neighbours is unknown. By comparing singing behaviour before and after simulated intrusions, I determined that subjects countersing with their neighbours more readily when highly aroused. Comparing responses to countersinging and noncountersinging playbacks showed that countersinging elicited more aggressive responses than did noncountersinging. Carolina wrens appear to exchange aggressive signals regularly through countersinging. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Song learning by chaffinches: how accurate, and from where?

Geographical variation in song is a common feature of populations of territorial songbirds. However, the factors responsible for the levels and patterns of variation have not been examined in detail. We measured geographical variation of songs in chaffinches, Fringilla coelebs, in samples from five locations in Scotland, U.K. We found substantial variation in the level of song sharing between locations. It was highest in isolated island areas and in a recently populated area. By comparing our data with a spatial simulation model designed to mimic the different areas, we calculated estimates of cultural mutation that ranged between 0.001 and 0.03, with a maximum possible value of 0.05, and found that chaffinches had a significant preference for learning songs not from immediate neighbours, but from males about 500m away. We discuss the implications of these findings for the process and function of cultural transmission of song in the chaffinch. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Noise-dependent song amplitude regulation in a territorial songbird

Some animals that use sound to communicate compensate for interference from background noise by adjusting the amplitude of their vocalizations as environmental noise levels vary. Territorial songbirds may have evolved a different strategy, since they can be expected to benefit from maximizing the amplitude of their songs to defend territories and attract females. We tested this hypothesis with calibrated measurements of the song level of male nightingales, Luscinia megarhynchos. All birds increased the sound level of their songs in response to an increase in white noise broadcast to them. A second experiment revealed that noise in the spectral region of their own songs was most effective in inducing the birds to increase vocal intensity. These findings show that nightingales do not maximize song amplitude but regulate vocal intensity dependent on the level of masking noise. The adjustment of vocal amplitude may serve to maintain a specific signal-to-noise ratio that is favourable for signal production. Concurrently, increasing the intensity of songs can maintain a given active space for communication. Thus, vocal amplitude in a territorial songbird can be interpreted as a flexible trait, which is individually regulated according to ecological demands from signal transmission. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Coding in the song of the wren: importance of rhythmicity, syntax and element structure

Communication between territorial songbirds usually involves a transfer of encoded information over long distances. We would expect coding and decoding strategies to be adaptive given the constraints imposed by the habitat. We used playback to examine some song parameters important for information transfer in the wren, Troglodytes troglodytes. Six stimuli were tested with various modifications to song rhythmicity, song composition, element structure, syntax and overall song spectra. Song features encoding information essential for eliciting a territorial response seemed to be embedded in the fine structure of song elements, that is, their variations in frequency and amplitude over time. To function, this strategy must be flexible enough to accommodate the deleterious effects of habitat-induced degradation. All stimuli composed of original song elements elicited responses regardless of their other alterations. Rhythmicity seemed not to be essential for eliciting territorial behaviour. However, alterations in song rhythmicity, syntax and spectra affected territorial responses, suggesting that these nonessential song parameters do contribute to the options for discrimination.

Ranging of songs with the song type on use of different cues in Carolina wrens: effects of familiarity

Male territorial song birds are usually spaced far apart and most often hear conspecific song after it has been degraded by propagation through the environment. Their ability to use the degradation of songs to assess the distance of a singing rival without approaching (called ranging) presumably increases the efficiency of defending a territory. In order to assess degradation in a song the receiver needs to compare the characteristics of the received song to its characteristics at the source or at different distances. Earlier experiments on ranging in species with song repertoires have suggested that prior familiarity with the particular song type is necessary for ranging. Here I show that male Carolina wrens (Thryothorus ludovicianus) can use either temporal or spectral characteristics for ranging song types which they were unlikely to have heard previously. Playbacks consisting of only one song prevented subjects' close-range experience with the loudspeaker, and flights beyond the loudspeaker provided direct evidence for over-assessment of distance when songs were degraded. Because ranging of songs was not affected by the degree of familiarity with the song type, this experiment provides no evidence that song repertoires hinder ranging in Carolina wrens, as suggested by Morton's ranging hypothesis. Instead, at least approximate ranging of songs is evidently possible by assessment of degradation in general features of a species' songs.

Tit for tat among neighboring hooded warblers

The “dear-enemy” relationship of territorial songbirds could be mutually beneficial to neighbors, as males who recognize neighbors and reduce their responses to these neighbors would require less time and energy for territorial defense. In order for this relationship to be evolutionarily stable, this reduction in response to a neighbor must be conditional on reciprocal restraint by that neighbor. This study examined the possibility of such conditional responses in hooded warblers (Wilsonia citrina). Responses of territorial hooded warblers to playbacks of neighbors' songs from shared boundaries were measured before and after playbacks that simulated intrusions of those same neighbors (NNNN treatment) or strange birds (NSSN treatment) into the subjects' territories. Each male received both treatments separated by at least 8 days. Males increased their responses to playbacks of a neighbor's songs at the boundary after simulated intrusions of that same neighbor (NNNN) but did not increase their responses to such playbacks after simulated intrusions of strangers (NSSN). This increased response to a “defecting” neighbor suggests that the relationship between neighboring territorial hooded warblers is based on a conditional strategy like tit-for-tat.

Signalling in territorial systems: a context for individual identification, ranging and eavesdropping

Territorial systems are characterized by the relative longevity and stability of interactions between neighbouring individuals. Two abilities of signal receivers that can be seen as adaptations increasing the efficiency of territory defence will be discussed: identifying neighbouring individuals and ranging (i.e. determining the distance to) signallers. The costs involved in such discriminations will also be outlined. Although signalling has been traditionally considered as occurring between two individuals (signaller and receiver), long-range signals will be received by many individuals. In territorial systems a group of neighbours could be considered as a communication network: consisting at its simplest of a signaller and a number of receivers. The scope for low cost, low-risk information gathering in such networks by eavesdropping will be discussed with particular reference to territorial songbirds and electric fish.

Long-term memory of individual neighbours in a migratory songbird

CAPABILITIES for long-term memory and recall of information have evolved in non-human animals primarily for special requirements such as for learning species-typical vocalizations and caching food1–6. Long-term memory of individual social partners has, however, not been demonstrated previously for non-human animals. The ability to recognize individuals has important consequences for the evolution of intricate social interactions7–11 and provides a basis for more sophisticated forms of cognition in animal societies12,13. Recognition of social partners has been documented for territorial songbirds, which discriminate between songs of different neighbours14–16 as well as between the songs of strangers and neighbours17. Here I show that male hooded warblers ( Wilsonia citrina, Parulinae) not only recognize their neighbours individually by song during the breeding season, but also retain the memory of neighbours' songs after an 8-month period during which they cease singing and migrate to Central America before they return to former breeding territories.