Abstract
ABSTRACTAcoustic monitoring of birds is developing rapidly as equipment, methods and analyses improve. However, most population monitoring studies still utilise traditional techniques like mark-recapture or line transects. Previous researchers used vocal recordings of male European nightjars, Caprimulgus europaeus, to identify individuals, finding that four acoustic parameters correctly assigned 98.5% of calls to individuals. We tested their methods on a population of European nightjars recorded over two successive breeding seasons and found that percentage of males correctly classified within a season reached a maximum of 73.5%, rising to 75% if full-length calls and 13 acoustic parameters were used. We tested whether males could be re-identified over a two-year period and found that only 20% of calls were assigned to the same putative territorial individuals, despite separate ringing data showing that males can maintain site fidelity for up to eight years. Our results indicate that the characteristics of male nightjar vocalisations may alter over time. We therefore recommend that vocal discrimination be used in conjunction with existing monitoring techniques when surveying for population monitoring, that as many call parameters as possible are used and that recording for automated presence/absence surveys takes place over a short time-frame.
Highlights
Birds of many species use auditory cues to recognise other individuals (Halpin 1991)
Peake et al (1998) found that by adding features of fine temporal structure, such as pulse to pulse duration, to their analysis they were able to improve classification from 87% to 100%
Our results indicate that vocal individuality as a monitoring technique is not currently suitable for use in longer-term studies, as variation in call parameters over time may cause misidentification, possibly leading to overcounting which could have negative consequences for nightjar conservation
Summary
Birds of many species use auditory cues to recognise other individuals (Halpin 1991). Successful individual recognition using vocalisations requires low intra- and high inter-individual variation (Falls 1982). This type of individuality in acoustic signals has been identified to some degree in most bird species investigated so far (Peake et al 1998; Terry et al 2005; Grava et al 2008; Li et al 2017; Zdenek et al 2017). Identification from vocalisations can provide a useful non-invasive means of monitoring birds that are normally difficult to observe (Terry et al 2005)
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