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Extreme Weather Affected Great Tit Chicks Differently as They Grew

Great tit holding a caterpillar at the entrance to a nest box
Research · Ecology

Extreme cold, heat and heavy rain affected great tit (Parus major) chicks differently depending on their age, according to a 60-year study from Wytham Woods in Oxfordshire, England. The analysis combined daily weather records with life-history data from more than 83,000 wild birds. It was published in Global Change Biology on 11 March 2026.

The great tit has been monitored at Wytham Woods since 1947. More than 1,000 nest boxes have been available across the mixed deciduous woodland since the 1960s, allowing researchers to follow breeding attempts, chick growth and the return of marked birds over many generations. For the new study, the team analysed 83,935 individuals hatched between 1965 and 2024 and used recruitment records extending into the 2025 breeding season.

The researchers divided development into two periods. The hatchling stage covered the first seven days after hatching, when chicks have few feathers and only a limited ability to regulate their body temperature. The nestling stage covered days eight to 15, when the growing young have greater energy requirements. Daily temperature and rainfall records were then used to identify unusually cold, hot or wet conditions during each period.

Extreme cold mattered most during the first week

The strongest responses changed as the chicks grew. Exposure to extreme cold during the first week was particularly strongly associated with lower mass at fledging. At this early stage, chicks have little protection against heat loss and must devote energy to maintaining body functions instead of growth. Parents may also spend more time brooding them during cold conditions, leaving less time for collecting food.

Heavy rain mattered more as nestlings grew

Rain became more important during the later nestling stage. Both high average rainfall and extreme rain events were associated with reduced fledging mass. The authors explain that heavy rain can restrict parental foraging and make caterpillars and other insect prey harder to find. This matters increasingly as the chicks grow and require more food.

Temperature and rainfall also acted together. During early development, the combination of extreme heat with consistently high rainfall, or extreme rain with higher average temperatures, produced some of the largest predicted reductions. Under the most severe combination assessed, fledging mass was predicted to be up to 27 percent lower.

Late broods exposed to repeated heat had the largest predicted losses

Breeding date shaped these patterns further. Late broods exposed to frequent hot events during their first week were predicted to produce fledglings up to 35 percent lighter than broods laid earlier in the season. Earlier broods generally coincide more closely with the seasonal peak in caterpillar abundance, which can help parents meet the high food demands of their young. When laying date was included in models of later recruitment into the breeding population, most direct associations between weather extremes and recruitment became weaker, identifying breeding date as the dominant seasonal influence on that outcome.

Sources and image credit

  1. Satarkar, D., López-Idiáquez, D., Sepil, I., & Sheldon, B. C. (2026). Developmental stage-specific responses to extreme climatic events and environmental variability in great tit nestlings. Global Change Biology, 32(3), e70794. https://doi.org/10.1111/gcb.70794Primary peer-reviewed open-access research article first published 11 March 2026. Using 60 years of Wytham Woods data from 83,935 great tits hatched during 1965–2024, the study found stage-specific effects of cold, heat and rainfall on fledging mass and later recruitment.

Image: Marie-Lan Taÿ Pamart — great tit (Parus major) holding a caterpillar at the entrance of a nest box in Paslières, Auvergne, France, photographed 3 May 2023; Wikimedia Commons; representative breeding image, not a bird from the Wytham Woods study · CC BY 4.0 · Source

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