Recent research has suggested sex hormones may be the reason behind variation in birds of the same species and sex. These sex hormones can be deposited at different concentrations into eggs by the mother and therefore result in different physical attributes as well as personality traits. This was tested on Black-headed gulls where eggs were inserted with different concentrations of the maternal hormone androgen. The results were an enhancement in the development of nuptial plumage (that which is developed by the males during courtship display) and frequency of aggressive sexual displays and these traits lasted beyond 1 year of age. These results correspond with a similar study involving House Sparrows where males developed a larger badge size which is essential in courtship displays to signal male’s fitness to females.
Concentrations of sex hormones can change in response to laying and therefore hatching timing as later eggs often have a higher levels of them. For instance, a study involving magpies demonstrated that explorative behaviour can alter with first hatched chicks being significantly bolder than last hatched ones. In this study, the results were particularly significant if the first hatched chick was female as they were much more explorative than their male siblings.
So, the question is what is the purpose of female birds altering their chick’s personalities? One of the most likely explanations is that this process reduces sibling rivalry. If a chick has a higher concentration of these hormones they are more likely to have higher reproductive success per year as they have more attractive traits such as sexual aggression. However, these sex hormones reduce a bird’s immune system so if there are unfavourable environmental conditions such as disease or difficult weather, these birds are less likely to survive. In contract, chicks with lower concentrations have less reproductive output but are more likely to survive. Therefore, the mother has effectively attempted to ensure that some of her chicks will survive and reproduce no matter the conditions. A very clever technique that requires a lot more research to truly understand the motives and techniques of these birds.
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