Hard work impinges on fitness: an experimental study with zebra finches

In experiments in which brood size is manipulated, it is impossible to disentangle the effects of changes in intensity and duration of parental care on parental fitness from those of changes in the parental energy budget. To determine whether a change in parental energy budget affects the costs of reproduction, we assessed the effects of increased levels of hopping activity, independent of parental care, on later reproductive decisions in zebra finches, Taeniopygia guttata. We imposed two levels of workload on individual, nonbreeding birds. After the workload periods, the birds were allowed to breed in ad libitum conditions. Each bird went through three alternating periods of working followed by breeding. The birds were more active, but weighed less and ate less, during the high workload than during the low workload periods. Clutch sizes and brood sizes did not vary with the previous work level. However, after a period on the high workload, reproduction, on average, was started 6 days later than after a low workload period. Females gained weight just before egg laying, but this might have been a result of egg production. If so, the delay in reproduction did not simply reflect extra time needed to restore body mass after a period of increased activity. We conclude that the level of daily activity itself, or the resulting energy budget during the workload periods, delayed reproduction. We suggest that these observations on the effects of increased activity demonstrate a causal mechanism for reduced residual fitness of parents. Copyright 1999 The Association for the Study of Animal Behaviour.