Senescence, the decline in reproductive value with age, is well documented in natural systems, but the underlying mechanisms remain an enigma. A decline in parental effort with age potentially explains reproductive senescence, but age effects on parental effort have been little investigated. I measured daily energy expenditure (DEE) of great tits raising offspring using doubly labelled water. Independent of sex, DEE declined with age. This correlation would also arise when individuals with high energy expenditure are more likely to die, but we have previously shown that survival was independent of DEE. The observed decline in DEE with age can therefore be attributed to changes within individuals over time. Although DEE declined with age, provisioning rate was independent of age in the same dataset, and neither did age significantly predict number and growth of offspring. I discuss potential explanations, and stress that variation in reproductive success, insofar as it reflects provisioning offspring, is an emergent property of parental effort and foraging efficiency. Depending on the relative strength of age effects on effort and efficiency, reproductive success can either decrease, increase or remain unchanged with increasing age. In our study, a negative effect of age on the capacity to work may have been offset by an increase in efficiency, yielding on balance no change in reproductive output.
Simon Verhulst (Thu,) studied this question.