Although a quarter of Europe’s breeding bird population crosses the Sahara on spring and autumn migration, when, where and how species prepare for and recover from this difficult and dangerous part of their journey remains poorly understood. Recent work by the Wetland Trust and the BTO has used ringing datasets from Portugal and Senegal to explore these questions in the Grasshopper Warbler. The data suggest that while many individuals use sites in Portugal in autumn and in Senegal in spring, they also stop and refuel at unknown sites in North Africa. Fuelling in Senegal for the north-bound migration was slow relative to rates recorded in Portugal, and likely to be constrained by food availability during the Sahelian dry season. A knowledge of the stopover sites used by migratory species is vital for their conservation, at a time when many trans-Saharan migrants are in decline.

1. Many patterns in macroecology are closely related to the total abundance of a species in a region. Here we show that interspecific differences in the pattern of population fluctuations of British bird species can be predicted from knowledge of their overall abundance and some basic life-history characteristics. 2. We identify a rarity syndrome that arises through an increased stochastic influence on population fluctuations with decreasing population size, mainly resulting from an inverse density-dependent effect of demographic stochasticity. This syndrome involves an increase in the annual changes in population size with increasing rarity in the United Kingdom. 3. The relationship between the magnitude of temporal variation and local mean population size differs between species dependent on their life history, i.e. species with larger clutch size and lower survival tended to have larger annual changes in population size than low-reproducing long-lived species. 4. The probability of local disappearance from a study plot depended on the population size and was hence closely related to the overall abundance of the species in UK. For a given population size, this probability was also related to species-specific life-history characteristics, being higher in species with larger clutch sizes and smaller survival rates. 5. Rareness results in a spatial decoupling of the temporal variation in population size. 6. These patterns show that once a species has become rare, e.g. due to human activities, key population dynamical characteristics will change because of density-dependent stochastic effects, which in turn are dependent on species-specific life-history characteristics.