Birds make laying eggs look easy. In fact, some species produce enormous clutches, weighing in at more than the mother's own body weight. But how much effort does it take a bird to produce an egg? François Vézina explains that it's not clear how costly it is for the mother. According to energetic theory, egg laying is extremely costly, yet some species produce eggs on a daily basis! Tony Williams, Katrina Salvante and Vézina decided to ask the birds directly. Having measured the resting metabolic rate as females prepare to lay eggs, the team already knew that it increased by over one fifth as they prepared to lay an egg. Vézina explains that 18%of the extra effort is funnelled into the oviduct to produce the albumin and eggshell as the follicle develops. But what about the remaining 82% of the egg layer's efforts? Could the liver, which produces yolk protein precursors,contribute to the metabolic deficit?

The team needed to isolate the liver's metabolic activity as zebra finches produced an egg, so they decided to compare the metabolic rates of birds whose livers were producing yolk proteins with birds that were not. They also had to rule out any contribution from the female's developing oviduct, so they turned to males instead. By injecting male zebra finches with a hormone,17β-Estradiol, they reproduced the female's yolk protein production pattern. Next they measured the zebra finch's resting metabolic rate while its liver pumped out yolk proteins, and compared it with untreated males. But it was the same!

What's going on? Vézina explains that just because he didn't record a metabolic increase doesn't mean it's not there. He suspects that other organs might take a metabolic cut, to subsidise the liver's increased demand. But he acknowledges that untangling that metabolic web could take the rest of his career.

Vézina, F., Salvante, K. G. and Williams, T. D.(
). The metabolic cost of avian egg formation: possible impact of yolk precursor production?
J. Exp. Biol.