Advantages and disadvantages
This is sometimes the only possible method and produces minimal disturbance. It is often advisable to pilot the method before embarking on a full study because there is no guarantee that it will work in all situations. It is non-intrusive; this might be particularly useful in studying rare and endangered species. The disadvantages are that it requires a good deal of hardwork to collect highquality recording of birds that often live at low densities across widely scattered sites and to analyse and distinguish among calls. Ideally, one also needs an independent means of identification, such as marking or radio tracking, to corroborate the findings. It requires specialist and quite expensive equipment; it often only tells us about breeding males; and it can be very time-consuming unless the analysis is automated (Rebbeck et al. 2001). For some species, the calls of individuals may vary between years, even though they are consistent within years.