Testing acoustic versus physical marking: two complementary methods for individual-based monitoring of elusive species

摘要

Individuals of some species differentiate each other on the basis of the acoustic features of their vocalizations, and this can be used in individual-based population monitoring studies. No research has tested for the effectiveness of individual marking through voice recognition as compared to traditional monitoring methods relying on physical marks. We compared voice recognition and physical marking using the Dupont's lark Chersophilus duponti as a study species. This bird needs to be attracted with playback in order to be seen (or captured). We first demonstrated that the territorial calls from a sample of banded males were individually distinctive and constant over time by means of discriminant function analysis, which correctly classified 100% of marked males. Then, we applied similarity techniques on call spectrotemporal features to define a threshold value of similarity within banded individuals, to be combined with qualitative spectrogram inspection for the classification of all recorded birds. Eventually, we compared the voice and the capture samples, to test for differences in relation to re-location rate, territory fidelity and dispersal movements both within and between years. Voice recognition was less time-consuming than capture-recapture method in the field, but it was useless for monitoring yearlings in call development stage. The two methods provided the same results in terms of territory fidelity and dispersal movements, but differed in re-location rates, which were significantly greater in the case of voice recognition method. By means of physical captures we possibly trapped a large sample of young and silent floaters, with low probability of recapture or recording. This mismatch between methods could bias the estimates of annual survival, which strongly depend on re-location rates. We suggest considering the two methods as complementary rather than alternatives for monitoring populations. Each technique offers unique information, and the two sources should be combined to provide correction factors that would eventually sharpen our knowledge on bird population ecology.