The deterrent effect of bird song in territory defense

Abstract

Using the responses of territory owners to playback to infer the territorial function of acoustic signals is common practice. However, difficulties with interpreting the results of such experiments have obscured our understanding of territorial signaling. For instance, a stronger response to playback is often interpreted as more aggressive, but there is no consensus as to whether this should be in response to the least or most threatening simulated intruder. Rather than following a gradual increase or decrease, the relationship between signal intensity and response strength may instead describe a peaked curve. We manipulated banded wren (Thryophilus pleurostictus) songs to simulate low, median and high performance singers and used these songs as stimuli in playback experiments. Banded wrens were less likely to approach the high performance stimulus compared to the low and median performance stimuli. However, the birds that did approach the high performance stimulus sang more than those that approached the low performance stimulus. In addition, birds were more likely to match the songs when exposed to the median and high performance stimuli compared to the low performance stimuli and song matching predicted approach behavior. These results are in accordance with theoretical models of aggressive encounters in which low performance opponents are challenged without further assessment. Median and high performance opponents however, may require further assessment and the latter may be perceived as too intimidating for approach.

Responses to song playback vary with the vocal performance of both signal senders and receivers

“…Schmidt et al [24], working with European nightingales (Luscinia megarhynchos), reported results similar to those for banded wrens [23], and also observed that males that remained unpaired during that breeding season showed low responses to fast trill playback, whereas males that did later pair with females maintained aggressive responses [24]. In another playback experiment on banded wrens, however, males were found to respond less aggressively to high-performance songs than to medium-performance songs [14]. These studies and others suggest that high-performance trills elicit wide-ranging responses, varying from increased aggression to avoidance.…”

“…Traditionally, researchers predicted that playback of threatening signals should result in defensive responses proportional to the level of threat signals represent [42], a prediction supported in empirical studies including some that have focused on trilled vocalizations [25]. More recent studies, however, report circumstances in which high-performance signals elicit less of an aggressive response than do signals of lower or more intermediate levels of performance [14,22]. A possible explanation for this outcome is that intruders signalling at particularly high-threat levels might present too great a risk for a territory holder to engage in conflict [13,14].…”

“…The x-axis represents the level of threat signalled by the intruder as the level of vocal performance, and the y-axis represents the aggressive response intensity of the receiver (the territorial male). Response intensity has recently been modelled as a parabolic function, to account for the expectation that territorial males should avoid rather than confront intruders with high-threat signals (solid line, from de Kort et al [14], modified from Collins [13]). We extend this model by accounting for expected variation in the quality of territorial subjects, here reflected in the vocal performance of the songs of playback subjects.…”

“…By contrast, receivers of relatively low quality might be expected to avoid or retreat from high-threat competitors to prevent costly conflict and risk of injury, even it if means giving up an established territory. Operationally, in playback studies in which high-quality intruders are simulated, one might expect to see both types of responses, with some birds attacking and others retreating [13,14].…”

What do territory owners defend against?

“…Although there is in many cases, a certain amount of information exchange between intruders and owners (e.g. [8,95,96]), we do therefore not know how well owners and intruder can predict each other's behaviour [97]. Limited information on both sides has not only the potential to drastically change the optimal behaviour for both parties [37,39] but it might make it impossible to separate different types of conflicts.…”