Most studies carried out on birds have shown that sexual as well as social behavioral repertoires in males are under strong control by gonadal secretions, most notably testosterone. The purpose of our research is to elucidate the effects of testosterone on the dominance hierarchy and mate attraction behavior in captive male European starlings (Sturnus vulgaris). For this purpose, we used the ‘phenotypic engineering method’. This means that we create novel phenotypes by manipulating the testosterone concentrations of the birds and then we compare the behavior of altered and unaltered individuals.

In our study, we used two ways to manipulate the plasma concentrations of testosterone. On the one hand, we castrated starlings and on the other hand we experimentally elevated testosterone levels by giving males implants of silastic tubing, packed with testosterone. Castrated males, implanted males and control males were placed together in an outdoor aviary to measure dominance. We observed the birds and we recorded every interaction. To evoke aggressive interactions between the starlings we took away the food a day before observation to make the birds very hungry during the observation period. Based on these observations we determined a dominance matrix which shows for each individual the total number of victories and defeats. Then we divided the total number of winning interactions by the total number of interactions that an individual participated in, to calculate the dominance index. Using this dominance index, we could determine the dominance hierarchy and dominance positions.

Testing the assumptions with a two-way ANOVA, it became clear that implanted males were dominant over castrated and control males. After removing the implants, we again determined the dominance hierarchy. Now, males that had previously been implanted, no longer had a higher dominance position than castrated and control males. In contrast to control males, castrated males did not show mate attraction and song behavior in the presence of a prospecting female. However, these behaviors were reinstated after the castrated males were treated with testosterone implants. We found this by observing a group of castrated males and a group of control males. Every minute, we recorded which males were singing and which were occupying a nest box. After some time we treated half of the castrated starlings with testosterone implants and we remeasured song activity and nest box occupation. For each individual we measured the proportion of time a male sang or occupied a nest box as an indication for his song activity and the time he spends in a nest box. Statistical analysis of our data was carried out using SAS.