The demography and age-specific annual survival of song thrushes during periods of population stability and decline

Abstract

1. The British song thrush (Turdus philomelos Brehm) population suffered a marked decline between 1975 and 1993, falling by 65% at an average rate of 5.7% per annum on farmland. We used ring recoveries to investigate whether this could have been brought about by changes in annual survival probabilities of fully grown birds. 2. The annual survival probabilities of first-year song thrushes averaged 0.484 prior to the population decline (1962-75), but dropped to an average of 0.405 in the years 1975-93. A long-term non-linear decrease accounted for 32% of the variance in the logit-transformed estimates of first-year annual survival probability. Adult annual survival probabilities did not show a similar decline; they increased then decreased over the same period, with 28% of this variance being described by a quadratic equation. 3. The changes in first-year annual survival probabilities alone were sufficiently large to account for the observed changes in population size. 4. First year annual survival probabilities were lower in years with cold (r2 = 11%) or dry (r2 = 20%) winters and together with the long-term decrease these effects explained 49% of the variance in the logit-transformed estimates of first-year annual survival probability. Adult annual survival probabilities were lower in years with cold winters (r2 = 34%), and together with the long-term trend, this accounted for 51% of the variance. These relationships with weather conditions explained part of the variation about the long-term trends but did not explain the trends themselves. 5. This study demonstrates that an Integrated Population Monitoring Programme can not only alert conservationists to declining species but also elucidate the demographic processes underlying population declines on a national scale.