Many organs can rapidly and reversibly change in size when the need arises,and one of the champions of such change must surely be the reproductive system of female birds. For example, birds reduce the size of their reproductive organs when not in use, as large organs are metabolically costly to maintain. However, they grow a large ovary and oviduct within a matter of days, when preparing to lay eggs and reproduce. François Vézina and Tony Williams from Simon Fraser University wondered how costly gearing up the reproductive system is and how it is paid for. Based on previous work, they expected to see an increase in energetic maintenance costs during egg production, but they were not sure about its mechanistic basis. Perhaps reducing the size of some nonreproductive organs could partially compensate for the costs of the reproductive system. Alternatively, increasing the size of nonreproductive organs, such as the food-processing system, might be necessary to supply the energy needed for egg production. In both scenarios,if egg production requires adjustments of nonreproductive organs, then the body composition of egg-laying females should consistently differ from that of nonbreeding and chick-rearing individuals.

To explore their ideas, Vézina and Williams carried out an impressively thorough study on wild starlings breeding in nest boxes. Three years in a row, they measured oxygen consumption of birds in the laboratory to determine resting metabolic rates of nonbreeding, laying and chick-rearing females. Afterwards, they dissected the birds to measure organ masses. In addition, they monitored vitellogenin and very low density lipoprotein, two yolk precursors.

The results showed that egg-laying female starlings undergo rapid and large changes in their reproductive system. Their oviduct gained 62% of its 22-fold increase in only 3 days! Its regression was just as rapid and began even before the last egg was laid. The change in reproductive organs was accompanied by major changes in the yolk precursors, and a 22% increase in energy expenditure.

So, gearing up the reproductive system was costly indeed! In fact, 18% of the increase in energy costs could be explained by changes in oviduct size. But how were these costs paid for? The answer was not straightforward. The proposed scenario that the nonreproductive organs could shrink to compensate for the expensive oviduct did not hold up. The alternative of increasing the size of the food-processing organs to supply the extra energy required for egg formation was not found either. Although several nonreproductive organs,including kidney and flight muscles, changed markedly in size, their changes did not follow the changes in reproductive system. Other organs, such as intestine and gizzard, showed different patterns in different years. These confusing results demonstrate that the authors' persistence, repeating their measurements 3 years in a row, paid off. Their work tells us that producing eggs does not require consistent changes in size of the nonreproductive organs, at least in starlings. Instead, the nonreproductive organs may simply be adjusting to ecological conditions that vary throughout the breeding season and among years. The study also shows that simply measuring organ size will not explain metabolic rate. It's time to look beyond size and focus on function.

Vézina, F. and Williams, T. D. (
). Plasticity in body composition in breeding birds: what drives the metabolic costs of egg production?
Physiol. Biochem. Zool.