Population Trends in the Steppe Birds of Castro Verde in the Period 2006-2011: Consequences of a Drought Event and Land Use Changes?


Castro Verde is the main area of cereal steppes in Portugal (ca. 80000 ha), having international importance for several steppe bird species with unfavourable conservation status. In spring 2006, a large-scale census of bird populations in the region was carried out. In the current paper, we (a) update the 2006 population estimates using correction for detectability, (b) report the results of a second census carried out in 2011, (c) explore the likely impacts of the 2005 drought event and land use changes on the observed population trends in the period 2006-2011. Correcting for the detectability bias of the 2006 counts resulted in an increase in population estimates ranging from 6% in the Stone Curlew to 210% in the Zitting Cisticola (mean= 75.4%, median = 61.4%, n=14 species), giving further significance to the importance of Castro Verde for the conservation of steppe birds in Portugal and Europe. Overall, between 2006 and 2011, species frequencies increased an average of 60% (median= 14%, range=-52.9% to 440.7%, n=14 species) and bird abundance increased 66% (median= 22.5%, range=-76.5% to 559.6%, n=14). This strongly suggests that bird populations in 2006 were still suffering the impact of the 2005 drought, and that the situation in 2011 is more representative of an average climatic context. The species registering the strongest population increases were the ones associated with cereal fields, in spite of the decreased availability of this habitat. This suggests that the 2005 drought was the main driver of a significant population crash and of the subsequent increasing population trend in spite of the ongoing habitat loss for this set of species.