Within brood trade-offs in reproductive effort: An experimental study on the common tern: Sterna hirundo

The optimal allocation of effort during reproduction is a key component of life history theory, with trade-offs predicted to operate both within and between reproductive attempts. Experimental work in this field has largely concentrated on the latter. The need to partition investment between the different phases of the current reproductive event, and how this varies between individuals, has received little empirical investigation. The primary aims of this project were to investigate the capacity of birds to adjust their reproductive effort in response to increasing demand, and to investigate within brood trade-offs between the different phases of the reproductive attempt. To do this the project involved experimental manipulation of effort (within the natural range) at the stages of egg production, incubation and chick rearing. Only those pairs that were given a free chick (incurring chick rearing costs only) were able to rear a significantly larger brood than unmanipulated controls. When parents incurred the full costs (egg production, incubation and chick rearing) of producing additional young to their intended clutch size, their capacity to rear an enlarged brood was negated. A within clutch trade-off was identified between producing and rearing extra young, with experimental parents subsequently showing reduced chick provisioning, growth and survival. In an experiment to increase the costs of incubation alone, which have often been considered relatively trivial, study birds showed a significantly depressed performance in the later stage of chick rearing. Again a within clutch trade-off in reproductive effort was identified with second hatched chicks in experimental nests growing at a significantly lower rate and Hedging at a significantly lighter mass than those in control broods. The effects of experimental manipulation on aspects of the dynamics of parental foraging suggested that an increase in reproductive demand may affect strategy, but the relationship was not clear. An increase in the costs of egg production alone was not found to decrease parental performance or the quality of the additional egg or chick. These results cast doubt on the interpretation of previous brood enlargement experiments as providing empirical evidence that observed clutch sizes are often less than the Lack value, as such experiments have failed to include the costs of egg production and much of the incubation costs also. They lend support to the Individual Optimisation Hypothesis, in that the parents appear to be raising the clutch size that maximises their number of recruits. The effect of increased cost in relation to individual quality was also examined. The capacity of individuals to compensate for deviations from their allocation of effort to different reproductive phases was found to differ. The negative fitness effects of an increase in incubation demand were most marked in lower quality pairs. Also, the capacity to lay additional eggs in response to experimental egg removal differed between individuals and between years. Only birds in a good enough condition prior to, and during, egg laying appeared able to increase their allocation of effort to the egg production phase and replace the removed egg. A significant proportion of (presumably poorer quality) birds simply deserted the nest site. The adaptive significance of a conditional response to egg loss is discussed, particularly in relation to the finding that egg production alone did not result in any significant within clutch trade-offs.