The colourations and patterns on a female great tit’s (Parus major) plumage indicates the ability to produce healthier offspring, finds an article published today in 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.
In a cross-fostering experiment, newly hatched broods were removed from their genetic mothers’ nests and swapped with another female’s brood. By 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 in relation to the ‘ornamentation’ of their mothers. They compared the offspring’s weight, size and immune strength as proxy measures of health and fitness. They found a correlation between the chicks’ weight at two weeks and the size of black breast stripe on the genetic mother- this correlation was independent of female size, which was assessed as a confounding variable. These findings indicate that the size of the black breast stripe may be an indicator of genetic fitness of the mother which also affects the fitness of the offspring.
The immaculateness of both genetic and foster mothers’ white cheek patches was related to the strength of the chicks immune responses suggesting that this could be due to nurture rather than direct genetic causes. Chick body size (measured by tarsus length) was only correlated to the size of the genetic mothers, with no relationship to ornamentation.
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. It is therefore likely that these ornaments have evolved to be used by males when choosing a potential mate, as a signal of reproductive fitness of females.
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.”