Effects of increasing nest temperature on blue tits incubation behavior and its impact on offsprings

Climate change is one of the most significant ecological challenges of the 21st century. Temperatures are predicted to increase by 2-4 °C in the future and exhibit greater variability. Birds are one of the most observable taxa, so consequences of climate change are various in bird populations and well documented, ranging from phenological differences, population declines, range shifts and other effects. In this thesis, the focus will be on the thermal environment experienced in the first phases of development, in particular incubation. Egg temperature needs to be in a specific temperature range (between 36.5 to 40 °C) to sustain optimal embryo development. Even fluctuations of a few degrees can harm hatching success and offspring phenotype. It is believed that changes in the prenatal environment can have carry-over effects in later stages of life, so parents are expected to buffer egg temperature changing their incubation rhythm. In birds this stage of the reproductive season is very costly, so parents need to balance the amount of parental care provided with their own needs. It is generally believed that higher nest attentiveness should be present at low ambient temperatures and gradually reduced when the temperature is increasing, however, behavioral analysis has given contrasting results. In this study, the effect of a higher nest microclimate on behavior was investigated in a population of blue tits. To check for behavioral differences correlated with high temperature an experimental manipulation was performed at nest level. The first hypothesis tested was that females from heated nests would spend more time carrying out activities to enhance their conditions because heat could alleviate them some of the costs of egg rewarming after each off-bout. Secondly, the thesis will consider whether behavioral decisions correctly buffer embryos from the higher temperature, avoiding deleterious fitness effects that could arise from an altered prenatal environment. Finally, it has been checked if there was a difference in the duration of the incubation period, as it is expected that higher temperatures could reduce it. As expected, increasing nest temperature had a significant effect on female behavior. The rhythm was adjusted by increasing the bouts number, furthermore, incubation sessions were on average shorter. It seems that parental role successfully protected embryos from temperature fluctuations and there are no apparent differences in reproductive success between groups. In the end, there were no significant differences in the length of the incubation; contrary to the expectation period was slightly longer in heated nests. In conclusion, birds possess the ability to adjust their incubation behavior in response to varying climatic conditions, a strategy that may confer advantages for both their immediate health and future reproductive success. By modifying the time spent incubating, birds can better regulate temperature and protect their offspring, ensuring optimal development. However, the flexibility of this behavioral adaptation is not without its constraints. As climate change continues to drive more extreme and unpredictable weather patterns, birds may face conditions that exceed their current capacity for adjustment. In such cases, more substantial shifts in life history traits, such as altered breeding timing or even changes in reproductive efficiency, might be necessary for their survival and continued reproductive success. This raises concerns about the long-term resilience of bird species, as their ability to adapt to increasingly severe environmental changes may be limited.