Proximate and ultimate aspects of sex ratio variation in the Black headed Gull L
03 / 2000 - 03 / 2004
Recent studies show increasing evidence for adaptive sex ratio variation in birds, as a function of a range of parameters like time of the year, food availiability, parental fitness or male attractiveness. It has been shown in several bird species that biased sex ratios are already present at hatching. Furthermore, there are indications for a relationship between offspring sex and its position in the laying sequence within a clutch. In addition, also maternal endocrine state varies systematically during the course of egg laying, as reflected by maternal testosterone levels in the yolk of the eggs. The correlation with laying sequence of (a) maternal testosterone levels in the yolk and (b) offspring sex, makes it attractive to investigate the possibility that maternal endocrine state is causally involved in offspring sex determination. Until now virtually nothing is known about the physiological mechanisms involved in avian sex ratio determination. This project focuses in the first year on the investigation of the offspring sex in relation to several parameters, e.g. laying sequence, laying date of the clutch, position of the nest in the colony, testosterone level of the yolk. And apart from this proximate approach we also aim to investigate the fitness consequences for the offspring by monitoring sex specific growth and survival related to e.g. different combinations of males and females in the brood, testosterone level of the yolk and position in the laying sequence.