A new study from the University of Oxford, published March 11, finds that sudden cold spells and heavy rainfall can slow growth and reduce survival chances for young great tits in the UK. The research also suggests that birds that begin breeding earlier in the season may avoid many of the harmful effects linked to these weather extremes.
The findings come from an unusually long dataset. Scientists analyzed 60 years of records covering more than 80,000 individual wild great tits in Oxford’s Wytham Woods. They paired this information with detailed daily weather records. By identifying the coldest, wettest, and hottest days in each breeding season, researchers measured how often these extremes occurred during critical stages of chick development and how they influenced body mass when nestlings fledged (left their nest), an important predictor of survival.
Cold and Rain Can Reduce Nestling Survival
The study revealed that severe cold during the first week after hatching is especially harmful. As chicks grow older, heavy rainfall becomes the greater threat. Both types of weather can reduce body mass at fledging by as much as 3%.
When intense heat occurs at the same time as heavy rain, the impact becomes much more severe. In those cases, fledging mass can drop by up to 27%, particularly for broods that hatch later in the breeding season.
Lead researcher Devi Satarkar (Department of Biology, University of Oxford) says: “In the Wytham population, great tits have adjusted to warmer springs by breeding earlier to track peak abundance of their main prey, caterpillars. This overall earlier laying is beneficial, buffering them against many impacts of extreme weather — but it also exposes them to cold spells early in the season. Even small early-life deficits can have large implications for survival. It will only get tougher for birds to keep up as extreme weather increases in frequency and intensity with climate change.”
Why Cold and Rain Affect Baby Birds
Newly hatched chicks cannot regulate their body temperature well because they lack feathers. During cold spells, they must use much of their energy simply to stay warm rather than to grow.
Bad weather also affects how much food they receive. Extreme cold and heavy rain can limit how often parents leave the nest to search for food. At the same time, rainfall can knock caterpillars off plants, reducing the main food source that growing chicks rely on to meet their high energy demands.
Mild Heat Can Sometimes Help Chicks Grow
One unexpected finding was that warmer extremes were linked to heavier fledging weights during the nestling stage. High temperatures are often associated with heat stress, but in this case the warmer periods in Oxfordshire appear relatively mild compared with extreme heat in southern Europe.
Devi explains: “Extreme weather events are affecting wild bird populations in complex ways. The level of warmth we see in these heat extremes in Oxfordshire might boosts growth because it can increase insect activity and visibility — making caterpillars easier to find — while letting parents forage more and reducing nestlings’ thermoregulatory costs. The high water content in caterpillars also helps against dehydration. This contrasts sharply with hotter regions like the Mediterranean, where similar events can exceed 35°C and harm nestlings.”
Early Breeding Helps Birds Cope With Weather Extremes
Broods that hatch earlier in spring tend to benefit from occasional warm spells, when caterpillars are abundant and temperatures remain within safe limits. Birds that breed later in the season face tougher conditions. Their fledglings are about one third lighter, even though the warmest days they experience reach similar temperatures of about 16-17ºC.
Over longer periods, extreme cold and rainfall slightly reduce the odds that young birds will survive to adulthood. In contrast, warm extremes can have small positive effects. Overall, breeding earlier within a season appears to shield many birds from the worst consequences of unpredictable weather.
Understanding Climate Impacts on Wildlife
As climate change intensifies weather extremes, scientists say it will become increasingly important to monitor small scale environmental conditions such as microclimates and habitat differences. This type of research can guide conservation strategies, including nestbox placement and woodland management, to better protect vulnerable chicks during key stages of development.
Researchers plan to continue monitoring the great tit population in Wytham Woods to understand how these weather effects may shift in the future. One key question is whether heatwaves that are currently moderate could eventually become harmful as temperatures continue to rise.
