Sturnus

Two extant species are recognized:[5]

Image	Scientific name	Common name	Distribution
	Sturnus vulgaris	Common starling	Native range is in temperate Europe and western Asia
	Sturnus unicolor	Spotless starling	Iberian Peninsula, northwestern Africa, southernmost France, and the islands of Corsica, Sardinia and Sicily

Male Sturnus unicolor individuals face a choice when it comes to the mating season. They can either invest in parental care, through helping feed the young, provide shelter, etc. and be monogynous or they can control many nests at one time, increase the number of matings they engage in and be polygynous.[6] To add to this, much of the time their choice is biological. Sturnus unicolor males are more likely to be polygynous and be less involved in parental care if they have higher levels of testosterone. In addition, females of the Sturnus vulgaris species see a decrease in their fitness if the male is polygynous because there is no assistance in nestling feeding.[7] Offspring success per nest, however, is significantly higher if there is parental care from the male, but males see greater overall fitness when they are polygynous. This is the trade off and the cost/benefit balance that each individual must go through.

To try and make the male stay faithful to her, a female will act aggressively towards another female that solicits his attention. If the male looks like he is going to leave, she will perform aggressive acts towards a prospecting female, in an attempt to have the male stay.[8] Males respond to this behaviour, and the resident female's aggressive acts towards prospecting females do play a role in whether the male stays monogamous.[9] The female then responds to whether or not the male sticks around to invest in parental care. If the father stays monogamous, she will allocate more maternal steroid hormones to the eggs she lays later.[10] Doing so negates the effects of hatching asynchrony. When the female is alone, she has difficulty finding enough resources to fully take care of all the offspring. Therefore, if the male is monogamous, she allows all the young to hatch at a similar time, and at a similar nutritional state. If the male is not around, she will allow asynchronous hatching and even allow a brood reduction if she cannot take care of all the young. This allocation of resources to the more desirable situation is differential allocation.

The sexy son hypothesis is an attempt to explain why a female may choose to mate with a previously mated (polygynous) male. This hypothesis suggests that the female mates with a polygynous male because he has already proven himself successful and will therefore be likely to pass these successful genes to his sons.[11] Although it may not be ideal for the female at the time, her genes are likely to get passed on for many generations due to her sons' success. Those sons are likely to exhibit many of the behavioral qualities of the father, causing them to also be highly successful in their sexual maturity.

This hypothesis has its critics, however. One counter-argument is that a female who chooses a polygynous mate, for a chance that her sons grow up to be successful and pass her genes on, would be risking her own health. The hypothesis has been shown to be flawed in some scenarios. A female that mates with a previously mated male, who is also likely to leave her nest, fledges far fewer young than one that mates with a monogynous male.[12]