Vocal individuality in the roding calls of Woodcock Scolopax rusticola and their use to validate a survey method.

Abstract

The Woodcock Scolopax rusticola is a difficult species to survey in the breeding season because of its cryptic plumage and, with the exception of courtship flights, secretive behaviour. Sparse data from general bird surveys prohibit reliable estimation of population sizes and trends, and a new species-specific method to provide baseline information on population status and to enable reliable future monitoring is required. Counts of displaying, or 'roding', males at dawn or dusk potentially provide such a method but their value is unclear because it is impossible for an observer to distinguish different birds. We examined the vocal individuality of 39 roding males and were able to attribute calls to individuals correctly in 95% of cases on the basis of five parameters measured from spectrograms. Using calls, we determined the number of individual males at 43 sites. Roding activity differed between males at one site, with the two most active birds accounting for, on average, 55 and 27% of passes. There was no evidence of sequential roding by different males. We quantified the relationship between numbers of males and numbers of passes of roding birds during a 1-h period at dusk. This relationship was not affected by region, month, habitat or woodland size class. We conclude that counts of roding males provide a suitable index for monitoring populations of this species.