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Published online 2 October 2008 | Nature | doi:10.1038/news.2008.1146
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Female birds sacrifice health to create more colourful eggs
Bright blue eggs keep males keen on fatherhood.
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.
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"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." I think there is a much better reason for males to abandon nests with insufficiently blue eggs than that the female pied flycatcher has failed to pass some sort of heath check. The pied flycatcher is one of a number of birds whose nests are parasitized by cuckoos, and these cuckoos do attempt to imitate the blue color of the other eggs in the nest: "Blue Cuckoo eggs occurred at significantly higher frequencies in the clutches of Redstarts Phoenicurus phoenicurus, Whinchats Saxicola rubetra, Wheatears Oenanthe oenanthe and Pied Flycatchers Ficedula hypoleuca than in host species which do not lay blue eggs." [http://www.jstor.org/pss/3677207] The possibility of cuckoo eggs in the nest is clearly a much more potent selective force for the evolution of a preference for intensely blue eggs among males than the possibility it may test the putative health of his mate. As LeVerrier said in another context, we "have no need of that hypothesis"
Is it possible that the biliverdin transfers some of its antioxidant properties to the fetus inside the egg?
Answer to the comment by Jim Morgan: That there is already a hypothesis on the evolutionary significance of a trait should not preclude us from exploring new explanations, that?s the way science progresses. Indeed, blue egg colour in brood parasites could have evolved as a counter-adaptation against host species that lay blue eggs. However, this does not explain why hosts themselves lay costly blue eggs. There is no clear evidence that the risk of brood parasitism is reduced for bluer clutches, in any case, for blue clutches with reduced colour variability. The fact that males adjust their effort according to blue egg colour intensity and that blue egg pigmentation is costly for females is why we needed the signaling hypothesis.