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

In experiments in which brood size is manipulated, it is impossible to dise ntangle the effects of changes in intensity and duration of parental care o n parental fitness from those of changes in the parental energy budget. To determine whether a change in parental energy budget affects the costs of r eproduction, we assessed the effects of increased levels of hopping activit y, independent of parental care, on later reproductive decisions in zebra f inches, Taeniopygia guttata. We imposed two levels of workload on individua l, nonbreeding birds. After the workload periods, the birds were allowed to breed in ad libitum conditions. Each bird went through three alternating p eriods of working followed by breeding. The birds were more active, but wei ghed less and ate less, during the high workload than during the low worklo ad periods. Clutch sizes and brood sizes did not vary with the previous wor k level. However, after a period on the high workload, reproduction, on ave rage, was started 6 days later than after a low workload period. Females ga ined weight just before egg laying, but this might have been;a result of eg g 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 b udget during the workload periods, delayed reproduction. We suggest that th ese observations on the effects of increased activity demonstrate a causal mechanism for reduced residual fitness of parents. (C) 1999 The Association for the Study of Animal Behaviour.