Egg sex ratio and paternal traits: using within-individual comparisons

Empirical studies of sex ratios in birds have been limited due to difficulties in determining offspring sex. Since molecular sexing techniques removed this constraint, the last 5 years has seen a great increase in studies of clutch sex ratio manipulation by female birds. Typically these studies investigate variation in clutch sex ratios across individuals in relation to environmental characteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratios did not vary in relation to a number of biological and environmental factors for 238 great tit Parus major nests. However, interesting sex ratio biases were revealed when variation in clutch sex ratios was analyzed within individual females breeding in successive years. There was a significant positive relationship between the change in sex ratio of a female's clutch from one year to the next and the relative body condition of her partner. Females mating with males of higher body condition in year x + 1 produced relatively male-biased sex ratios, and the opposite was true for females mated with lower condition males. Within-individual analysis also allowed investigations of sex ratio in relation to partner change. There was no change in sex ratios of females pairing with the same male; however, females pairing with a new male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be a powerful method for detecting sex ratio variation, and perhaps female birds may indeed manipulate egg sex but require personal contextual experience for such decisions. Copyright 2002.