Assessing population changes from disparate data sources: the decline of the Twite Carduelis flavirostris in England

Abstract

Conservationists often find it difficult to assess long-term population change in a species when the only data available are from disparate sources. This is especially the case when a range of survey methodologies and reporting units have been utilised and the results have been published in the grey literature. Although the production of a cohesive assessment of change may be a daunting task, in such circumstances, a sound assessment of change is often possible. We illustrate this by considering the decline of the Twite Carduelis flavirostris in England. Whilst there is evidence of decline, it is widely dispersed and the losses have yet to be formally documented. To assess longer-term change, we reviewed information available in county avifaunas and historical accounts of the status of the species throughout its former English range. Twite now only breed regularly in six of their 12 historical range counties, and in all of these six, the birds have declined markedly in abundance. We collated and analysed data drawn from a diverse range of surveys and county bird reports to assess more recent change and assessed contemporary distribution and abundance during our own surveys of breeding colonies in the South Pennines, an area supporting the last known nesting colonies in England. Combined, the data clearly indicate that the range and numbers of breeding Twite have declined considerably since the 1970s. Recent re-surveys in the South Pennines indicate a significant range contraction, with a loss of Twite from 83% of 1-km grid squares found occupied during a 1990 survey. A detailed resurvey of historical breeding sites in east Lancashire revealed a similar pattern, with 77% of sites having lost their colonies over the last four decades. We also constructed an index of change in numbers on the east and south-east coastal wintering grounds used by English breeding birds and this shows that numbers have severely declined since the 1970s, mirroring change on the breeding grounds.