Nest Predation and Delayed Cost of Reproduction in the Great Tit

1. During a study on the great tit at Wytham Wood, Oxfordshire, nest predation by weasels was as high as 50% in some years prior to 1976. In 1976, nest-boxes were made virtually predator proof. 2. The change in nest-box type increased local recruitment rate and total population size of great and blue tits. We examined here the consequences of reduced nest predation on the survival rate of the great tit late in life. 3. Prior to 1976 (during the nest predation period), survival rate after the age of 5 years showed no decline and no relationship to the number of successful breeding attempts in either sex. 4. After 1976 (predator-proof period), both sexes showed increased mortality rates after the age of 5. However, only female survival was related to the number of successful breeding attempts, with a negative correlation. 5. This enhanced mortality late in life is likely to result from an increased cost of reproduction due to the combined effects of increased competition (higher adult density) and increased investment in reproduction (more young to raise because of reduced nest predation). These results fit predictions from life history theory, where increased effort early in life is predicted to trade with survival late in life.