Cetti's Warbler Cettia cetti:: analysis of an expanding population

Capsule Productivity in the UK Cetti's Warbler population is constant, but overwinter survival has become increasingly dependent on winter temperatures. Aims To demonstrate how constant effort (CE) ringing can be used to estimate productivity of breeding populations, using the expanding Cetti's Warbler population as an example. Methods A logistic generalized linear model is developed from an established method of estimating abundance to generate an annual index of productivity from CE catches. Dispersal of birds from ring-recapture data is modelled using a simple exponential model of dispersal. Results The number of Cetti's Warblers on CE sites has increased markedly, particularly since 1998, but productivity has remained largely constant. The rate of population increase has slowed in recent years, in part because of an increasing sensitivity of the population to cold winters. Juveniles disperse further than adults, and the distance over which they disperse has increased. Conclusions CE catching provides a way of easily measuring whole-season productivity. The future of Cetti's Warblers in Britain looks secure, but the population may be approaching its maximum size under current conditions.