A test of the eavesdropping avoidance hypothesis as an explanation for the structure of low-amplitude aggressive signals in the song sparrow

Abstract

Low-amplitude signals function in private exchanges of information between signalers and nearby receivers. The eavesdropping avoidance hypothesis proposes that selection favors quiet threat signals in order to avoid the costs of eavesdroppers. If true, then selection should favor other acoustic traits in addition to low amplitude that lead to quiet signals transmitting less effectively through the environment compared to broadcast signals. The “warbled” soft songs of male song sparrows differ from “crystallized” soft songs and from broadcast songs in a number of acoustic traits, suggesting that these songs may transmit less effectively. We tested this prediction in a field experiment by playing broadcast songs, crystallized soft songs, and warbled soft songs through a loudspeaker at the same amplitude and recording the propagated songs at five distances, at two heights, and in two different habitat types. Counter to our prediction, we found no evidence that either form of soft song transmits differently than broadcast song when all were played loudly. If anything, soft songs transmitted more effectively when all songs were played quietly. Our results do not support one prediction made by the eavesdropping avoidance hypothesis, although the possibility remains that reduced amplitude alone is sufficient to reduce eavesdropping. The question of why warbled soft song differs in acoustic structure remains unresolved.

Significance statement

Quiet aggressive signals are a puzzle because their meek form seems counter to their purpose—to threaten and intimidate rivals. One explanation for quiet signals is that reducing the signal’s transmission range reduces the costs imposed by eavesdroppers, which predicts that quiet signals will transmit through the environment poorly relative to louder, broadcast signals. We tested this prediction by playing song sparrow “soft songs” and “broadcast songs” at the same amplitudes and measured their transmission properties at different distances and heights, and in two habitats. We found that soft and broadcast songs did not transmit differently when played loudly, and soft songs transmitted slightly better when played softly. Our results do not support the idea that quiet signals are explained by eavesdropping avoidance, at least not in the song sparrow.