Risk-based alarm calling in a nonpasserine bird

Abstract

Recent studies have demonstrated the presence of risk-based variation in alarm calling in many vertebrate species. Most of the studies on birds, however, have focused on passerine systems perhaps due to the traditional view that passerine vocalizations are more complex. We investigated the presence of a risk-based alarm calling system in a nonpasserine, the herring gull, Larus argentatus, by recording birds presented with varying degrees of threat, and experimentally exposing and quantifying responses to manipulated alarm calls. We found that herring gulls communicate threat urgency in their alarm calls using both frequency and time parameters. Sound recordings indicated that herring gulls change centre frequency modulation patterns of their alarm calls, generate frequency discontinuities in notes and increase the rate of calling with increases in perceived threat level. Playback experiments showed that conspecifics pay attention to both frequency and time parameters and respond most urgently to playbacks of high-threat calls at a high call rate. A less urgent response to high-threat calls at a low call rate and to low-threat calls at a high call rate suggests that threat urgency information is reinforced by both call type and call rate in the herring gull system. This study is one of the first demonstrations of a risk-based alarm calling system in a nonpasserine.