Published online 2 October 2008 | Nature | doi:10.1038/news.2008.1146

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Bright blue eggs keep males keen on fatherhood.

Did the effort of colouring these eggs damage the health of their mother? PhotoStockFile / Alamy

Great artists are said to pour all their energies onto the canvas, leaving them exhausted after a flurry of creativity. Now, researchers have found that female birds make a similar sacrifice when colouring their eggs, creating vivid hues at the expense of their health.

The blue in many birds' eggs comes from the compound biliverdin, a breakdown product of the heme unit in haemoglobin, which circulates freely in the blood. But biliverdin is not just a pigment, it is also an antioxidant used by the body to prevent cellular damage.

Previous research has proven that when females lay vibrant blue eggs, their partners are more likely to stick around and help rear the young1. So researchers speculated that because the blue comes from an antioxidant, it is a signal to males of the female's health status. Some scientists have argued that the female is making a dangerous trade-off, giving up resources needed to sustain her health to convince her partner that her offspring are worth looking after.

“To the best of our knowledge, this is the first evidence that blue eggs are not free — there is a big price that the females are paying.” Judith Morales

University of Vigo, Spain

Determined to find out whether females were in fact sacrificing their health to lay blue eggs, Judith Morales at the University of Vigo in Spain and her colleagues monitored 100 boxes near the village of Lozoya in central Spain, where pied flycatchers (Ficedula hypoleuca) commonly nest. The team tracked the progress of 48 females from when they started nesting.

Once the nests were completed, they were either removed permanently from the nesting box, or removed and then reinserted immediately — a control measure to ensure that all birds experienced some human interference.

Females that had their nests confiscated started rebuilding within two days, and were allowed to completely reconstruct their nests. Once females had completed their nests, they began laying eggs. The scientists measured the colour of the eggs with a portable spectrophotometer, and one week after egg-laying was completed the team took blood samples from the birds.

Stressed and blue

Morales and colleagues report in Behavioural Ecology and Sociobiology2 that neither laying intensely blue eggs nor having to rebuild a nest triggered a noticeable decrease in plasma concentrations of the antioxidant. But both factors combined did.

The scientists suggest that the birds somehow shift their allocation of biliverdin towards the eggs, depleting their own antioxidant defences in the process. The effect becomes measurable in birds that are already stressed by having to rebuild their nest, something that is itself expected to decrease antioxidant levels.

"To the best of our knowledge, this is the first evidence that blue eggs are not free — there is a big price that the females are paying," says Morales.

"This makes a good case that the blue eggs are affecting the health of the females," says evolutionary ecologist, Martin Schaefer at the University of Freiburg in Germany.

Schaefer suggests that the next step is to quantify a direct link between low biliverdin concentrations and the birds' chances of survival. "Running the study again while monitoring long-term survival of the mothers and their offspring would solidify this," Schaefer says.

Whether other bird species pay the same stiff penalty remains to be determined, but Morales suspects that those with bi-parental care and blue eggs will be in a similar situation.