Blue-green eggs in pied flycatchers: An experimental demonstration that a supernormal stimulus elicits improved nestling condition

Abstract

It has been proposed that blue-green egg colouration in certain avian species has evolved as a signal of female and egg quality to males, affecting their investment in the brood according to the differential allocation scenario. A recent experiment has successfully manipulated male investment by placing dummy eggs simulating the extremes in the blue-green natural reflectance. Here we have substituted one egg in certain pied flycatcher clutches with a single deep-blue (blue) or light-blue (pale) dummy egg and used other clutches as controls. Blue dummy eggs reflected more in the blue-green part of the spectrum than natural and pale dummy eggs, which were similar in reflectance. Nestlings in nests with one blue dummy egg during laying and incubation attained a significantly higher mass and condition than those in nests with one pale dummy egg or in control broods, when controlling for phenology, brood size and blue-green chroma of the clutch. Only a shift in parental investment can have induced this highly significant effect. Preference for deeply coloured clutches associated with high-quality females and their maternal effects may explain stimulation by a single supernormal stimulus emanating from multiple objects like clutches. © 2008 The Authors.