The woodcock survey requires an observer to record the number of woodcock ‘passes’ seen within a fixed time frame. But by counting just the number flypasts it is not immediately clear how many different individuals have been observed and how this relates to overall population size.
In order to use these counts of roding males as a measure of abundance, it is important to establish the relationship between numbers of individual males and numbers of observations, and to check for differences between regions and habitats. We were able to devise a calibration equation that could provide us with reliable estimates of true population size based on the numbers of birds seen.
This calibration equation was devised by the GWCT following a study into the vocal individuality of male woodcock. At a sample of 47 sites, about half in Durham and Northumberland and half in Hampshire and Wiltshire, we recorded the calls of all displaying birds. From these recordings, sonograms were created. Sonograms are plots of call frequency against time that can be used to measure different vocal characteristics. Based on differences in the 'squeak' element of the call, we found that individual males had distinctive, recognisable calls. This meant that we could plot the number of separate individuals against the number of recordings or the number of roding observations at each site.
As expected, the higher the number of woodcock passes, the higher the predicted number of displaying males. We were able to produce a predictive equation for converting the number of woodcock passes into an estimate of local abundance. To account for regional differences in the length of the dusk roding period and hence potential differences in the slope of the relationship, for the national surveys we limited the count period to one hour.
Figure 1: Sonogram of a typical woodcock roding call, consisting of three low frequency ‘croak’ elements followed by a much higher frequency ‘squeak’ note
Figure 2: Examples of ‘squeak’ notes from four different woodcock