Hard life for sons in the nest? Sex-dependent offspring mortality in great tits in relation to breeding habitat and age

Abstract

Sex-biased mortality can occur in birds during development, for example due to sexual differences in energy requirement and/or environmental sensitivity, or the effects of sex hormones or sex differences in expression of mutations linked to sex chromosomes. Urban habitats often provide poorer conditions for nestling development resulting in higher offspring mortality compared to natural habitats, which may accelerate sex differences in offspring mortality in cities. To test this hypothesis, we examined sex-specific offspring mortality in great tits (Parus major), using 660 samples of dead offspring collected in two urban and two forest sites between 2013-2019. Overall, the sex ratio of dead offspring was significantly male-biased (56.80%). When habitats and age groups were analysed separately, the sex ratio of dead offspring was significantly male-biased in urban habitat (57.60%) and in young nestlings (58.62%), and non-significantly in the forest habitat (56.58%) and in unhatched embryos (54.11%) or in old nestlings (54.55%). However, these estimates were associated with wide confidence intervals, thus sex ratios of dead offspring did not differ significantly among study sites and between age groups when these were analysed together. 70.30% of unhatched eggs was not fertilized, and their proportion did not differ between urban and forest habitats. These results suggest male-biased offspring mortality in great tits, and highlight the need of large datasets to detect subtle differences between habitats and developmental stages.