Delayed cost of reproduction and senescence in the willow tit Parus montanus

Abstract

Summary We studied age‐specific survival rates in the willow tit Parus montanus in northern Finland using 15 years of capture–recapture data obtained from birds during the breeding seasons 1986–2000. In addition, short‐ and long‐term costs of reproduction were investigated by comparing survival probabilities of breeding and non‐breeding individuals. We did not find evidence supporting age‐specific survival probabilities in males. However, in females there was a significant decline in survival after the age of 5 years. Reproduction did not impair individuals’ chances of being alive in the subsequent year (short‐term cost) because breeding males and females had similar survival rates as non‐breeders. Demographic costs of breeding appeared later in life. Females skipping breeding earlier in life showed a higher probability of survival after the age of 5 years than females that bred every year until that age. This effect was non‐significant in males. The observed decline in survival probability late in life is likely to result from an increased cost of reproduction due to higher allocation of resources to breeding earlier in life, i.e. increased effort early in life is traded with survival late in life. The results also suggest that income breeders, such as small passerines, may pay long‐term costs of reproduction. This is in agreement with the disposable soma theory of ageing.