Survival to first breeding is not sex-specific in the Coal Tit (Parus ater)

Dietrich VCJ, Schmoll T, Winkel W, Lubjuhn T. Survival to first breeding is not sex-specific in the Coal Tit (Parus ater). JOURNAL FUR ORNITHOLOGIE. 2003;144(2):148-156.Differential survival of males and females affects the structure and dynamics of avian populations, but studying sex-specific survival rates is difficult. This is especially true for offspring if the period between hatching and first breeding is considered. Under certain conditions, however, the determination of offspring sex ratio, recruitment and dispersal may help in the investigation of sex-specific survival rates to the age of first breeding. Applying a molecular technique we sexed a large sample of Coal Tit (Parus ater) nestlings from a nest box population in a coniferous forest near Lingen/Emsland (Lower Saxony, Germany). The study covered a period of two years and included first and second broods. We found that the sex ratio did not deviate from unity. Through capturing breeding adults in the second year, we were able to examine local recruitment rates and natal dispersal distances of male and female offspring. The sexes differed significantly neither in recruitment probability, which was generally high, nor in dispersal distance, which was generally low. Our results indicate that there is no difference in the survival rate of male and female Coal Tits during their first year of life. The relevance of our findings are discussed with regard to the characteristics of the study population

Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater)

Dietrich-Bischoff V, Schmoll T, Winkel W, Krackow S, Lubjuhn T. Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater). BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY. 2006;60(4):563-571.Females of many socially monogamous bird species commonly engage in extra-pair copulations. Assuming that extra-pair males are more attractive than the females' social partners and that attractiveness has a heritable component, sex allocation theory predicts facultative overproduction of sons among extra-pair offspring (EPO) as sons benefit more than daughters from inheriting their father's attractiveness traits. Here, we present a large-scale, three-year study on sex ratio variation in a passerine bird, the coal tit (Parus ater). Molecular sexing in combination with paternity analysis revealed no evidence for a male-bias in EPO sex ratios compared to their within-pair maternal half-siblings. Our main conclusion, therefore, is that facultative sex allocation to EPO is absent in the coal tit, in accordance with findings in several other species. Either there is no net selection for a deviation from random sex ratio variation (e.g. because extra-pair mating may serve goals different from striving for 'attractiveness genes') or evolutionary constraints preclude the evolution of precise maternal sex ratio adjustment. It is interesting to note that, however, we found broods without EPO as well as broods without mortality to be relatively female-biased compared to broods with EPO and mortality, respectively. We were unable to identify any environmental or parental variable to co-vary with brood sex ratios. There was no significant repeatability of sex ratios in consecutive broods of individual females that would hint at some idiosyncratic maternal sex ratio adjustment. Further research is needed to resolve the biological significance of the correlation between brood sex ratios and extra-pair paternity and mortality incidence, respectively