Rainfall on wintering grounds affects population change in many species of Afro-Palaearctic migrants

Survival and population growth rates in several species of Afro-Palaearctic migrant have been shown to correlate with rainfall regimes and/or vegetation level on their African wintering grounds, implying that seasonal food resources may contribute to population limitation. Here we explore the generality of this relationship across 16 migrant species breeding in England by investigating the proportion of variability in annual population change that is explained by the impact of environmental variables on wintering and staging grounds over 25- and 40-year time periods. In the 40-year time-series, rainfall in the arid Sahel region of West Africa had the strongest and most consistent effects on migrant populations, positively influencing the population growth rate in six of nine species which winter in this area and in three of seven species that use the region during their migratory passage but which over-winter further south. The effects of precipitation in other regions of Africa and satellite-derived measures of vegetation quality in all regions were weaker and less consistent in direction. Over the 25-year period for which data on both rainfall and vegetation were available, 12 of the 16 study species showed significant weather effects; the mean deviance explained by environmental variables among these 12 species was 32 %, increasing to 41 % when density-dependence was added to the models. For the 40-year time period, 11 species showed significant effects of rainfall, with the mean deviance explained by environmental variables of 14 % (23 % including density-dependence). Our results demonstrate that in many long-distance migrant species, precipitation in the Sahel is a significant driver of changes in abundance at the large-scale population level.