| Acoustic signaling is one of the important ways of information communication among animals,which contains rich biological meaning.Acoustic signal is widely concerned because of its long distance,high diversity and similarity with human language.Birds commonly use acoustic signals as a mean of communication.Many birds produce alarm calls to convey information of danger when they encounter threats.At present,the characteristics and the function of information transmission of alarm calls in passerine birds have been widely studied.However,little is known about the characteristics,plasticity,function,and impact factors of alarm calls during the reproductive period in raptors.In fact,raptors can also produce complex and functional communication calls under a variety of behavioral backgrounds,especially in the breeding season,they always emit courtship calls and alarm calls.Whether the alarm calls of raptors can change with the breeding stage,and also convey relevant information about the environment and whether the alarm information can be identified by offspring need to be resolved.In this study,common kestrel(Falco tinnunculus),a small-sized raptor,was taken as the research object,and Zuo Jia Nature Reserve was selected as the research area.The alarm calls of common kestrels during the breeding period were recorded using a portable recorder TASCAMHD-P2 with a strongly directed microphone Sennheiser MKH P48.The characteristics,plasticity and information transfer mechanism of alarm calls were studied by the methods of sound analysis,playback experiment and behavior observation.Our results indicated that the alarm call of common kestrels is a monosyllabic repetitive call composed of multi-harmonic arched frequency modulation,its maximum energy is distributed in the second harmonic.The acoustic characteristics of alarm calls during different breeding stages were significantly different.The frequency of alarm calls increased significantly as the breeding progressed.The duration and rate of alarm call increased during the whole breeding period,and the intensity of alarm call during the nestling period was significantly higher than that during the incubation period.The results of discriminant analysis showed that the alarm calls in the breeding period were significantly classified into different stages,indicating that the characteristics of alarm calls in different breeding stages were different which indicated that the parents could weigh the costs and benefits of defense to adjust the defense investment.In addition,the difference of acoustic parameters of alarm call depends on offspring number,indicating that the offspring number affects the characteristics of parents alarm call.When kestrels are under different threat levels,the acoustic characteristics of their alarm calls are significantly different.The duration of syllables increased significantly with the increase in threat level,while the intevel of calls decreased with the increase in threat level and increased the call duration.The frequency parameters of medium threat level were significantly higher than those of low and high threat level.As the threat level increases,the call rate increased accordingly.This suggested that parents will increase its nest defenses,sending a message about the urgency of the threat,when parent birds perceive a higher threat level.The results of playback experiment on nestlings showed that nestlings responded more to their parent’s alarm calls and that age had an effect on the intensity of nestlings’ responses.To sum up,this study showed that kestrel can adjust the acoustic characteristics of alarm calls,and changing the characteristics of alarm calls in nest defense according to the breeding stage and threat urgency which is helpful to weigh parental investment.The results of this study will promote the study of communication calls in non-songbirds,deepen the understanding of bird acoustic communication behavior,and provide a scientific basis for the conservation of raptor species diversity from the acoustic and cultural aspects. |
