Importance of sampling frequency to detect differential timing of migration: a case study with Blackcaps Sylvia atricapilla

Our aim was to assess to what extent detection of differential timing of passage in migrant birds depends on the sampling frequency (SF). We determined if Blackcaps Sylvia atricapilla performed differential migration by age and sex when passing over N Iberia. We mist netted and ringed migrating Blackcaps at a stopover site, both during the autumn (2005) and the spring (2006) migrations. During autumn, adult female Blackcaps passed later than juvenile birds and adult males. In spring, however, migration differed mainly by sex rather than by age. In particular, males passed over earlier than females, likely due to the pressure on males to earlier arrival at their breeding areas. A simulation based on daily captures at the Constant Effort Site showed that the ability to detect differential timing of passage was affected by SF, with P-values being higher with decreasing SF However, the proportion of variance explained by differences in timing of passage between sex or age classes was not affected by SF. This implies that at Constant Effort Sites a low SF may be adequate to detect differential timing of passage if a higher sampling effort per day (e.g. by sampling with more mist nets, and/or for a longer period) is maintained.