Egg pigmentation reflects female and egg quality in the spotless starling Sturnus unicolor
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It has been proposed that blue colouration in eggs has evolved as a signal of female quality that males can use to modulate their parental investment. This hypothesis is based in the antioxidant properties of biliverdin whose costly deposition in the eggshell is expected to signal female antioxidant capacity and egg quality. Since maternally derived androgens are costly to produce and may adaptively affect offspring phenotype, high-quality females may benefit by signalling their androgen investment through egg colouration. Our aim was to investigate whether egg colour variation in the spotless starling reflected the amount of pigments on the eggshell and whether egg pigmentation was related to female and egg quality. Chromatography analyses revealed that spotless starling eggshells contained two different pigments: biliverdin and protoporphyrin IX with no correlation between them. Biliverdin contents correlated positively with egg colouration indicating that darker eggs with a higher peak in the blue–green segment of the spectrum contained higher amounts of biliverdin. Eggs containing more biliverdin were laid by high-quality females and contained higher yolk testosterone levels. However, despite the strong correlation between biliverdin and colorimetric variables, egg colouration did not reflect accurately female and egg quality. Our results provide evidence that eggshell pigmentation in the spotless starling is related to female and egg quality as shown by the yolk testosterone levels. However, the lack of relation between egg colour and female condition and egg quality do not provide evidence to support the signalling function of egg colouration.
KeywordsEgg pigments Biliverdin Yolk androgens Spotless starling
We are grateful to Montserrat Gil for helping at the field and to Ana Martinez for assistance with colorimetric measurements. IL was supported by Ph.D. grants from CONACYT (México). Research was funded by a grant (BOS2002-00105) and a Ramon y Cajal fellowship from the Ministerio de Educación y Ciencia (Spain) to DG. Permission to work in the study area was granted by the Ayuntamiento de Soto del Real and the Consejería de Medio Ambiente of the Comunidad Autónoma de Madrid. This manuscript is a contribution of the field station “El Ventorrillo”. The experimental procedures for this study comply with the current laws for the treatment of animals in behavioural research and were approved by the Spanish Ministry of Education and Science (BOS2002-00105).
- Gil D (2003) Golden eggs: maternal manipulation of offspring phenotype by egg androgen in birds. Ardeola 50:281–294Google Scholar
- Hiraldo F, Herrera CM (1974) Dimorfismo sexual y diferenciación de edades en Sturnus unicolor Temm. Doñana Acta Vertebrata 1:149–170Google Scholar
- Kennedy GY, Vevers HG (1973) Eggshell pigments of the Araucano fowl. Comp Biochem Physiol 44B:11–25Google Scholar
- Kennedy GY, Vevers HG (1975) A survey of avian eggshell pigments. Comp Biochem Physiol 55:117–123Google Scholar
- López-Rull I, Celis P, Gil D (2007) Egg colour covaries with female expression of a male ornament in the spotless starling (Sturnus unicolor). Ethology 113:926–933Google Scholar