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.