Biology session

The offspring sex ratio of broods is usually close to even in most bird species, and male- or female-biased brood sex ratios are relatively rare. In contrast, sex-specific differences can be more often seen in juvenile mortality which could be caused by various mechanisms. For example, differences in sex-specific mortality may be caused by the immunosuppressant effect of testosterone, sexual differences in energy requirement originated from sexual dimorphism and environmental sensitivity (e.g. to scarcer food), or mutations linked to sex chromosomes. Population viability is influenced greatly by mortality patterns, and the sex-ratio of a population is an important demographic trait which impacts several ecological and population dynamic features. Thus, it could be useful to study how sex-dependent differences in mortality in different ages could contribute to forming sex ratios. Studies in songbirds showed large variations among species: female-biased or male-biased mortality, and no sex-biased differences in mortality patterns also existed.

In this study, I examined sex-specific mortality patterns in great tit (Parus major) nestlings, from tissue samples collected from two urban and two rural nesting areas in between 2013 and 2018. Every nestling has been ringed after the age of 14-16 days of hatching, and a small blood sample was taken from the brachial vein. After fledging of the juveniles, we registered if ringed individuals died in the nest and cannot fledge. We also had tissue samples (i.e. one finger) from those nestlings that died before the ringing. Altogether I studied 342 tissue samples from two age groups, 240 that died before ringing and 102 that died after ringing. The sex determination was made by molecular methods, with the use of CHDi16 and p2p8 primers. I identified 202 (59,06%) males and 138 (40,35%) females from these samples (offspring sex cannot be determined for two samples). Overall, more males than females died both before (60,83%) and after (54,90%) ringing. Comparing urban and rural populations, more males died before ringing in both urban (61,58%) and rural (58,73%) populations, while after ringing there was more dead males in urban (56,18%) but more dead females in rural habitat (53,84%), although for the latter we had low sample size (n=13).

I found overall significantly higher juvenile male mortality in great tit nestlings. In addition, I also found that sex-dependent mortality of nestlings didn’t differ between urban and rural habitats, thus sex-dependent juvenile mortality seems to be unrelatable to environmental changes. In the near future, I also plan to examine unhatched eggs, which were collected after 5 days of when the first hatchling hatched. Hence, we will be able to analyze sex differences in embryo mortality and would obtain a broader picture in sex-dependent mortality from early ontogeny to independence.