THE DEMOGRAPHY AND AGE-SPECIFIC ANNUAL SURVIVAL OF SONG THRUSHES DURING PERIODS OF POPULATION STABILITY AND DECLINE

1. The British song thrush (Turdus philomelos Brehm) population suffer ed a marked decline between 1975 and 1993, falling by 65% at an averag e rate of 5.7% per annum on farmland. We used ring recoveries to inves tigate whether this could have been brought about by changes in annual survival probabilities of fully grown birds. 2. The annual survival p robabilities of first-year song thrushes averaged 0.484 prior to the p opulation 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 t he variance in the logit-transformed estimates of first-year annual su rvival probability. Adult annual survival probabilities did not show a similar decline; they increased then decreased over the same period, with 25% of this variance, being described by a quadratic equation. 3. The changes in first-year annual survival probabilities alone were su fficiently large to account for the observed changes in population siz e. 4. First year annual survival probabilities were lower in years wit h cold (r(2) = 11%) or dry (r(2) = 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 win ters (r(2) = 34%), and together with the long-term trend, this account ed for 51% of the variance. These relationships with weather condition s explained part of the variation about the long-term trends but did n ot explain the trends themselves. 5. This study demonstrates that an I ntegrated Population Monitoring Programme can not only alert conservat ionists to declining species but also elucidate the demographic proces ses underlying population declines on a national scale.