OPTIMAL ALLOCATION OF EFFORT BETWEEN REPRODUCTIVE PHASES - THE TRADE-OFF BETWEEN INCUBATION COSTS AND SUBSEQUENT BROOD REARING CAPACITY

The optimal allocation of effort during reproduction is a key componen t of life history theory, with tradeoffs predicted to operate both wit hin and between reproductive attempts. Experimental work in this held has largely concentrated on the latter. The need to partition investme nt between different phases of reproduction, and how this varies betwe en individuals, has received little empirical investigation. In this s tudy, the costs of the incubation phase in common terns Sterna hirundo were increased independently of those of the egg production and brood rearing phases. Incubation of a clutch of three eggs, rather than the two originally laid, reduced the subsequent capacity of parents to pr ovision their brood of two, demonstrating an important trade-off betwe en reproductive phases that has generally been omitted in estimations of optimal clutch size. These results show for the first time that an increase in the costs of incubation alone, which have often been consi dered relatively trivial, can significantly depress parental performan ce later in the same breeding attempt. The effect of increased incubat ion costs was found to be most marked in the lower quality pairs, whic h demonstrates that individuals differ in their capacity to compensate for deviations from their allocation of effort to different reproduct ive phases.