Long-term variation in survival rates of Sand Martins Riparia riparia: dependence on breeding and wintering ground weather, age and sex, and their population consequences

Abstract

Capsule Survival rates of Sand Martins are driven by variation in rainfall on the wintering grounds prior to their arrival and in the breeding area in the previous summer; inter-annual changes in abundance also show strong correlations with survival. Aims To investigate the demography underlying long-term changes in population size in a large study area in central England, focusing primarily on describing the variation in survival and identifying its causes and demographic consequences. Methods An intensive mark–recapture effort was undertaken on Sand Martin colonies in an area of over 400 km2 of Nottinghamshire, England, from 1967 to 1992. The resulting data were analysed using the program MARK to investigate variation in survival with respect to sex, age, time and variables describing the key climatic conditions on the breeding and wintering grounds. The same variables were also tested against data on the age-structure of the population. Results The average annual apparent survival probabilities of adult males and females were not significantly different (males 0.312 ± 0.026 se; females 0.289 ± 0.026 se). There was no obvious long-term temporal trend in survival rates, but considerable fluctuations occurred between years. There was a positive relationship between annual survival rates and Sahel rainfall, but a stronger, negative, relationship between survival and the previous summer's rainfall in the study area. These relationships were reflected in correlations with the annual population level and the annual percentage change in the population, but other evidence suggests that rainfall on the breeding grounds during the breeding season has a further important effect on productivity. Conclusion The Sand Martin population in the study area was limited by climatic conditions, primarily rainfall in the breeding area, but also rainfall on the wintering grounds. These climatic effects operated mostly by influencing annual survival, but also probably affected productivity. This contrasts with published results for Sand Martins in central Europe, which have been found to be more strongly affected by rainfall on the wintering grounds and for which relationships between survival and abundance are obscured by fluctuations in immigration and emigration. The importance of rainfall on the breeding grounds as a negative influence on over-winter survival is a novel finding both in Sand Martin biology and in the context of the environmental influences on passerine demography in general.