A female great tits' (Parus major) appearance is shown to signal healthy attributes in offspring in a paper in BioMed Central's open access journal Frontiers in Zoology. The black stripe across her breast and white patches on her cheeks correlate to a chick's weight at two weeks and immune strength respectively – though the former seems to signal a genetic benefit and the latter can affect an 'adopted' chick's health, suggesting nurture is involved.
Taking two mothers with different patterning, and swapping their chicks, researchers from Palacky University in the Czech Republic were able to investigate the growth and health of the infants and the 'ornamentation' of their mothers. They compared the offspring's weight, size and immune strength and found a correlation between the chick's weight at two weeks and the size of black breast stripe on the genetic mother.
The immaculateness of both genetic and foster mother's white cheek patch was related to the strength of chick's immune response suggesting that this was due to both nurture and genetics. In contrast the body size of a chick was related only to the body size of its genetic mother and not to ornamentation at all.
In these socially monogamous birds both the males and females are brightly coloured, however neither the cheek patch nor the stripe in males affected the health of the babies.
Talking about how the ornaments can have evolved to signal reproductive fitness, Vladimír Remeš and Beata Matysioková who performed this study explained, "Bigger healthier babies are important to the reproductive success of individuals, because they are more likely to survive to adulthood - so it is useful for birds to be able to work out which potential mates will produce the best babies. Maintaining bright colouration uses up resources which could otherwise be invested in reproduction or self-maintenance - consequently the evolution and maintenance of ornamentation in female great tits is probably due to direct selection by males."
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