It doesn't seem to make sense to be at a low ebb during a physiologically demanding time, but this is what happens to female birds when laying eggs. Their red blood cell numbers plummet and they become anaemic, a metabolic hit that the birds can ill afford when reproducing. But what causes this loss of oxygen carrying capacity at such a challenging time? Emily Wagner and her colleagues, Christine Stables and Tony Williams from Simon Fraser University,Canada, explain that there could be several reasons for the birds' anaemia:the birds' blood has become diluted by an influx of materials destined for egg yolk; the birds have redirected the energy required for red blood cell production to reproduction; the birds reduce red blood cell production while laying eggs; or several of these factors conspire to compromise the birds'fitness. Wagner and her colleagues set out to find out why birds become anaemic when they lay a clutch(p. 2960).
Working with zebra finches, the team fed one group of birds on a high quality diet, while the others were fed on regular birdseed as they mated and laid eggs. Then the team swapped the groups, so that the poorly fed birds received the high quality diet, while the previously well-fed birds switched to birdseed before mating and laying again. Monitoring the females' blood as they laid and incubated their eggs, the team found that even the birds on the poorest diet experienced the same level of anaemia, with all of the females'red blood cell counts falling by ∼8%. Wagner explains that as all of the females experienced the same reduction in red blood cell levels, regardless of their diet, the anaemia is probably caused by dilution with compounds destined for egg yolk.
However, this wasn't the only cause of the birds' anaemia. The team suspect that egg-laying female zebra finches also become anaemic because they reduce red blood cell production. Wagner found that the proportion of immature red blood cells released into the bird's blood continued rising after the females stopped laying their eggs, peaking around the time when the eggs hatched.`This is consistent with enhanced production and release of larger immature cells into the circulation following suppression of erythropoiesis,' says Wagner, before speculating that the hormone oestrogen could be responsible for reducing the birds' red blood cell production levels while laying eggs.
Wagner adds that the metabolic burden associated with recovery from a bout of egg-laying anaemia could account for the negative impact that egg laying has on brooding mothers.
