# Developmental Stage‐Specific Responses to Extreme Climatic Events and Environmental Variability in Great Tit Nestlings

## TL;DR

Great tit nestlings are affected differently by extreme weather events depending on their age, with cold snaps harming young chicks and rain and heat impacting older ones.

## Contribution

The study reveals stage-specific and context-dependent effects of extreme climatic events on great tit nestlings using long-term data.

## Key findings

- Extreme cold events in the first week of development reduce fledging mass in great tit nestlings.
- Extreme rain and heat have stronger negative effects on older nestlings with increasing energetic demands.
- Combined exposure to extreme heat and rainfall during early development can reduce fledging mass by up to 27%.

## Abstract

Climate change poses a pervasive threat to many aspects of natural systems, and while impacts of changes in average conditions have been extensively studied, the effects of increased climate variability and extreme events on natural populations are less understood due to the challenges of studying these rare and unpredictable occurrences. Using 60 years of life‐history data from over 83,000 individuals and historical daily climate records, we show that developmental stages in wild great tits (
Parus major
) differ in their sensitivity to extreme climatic events (ECEs). Exposure to extreme cold events during the first week of development is particularly detrimental to fledging mass, while extreme rain events have a stronger negative impact as nestlings grow older and their energetic requirements increase. Synergistic effects of ECEs and average climatic conditions can be particularly severe, exacerbating the challenges faced by these birds. Our findings indicate that combined exposure to extreme heat and heavy rainfall during early development is associated with a predicted reduction in fledging mass by up to 27%. Additionally, birth timing may further modulate these effects, since late‐season broods exposed to frequent hot ECEs during early development are predicted to fledge nestlings up to 4.27 standard deviations (35%) lighter than broods laid earlier in the season. Moreover, phenotypic plasticity has enabled many similar populations to shift towards an overall earlier laying date, which may have increased susceptibility to cold extremes during development. However, our analyses suggest that the benefits of being part of an early‐laid clutch within a season may, to some extent, offset the negative effects of extreme climate on fledging mass and apparent survival. In climate scenarios where ECEs are predicted to increase in frequency, duration and severity, these developmental stage‐specific insights are important for understanding how climate change may be influencing wild avian populations.

Extreme climatic events (ECEs) impact great tit nestlings in a developmental stage‐specific and context‐dependent manner. Using 60 years of data on 83,000+ great tit nestlings from Wytham Woods, UK, we show that cold snaps are detrimental to hatchlings, while older nestlings are more affected by extreme rain and heat. Combined effects of ECEs with ambient weather conditions can worsen outcomes. Our findings indicate that combined exposure to extreme heat and heavy rainfall during early development is associated with a predicted reduction in fledging mass by up to 27%. These insights are important for predicting how wild populations may fare amid the ongoing climate crisis, where ECEs continue to increase in frequency and intensity.