London, Oct 3 : Researchers at the University of Vigo in Spain have found that female birds sacrifice their health to lay colourful eggs.
The colour blue in many birds' eggs comes from the compound biliverdin - a breakdown product of the heme unit in haemoglobin - that circulates freely in the blood.
However, biliverdin is not just a pigment, it is also an antioxidant used by the body to prevent cellular damage.
Previous studies have shown that when females lay vibrant blue eggs, their partners are more likely to stick around and help rear the young one.
Therefore, researchers thought that since the blue colour comes from an antioxidant, it is an indication to male partners of the female's health status.
For the study, 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.
They observed the progress of 48 females from when they started nesting. Once the nests were completed, the female birds began laying eggs.
The researchers measured the colour of the eggs with a portable spectrophotometer, and one week after egg-laying was completed the scientists took blood samples from the birds.
They found 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 researchers 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," Nature quoted Morales, as saying.
The study is published in Behavioural Ecology and Sociobiology2.