Animal communication can come in many forms and can have many different receivers. Sometimes, however, information is picked up by unintended receivers. These are referred to as eavesdroppers. If this eavesdropper is a predator, a sent signal can be quite costly. As a result, animals have evolved many strategies to manage the risk of predation
while signaling. “Soft song” is hypothesized to be one of these strategies. Just a quieter song than what a bird normally produces, soft song is thought to be used to convey aggressive intent while reducing the risk of eavesdropping by predators. However, there has yet to be any solid evidence of this being its purpose.
This experiment focused on aggressive calling and the use of soft song under the risk of predation. Song sparrows, a songbird native to North America, were used to determine whether males defending their territory against an intruder would adjust their signaling behavior to a perceived increase in the risk of predation. Their signaling system to convey aggressive intent to other conspecifics (birds of the same species) has two close-range signals that can be used to predict attack: soft songs and a rapid fluttering of wings, called wing waves.
Each test subject was presented with a playback of a song sparrow song within his own territory and his response was recorded. A recorded call from a heterospecific(belonging to a different species) was then played. Calls were from Cooper’s hawks, the song sparrow’s primary predator, or from a northern flicker, a species that does not prey upon the sparrows (used as a control). The conspecific playback was then resumed with the sparrow’s response again being recorded. The presence of the “hawk” was expected to increase the perceived risk of predation and cause the sparrow to change his signaling behavior. A decrease in loud songs and either an increase or no change in soft song was expected. This change was predicted to occur during the heterospecific playback and continue into the second conspecific playback. The authors also expected an increase in alarm calls during and after heterospecific playbacks.
When presented with a hawk call, sparrows sang far fewer songs, gave fewer wig waves, and spent more time producing alarm calls. The sparrows did not have this response toward the northern flicker call. These results indicate that song sparrows can recognize a Cooper’s hawk from its call and are able to, in response to this threat, alter their signaling behavior during aggressive interactions.
There was no significant difference between the amount of loud songs and soft songs before and after the heterospecific playback. However, the perceived risk of predation did have an effect on the proportion of time spent alarm calling, with more alarm calls after the hawk playback. These results do not agree with the hypothesis that the changes in signaling would persist during the second conspecific playback. Instead, the renewed challenge from the “intruder” is thought to perhaps be enough to incite a recovery of aggressive signaling behavior even under the risk of predation.
The results also failed to support the prediction that there would be an increase in soft songs and a decrease in loud songs under the risk of predation. There could have been an increase in soft song during the heterospecific playback, when there was no song classification, as that would have been when the playback would have had the strongest effect. However, the main effect of the hawk call appeared to be a cessation of calling in general, loud or soft.
This experiment is one of the first tests of the hypothesis that soft song is an adaptation to combat eavesdropping and, like previous experiments, there was no support found for this hypothesis. The main purpose of soft song is therefore unlikely to be predator eavesdropping avoidance. It may be instead that the song is used to avoid eavesdropping by other conspecifics or that a loud song could interfere with visual tracking of the opponent. Both of these alternative hypotheses are believed to merit testing.
This experiment not only shows evidence disputing a current hypothesis, but also adds to the rapidly growing body of evidence showing the ability of signalers to assess predation risks and adjust signaling behaviors accordingly. Song sparrows have been shown to be able to make flexible signaling decisions regarding the trade-off between the risk of predation and the need to protect one’s territory.
These findings bring sensory ecologists one step closer to understanding signaling behavior in both song sparrows and animals in general. Even humans encounter trade-offs in everyday life, and animals may have a thing or two to teach us about risk assessment.
Akcay, C., Clay, A., Campbell, S.E., Beecher, M.D. (2016) The sparrow and the hawk: aggressive signaling under risk of predation. Behavioral Ecology, 27, 601-607.